title: Robert Hanssen

text: Hanssen was recalled yet again to Washington, D.C., in 1987. He was given the task of making a study of all known and rumored penetrations of the FBI in order to find the man who had betrayed Martynov and Motorin; this meant in effect that he was charged with searching for himself. Hanssen ensured that he did not reveal himself with his study, but in addition, he gave the entire study—including the list of all Soviets who had contacted the FBI about FBI moles—to the KGB in 1988. That same year, Hanssen, according to a government report, committed a "serious security breach" by revealing secret information to a Soviet defector during a debriefing. The agents working for him reported this breach to a supervisor, but no action was taken.


title: Robert Hanssen

text: In 1989, Hanssen compromised the FBI investigation of Felix Bloch, a Department of State official who had become suspected for espionage. Hanssen warned the KGB that Bloch was being investigated, causing the KGB to abruptly end contact with him. The FBI was unable to produce any good evidence, and as a result, Bloch was never charged with a crime, although the State Department later terminated his employment and denied his pension. The failure of the Bloch investigation and the FBI's investigation of how the KGB learned that they were investigating Bloch caused the mole hunt that eventually resulted in the arrest of Hanssen.


title: Robert Hanssen

text: Later that year, Hanssen gave extensive information about American planning for measurement and signature intelligence (MASINT), a general term for intelligence collected by a variety of electronic means, such as radar, spy satellites, and signal intercepts. When the Soviets began construction on a new embassy during 1977, the FBI dug a tunnel beneath their decoding room. The FBI planned to use it for eavesdropping, but never did for fear of being caught. Hanssen disclosed this information to the Soviets during September 1989 and received a $55,000 payment the next month, . On two occasions, Hanssen gave the Soviets a complete list of American double agents.


title: Robert Hanssen

text: In 1990, Hanssen's brother-in-law, Mark Wauck, who was also an FBI employee, recommended to the FBI that Hanssen be investigated for espionage because his sister, Hanssen's wife, told him that her sister, Jeanne Beglis, had found a pile of cash on a dresser in the Hanssens' house. Bonnie had previously told her brother that Hanssen once talked about retiring in Poland, then part of the Eastern Bloc. Wauck also knew that the FBI was hunting for a mole and so spoke with his supervisor, who took no action.


title: Robert Hanssen

text: When the USSR disbanded in December 1991, Hanssen, possibly worried that he could be exposed during the ensuing political upheaval, ended communications with his handlers for a time. The next year, after the Russian Federation assumed control of the defunct Soviet spy agencies, Hanssen made a risky approach to the GRU, with whom he had not been in contact in ten months. He went in person to the Russian embassy and physically approached a GRU officer in the parking garage. Hanssen, carrying a package of documents, identified himself by his Soviet code name, "Ramon Garcia", and described himself as a "disaffected FBI agent" who was offering his services as a spy. The Russian officer, who evidently did not recognize the code name, drove away. The Russians then filed an official protest with the State Department, believing Hanssen to be a triple agent. Despite having shown his face, disclosed his code name, and revealed his FBI affiliation, Hanssen escaped arrest when the FBI's investigation into the incident did not advance.


title: Robert Hanssen

text: Hanssen continued to take risks in 1993 when he hacked into the computer of a fellow FBI agent, Ray Mislock, printed out a classified document from Mislock's computer, and took the document to Mislock, saying, "You didn't believe me that the system was insecure." Hanssen's superiors were not amused and began an investigation. In the end, officials believed his claim that he was merely demonstrating flaws in the FBI's security system. Mislock has since theorized that Hanssen probably went onto his computer to see if his superiors were investigating him for espionage, and invented the document story to cover his tracks.


title: Robert Hanssen

text: In 1994, Hanssen expressed interest in a transfer to the new National Counterintelligence Center, which coordinated counterintelligence activities. When told that he would have to take a lie detector test to join, Hanssen changed his mind. Three years later, convicted FBI mole Earl Edwin Pitts told the FBI that he suspected Hanssen due to the Mislock incident. Pitts was the second FBI agent to mention Hanssen by name as a possible mole, but superiors were still unconvinced and no action was taken.


title: Robert Hanssen

text: IT personnel from the National Security Division's IIS Unit were sent to investigate Hanssen's desktop computer after a reported failure. NSD chief Johnnie Sullivan ordered the computer impounded after it seemed to have been tampered with. A digital investigation found that an attempted hacking had occurred using a password cracking program installed by Hanssen, which caused a security alert and lockup. After confirmation by the FBI CART Unit, Sullivan filed a report with the Office of Professional Responsibility requesting further investigation of Hanssen's attempted hack. Hanssen claimed that he was attempting to connect a color printer to his computer, but needed the password cracker to bypass the administrative password. The FBI believed his story and Hanssen was merely given a warning.


title: Robert Hanssen

text: During the same time period, Hanssen searched the FBI's internal computer case record to see if he was being investigated. He was indiscreet enough to type his own name into FBI search engines. Finding nothing, Hanssen decided to resume his spy career after eight years without contact with the Russians. He established contact with the SVR (the successor to the Soviet-era KGB) during the autumn of 1999. He continued to perform incriminating searches of FBI files for his own name and address.


title: Robert Hanssen

text: The existence of two Russian moles working in the U.S. security and intelligence establishment simultaneously—Ames at the CIA and Hanssen at the FBI—complicated counterintelligence efforts during the 1990s. Ames was arrested in 1994. His exposure explained many of the asset losses U.S. intelligence suffered during the 1980s, including the arrest and execution of Martynov and Motorin. However, two cases—- the Bloch investigation and the embassy tunnel—- remained unsolved. Ames had been stationed in Rome at the time of the Bloch investigation, and could not have had knowledge of that case or of the tunnel under the embassy, as he did not work for the FBI.


title: Robert Hanssen

text: The FBI and CIA formed a joint mole-hunting team in 1994 to find the suspected second intelligence leak. They formed a list of all agents known to have access to cases that were compromised. The FBI's codename for the suspected spy was "Graysuit". Some promising suspects were cleared, and the mole hunt found other penetrations, such as CIA officer Harold James Nicholson. However, Hanssen escaped notice, likely because these efforts concentrated on CIA agents rather than FBI agents. The idea that an FBI agent was the second mole was not considered early enough.


title: Robert Hanssen

text: By 1998, using FBI criminal profiling techniques, the pursuers suspected an innocent man: Brian Kelley, a CIA operative involved in the Bloch investigation. The CIA and FBI searched his house, tapped his telephone and surveilled him, following him and his family everywhere. In November 1998, they had a man with a foreign accent come to Kelley's door, warn him that the FBI knew he was a spy and tell him to show up at a Metro station the next day in order to escape. Kelley instead reported the incident to the FBI. In 1999, the FBI even interrogated Kelley, his ex-wife, two sisters and three children. All denied everything. He was eventually placed on administrative leave, where he remained falsely accused until after Hanssen was arrested.


title: Robert Hanssen

text: FBI investigators later made progress during an operation in which they paid disaffected Russian intelligence officers to deliver information on moles. They paid $7 million to KGB agent Alexandr Shcherbakov who had access to a file on "B". While it did not contain Hanssen's name, among the information was an audiotape of a July 21, 1986, conversation between "B" and KGB agent Aleksander Fefelov. FBI agent Michael Waguespack thought the voice was familiar, but could not remember who it was. Rifling through the rest of the files, they found notes of the mole using a quote from General George S. Patton about "the purple-pissing Japanese". FBI analyst Bob King remembered Hanssen using that same quote. Waguespack listened to the tape again and recognized the voice as belonging to Hanssen. With the mole finally identified, locations, dates, and cases were matched with Hanssen's activities during the time period. Two fingerprints collected from a trash bag in the file were analyzed and proved to be Hanssen's.


title: Robert Hanssen

text: The FBI surveilled Hanssen and soon discovered that he was again in contact with the Russians. In order to bring him back to FBI headquarters, where he could be closely monitored and kept away from sensitive data, they promoted him in December 2000 and gave him a new job supervising FBI computer security. In January 2001, Hanssen was given an office and an assistant, Eric O'Neill, who, in reality, was a young FBI surveillance specialist who had been assigned to watch Hanssen. O'Neill ascertained that Hanssen was using a Palm III PDA to store his information. When O'Neill was able to briefly obtain Hanssen's PDA and have agents download and decode its encrypted contents, the FBI had decisive evidence.


title: Robert Hanssen

text: During his final days with the FBI, Hanssen began to suspect that something was wrong. In early February 2001, he asked his friend at a computer technology company for a job. He also believed he was hearing noises on his car radio that indicated that it was bugged, although the FBI was later unable to reproduce the noises Hanssen claimed to have heard. In the last letter he wrote to the Russians, which was found by the FBI when he was arrested, Hanssen said that he had been promoted to a "do-nothing job ... outside of regular access to information," and that, "Something has aroused the sleeping tiger".


title: Robert Hanssen

text: However, Hanssen's suspicions did not stop him from making one more dead drop. After leaving a friend at an airport on February 18, 2001, Hanssen drove to Virginia's Foxstone Park. He placed a white piece of tape on a park sign, which was a signal to his Russian contacts that there was information at the dead drop site. He then followed his usual routine, taking a package consisting of a sealed garbage bag of classified material and taping it to the bottom side of a wooden footbridge over a creek. When FBI agents observed this incriminating act, they rushed in to arrest Hanssen. Upon being arrested, Hanssen asked, "What took you so long?" The FBI waited two more days to see if any of Hanssen's SVR handlers would show up at Foxstone Park. When they failed to appear, the Justice Department announced the arrest on February 20.


title: Robert Hanssen

text: With the representation of Washington lawyer Plato Cacheris, Hanssen negotiated a plea bargain that enabled him to avoid the death penalty in exchange for cooperating with authorities. On July 6, 2001, he pleaded guilty to 13 counts of espionage, one count of attempted espionage, and one of conspiracy to commit espionage in the U.S. District Court for the Eastern District of Virginia. On May 10, 2002, Hanssen was sentenced to 15 consecutive sentences of life in prison without the possibility of parole. "I apologize for my behavior. I am shamed by it," Hanssen told U.S. District Judge Claude Hilton. "I have opened the door for calumny against my totally innocent wife and children. I have hurt so many deeply."


title: Robert Hanssen

text: Hanssen is Federal Bureau of Prisons prisoner #48551-083. He is serving his sentence at the ADX Florence, a federal supermax prison near Florence, Colorado, in solitary confinement for 23 hours a day.


title: Robert Hanssen

text: Hanssen never told the KGB or GRU his identity and refused to meet them personally, with the exception of the abortive 1993 contact in the Russian embassy parking garage. The FBI believes that the Russians never knew the name of their source. Going by the alias "Ramon" or "Ramon Garcia", Hanssen exchanged intelligence and payments through an old-fashioned dead drop system in which he and his KGB handlers left packages in public, unobtrusive places. He refused to use the dead drop sites that his handler, Victor Cherkashin, suggested and instead chose his own. He also designated a code to be used when dates were exchanged. Six was to be added to the month, day, and time of a designated drop time, so that, for example, a drop scheduled for January 6 at 1:00p.m. would be written as July 12 at 7:00p.m..


title: Robert Hanssen

text: Despite these efforts at caution and security, Hanssen could at times be reckless. He once said in a letter to the KGB that it should emulate the management style of Mayor of Chicago Richard J. Daley—a comment that easily could have led an investigator to look at people from Chicago. Hanssen took the risk of recommending to his handlers that they try to recruit his closest friend, a colonel in the United States Army.


title: Robert Hanssen

text: According to "USA Today", those who knew the Hanssens described them as a close family. They attended Mass weekly and were very active in Opus Dei. Hanssen's three sons attended The Heights School in Potomac, Maryland, an all-boys preparatory school. His three daughters attended Oakcrest School for Girls in Vienna, Virginia, an independent Roman Catholic school. Both schools are associated with Opus Dei. Hanssen's wife, Bonnie, retired from teaching theology at Oakcrest in 2020.


title: Robert Hanssen

text: A priest at Oakcrest said that Hanssen had regularly attended a 6:30 a.m. daily Mass for more than a decade. Opus Dei member C. John McCloskey said he also occasionally attended the daily noontime Mass at the Catholic Information Center in downtown Washington. After being imprisoned, Hanssen claimed he periodically admitted his espionage to priests in confession. He urged fellow Catholics in the FBI to attend Mass more often and denounced the Russians as "godless", even though he had been spying for them.


title: Robert Hanssen

text: At Hanssen's suggestion, and without the knowledge of his wife, a friend named Jack Hoschouer, a retired Army officer, would sometimes watch the Hanssens having sex through a bedroom window. Hanssen then began to secretly videotape his sexual encounters and shared the videotapes with Hoschouer. Later, he hid a video camera in the bedroom that was connected via closed-circuit television line so that Hoschouer could observe the Hanssens from his guest bedroom. He also explicitly described the sexual details of his marriage on Internet chat rooms, giving information sufficient for those who knew them to recognize the couple.


title: Robert Hanssen

text: Hanssen frequently visited D.C. strip clubs and spent a great deal of time with a Washington stripper named Priscilla Sue Galey. She went with Hanssen on visits to Hong Kong and to the FBI training facility in Quantico, Virginia. Hanssen gave her money, jewels, and a used Mercedes-Benz, but ended contact with her before his arrest when she began abusing drugs and doing sex work. Galey claims that although she offered to have sex with him, Hanssen declined, saying that he was trying to convert her to Catholicism.


title: Robert Hanssen

text: The Hanssen spy case is told in Ronald Kessler's book "The Secrets of the FBI" in chapter 15, "Catching Hanssen," chapter 16, "Breach", and chapter 17, "Unexplained Cash", based in part on interviews with Michael Rochford, who directed the FBI team that eventually caught Hanssen after initially wrongly assuming a CIA officer was the master spy.


title: Robert Hanssen

text: Hanssen was the subject of a 2002 made-for-television movie, "", with the teleplay by Norman Mailer and starring William Hurt as Hanssen. Hanssen's jailers allowed him to watch this movie, but he was so angered by it that he turned it off.


title: Robert Hanssen

text: Eric O'Neill's role in the capture of Robert Hanssen was dramatized in the 2007 movie "Breach", in which Chris Cooper played the role of Hanssen and Ryan Phillippe played O'Neill.


title: Robert Hanssen

text: Hanssen is mentioned in chapter 5 of Dan Brown's book "The Da Vinci Code", as the most noted Opus Dei member to non-members. Because of his sexual deviancy and espionage conviction, the organization's reputation was badly hurt.


title: Robert Hanssen

text: The American Court TV (now TruTV) television series "Mugshots" released an episode on the Robert Hanssen case titled "Robert Hanssen – Hanssen and the KGB".


title: Robert Hanssen

text: The investigation is covered in O'Neill's memoir "", published by Penguin Random House in spring 2019. David Wise wrote "".


title: Robert Hanssen

text: Hanssen's story was featured in episode 4, under the name of "Perfect Traitor", of Smithsonian Channel's series "Spy Wars", aired end of 2019 and narrated by Damian Lewis.


title: Robert Hanssen

text: Hanssen is mentioned in the seventh episode of The History Channel series "America's Book of Secrets", as well as in the fifth episode of Netflix series "Spycraft".


title: Mil Mi-17

text: The Mil Mi-17 (NATO reporting name: Hip) is a Soviet-designed Russian military helicopter family introduced in 1975 (Mi-8M), continuing in production at two factories, in Kazan and Ulan-Ude. It is known as the Mi-8M series in Russian service. The helicopter is mostly used as a medium twin-turbine transport helicopter, as well as an armed gunship version.


title: Mil Mi-17

text: Developed from the basic Mi-8 airframe, the Mi-17 was fitted with the larger Klimov TV3-117MT engines, rotors, and transmission developed for the Mi-14, along with fuselage improvements for heavier loads. Optional engines for "hot and high" conditions are the 1545 kW (2070 shp) Isotov TV3-117VM. Recent exports to China and Venezuela for use in high mountains have the new Klimov VK-2500 version of the Klimov TV3-117 engine with FADEC control.


title: Mil Mi-17

text: The designation Mi-17 is for export; Russian armed forces call it Mi-8MT. The Mi-17 can be recognized because it has the tail rotor on the port side instead of the starboard side, and dust shields in front of the engine intakes. Engine cowls are shorter than on the TV2-powered Mi-8, not extending as far over the cockpit, and an opening for a bleed air valve outlet is present forward of the exhaust.


title: Mil Mi-17

text: Actual model numbers vary by builder, engine type, and other options. As an example, the sixteen new Ulan-Ude-built machines delivered to the Czech Air Force in 2005 with -VM model engines were designated as Mi-171Sh, a development of the Mi-8AMTSh. Modifications include a new large door on the right side, improved Czech-built APU, and Kevlar armor plates around the cockpit area and engines. Eight have a loading ramp in place of the usual clamshell doors and can load a vehicle up to the size of an SUV.


title: Mil Mi-17

text: In March 2007, licensed production of the Mi-17 started in China, with production being led by Mil Moscow Helicopter Plant JSC and the Sichuan Lantian Helicopter Company Limited in Chengdu, Sichuan province as a joint venture. The plant built 20 helicopters in 2008, using Russian Ulan-Ude-supplied kits; annual production was expected to increase to 80 helicopters. The variants planned to be built by Lantian include the Mi-171, Mi-17V-5, and Mi-17V-7. In 2021 it was reported that China was replacing the Russian-made Mi-17 with their own Z-20, except possibly for the Mi-171Sh assault helicopter; the last Mi-17 order was in 2014.


title: Mil Mi-17

text: In 2021 the Web site of Russian Helicopters, the manufacturer, said that the Mi-8/17 was "the most widely operated helicopter in history."


title: Mil Mi-17

text: Royal Cambodian Air Force Mi-17s were used during the Cambodian government's offensives; by 1994 ten operational airframes five were converted to helicopter gunships equipped with 57 mm rocket pods and providing air support for ground forces attacking the Khmer Rouge positions. In 1996 the Government launched an offensive during the dry season at the Khmer Rouge stronghold of Anlong Veng and Pailin, using five Mi-17 gunships and eight Mi-17-Mi-8 troops transports.


title: Mil Mi-17

text: In May 1999, during Operation Safed Sagar, the Mi-17 was used in the first air phase of the Kargil War by 129HU of the Indian Air Force against Pakistani regular. One Mi-17 was downed by a shoulder-fired missile, and a fighter aircraft was lost in combat. This led to the withdrawal of armed helicopters and attacks by fixed-wing aircraft began.


title: Mil Mi-17

text: The Mi-17 was used extensively by the Sri Lanka Air Force in Sri Lanka's war on terrorism by the LTTE. Seven of them were lost in combat and attacks on airports.


title: Mil Mi-17

text: Freelance pilot Neall Ellis operated an Mi-17 in support of the Sierra Leone government in the Sierra Leone Civil War, ferrying ammunition and other supplies to government troops.


title: Mil Mi-17

text: US Army Special Forces in Afghanistan extensively used CIA-operated Mi-17s during the initial stages of Operation Enduring Freedom.


title: Mil Mi-17

text: The Mi-17 is used for passenger transport by Air Koryo, national airline of North Korea. Previous flights include those between Pyongyang and Kaesong and Pyongyang and Haeju.


title: Mil Mi-17

text: The Mexican Navy uses its Mi-17s for anti-narcotic operations such as locating marijuana fields and dispatching marines to eradicate the plantations.


title: Mil Mi-17

text: Mi-17s are operated by the Afghan Air Force. In July 2010 two Mi-17 were flown by a mixed crew of United States Air Force and Afghan Air Force personnel in a 13-hour mission that rescued 2,080 civilians from flood waters. This was the largest rescue by two helicopters in USAF history. USAF pilot Lt Col Gregory Roberts received the Distinguished Flying Cross for the mission.


title: Mil Mi-17

text: During the Tham Luang cave rescue in July 2018, the Thai army used the Mi-17 helicopter for searching operations, and evacuating the first batch of survivors from Tham Luang to Chiang Rai hospital; on 10 July 2018 a Mil Mi-17 helicopter took the last evacuated boy to the hospital.


title: Mil Mi-17

text: In October 2007, the Saudi Arabian Government cancelled the purchase of 64 NHIndustries NH90 helicopters and agreed to buy 150 Russian-made Mil Mi-17 and Mi-35 helicopters instead.


title: Mil Mi-17

text: On 28 October 2008, the Royal Thai Army announced a deal to buy six Mi-17s to meet its requirement for a medium-lift helicopter. This is the first time the Thai armed forces have acquired Russian aircraft instead of American aircraft. "Flight International" quotes the Thai Army's rationale: "We are buying three Mi-17 helicopters for the price of one Black Hawk. The Mi-17 can also carry more than 30 troops, while the Black Hawk could carry only 13 soldiers. These were the key factors behind the decision."


title: Mil Mi-17

text: On 15 December 2008, it was reported that India ordered 80 Mi-17V-5 helicopters worth $1.375 billion, which would be delivered to the Indian Air Force between 2011 and 2014 to replace aging Mi-8s. In August 2010, it was reported that India planned to order another 59 Mi-17s. The first Mi-17V-5s entered service with India in February 2012. In December 2012, India signed a contract for 71 aircraft at a reported cost of US$1.3 billion. In December 2014 it was reported that India is in agreement with the Russian Federation to produce on its territory Mi-17s and Ka-226Ts. All 151 helicopters were delivered as of February 2016.


title: Mil Mi-17

text: On 11 June 2009, it was announced that the United States had handed over four Mi-17 cargo helicopters to the Pakistan Army to facilitate its counter-terrorism operations. This followed an urgent request for helicopters by Pakistan's Chief of Army Staff General Ashfaq Parvez Kayani in a leaked US embassy cable published on WikiLeaks.


title: Mil Mi-17

text: On 10 July 2009, it was announced that Chile would pursue talks with Russia to purchase five Mi-17 multi-role helicopters for the Chilean Air Force, despite pressure from the United States. However, as of January 2013, it seems that these plans were canceled.


title: Mil Mi-17

text: On 16 September 2009, the United States Navy delivered the last two of four Mi-17s to the Afghan National Army Air Corps. On 19 June 2010, it was announced that the US government would buy and refurbish 31 more Mi-17 helicopters from Russia to supply the Afghan Air Force.


title: Mil Mi-17

text: The US was reportedly considering adding the helicopter to the US military for special forces use in order to obscure troop movements. The US has used some Mi-8s and Mi-17s for training, and has purchased units for allies in Iraq, Afghanistan and Pakistan.


title: Mil Mi-17

text: In August 2010 a contract was signed by the Argentine Air Force for two Mi-17Es, plus an option on another three, to support Antarctic bases.


title: Mil Mi-17

text: In September 2010, the Polish Defense Minister announced that his country would buy five new Mi-17s from Russia, to support Polish operations in Afghanistan. All five Mi-17-1Vs were delivered by 2011.


title: Mil Mi-17

text: In 2011, Chief of Staff of the Afghan National Army Abdul Wahab Wardak announced that the US government would buy Mi-17s for use by Afghanistan's troops. He explained the choice of the Russian helicopter over the American Chinook was due to the familiarity of the Afghan technical and pilot staff with the helicopter type and that it is better suited for Afghanistan's environment. The United States continued to purchase the helicopters for Afghanistan in 2013, despite a congressional prohibition. Overall, 63 Mi-17s were acquired through the 2011 contract at a cost of US$16.4 to US$18.4 million each, or US$4 to US$6 million more each than a refurbished American Chinook.


title: Mil Mi-17

text: China signed two contracts with Rosoboronexport in 2009 and 2012 for 32 and 52 Mi-171E, respectively.


title: Mil Mi-17

text: The Helicopters of Russia has concluded a contract with the Defense Ministry of Belarus for the supply of twelve Mi-8MTV-5 military transport helicopters in 2016–2017. The Belarusian military will get the helicopters possessing the same parameters as those used by the Russian military. The contract was executed in April 2017.


title: Mil Mi-17

text: In the course of the Army-2017 International Military Technical Forum signed a contract to supply two Mi-171Sh helicopters to Burkina Faso. Also signed a contract for the supply of helicopters to Russian state special purpose aviation. Three Mi-8AMTSh military transport helicopters were produced and three more were ordered later.


title: Mil Mi-17

text: Royal Thai Army ordered 2 Mi-17V-5s in September 2017 which received in December 2018 plus 3 more received in March 2021 for a total of 10 delivered since 2011. RF National Guard ordered two Mi-8AMTSHs in April 2018.


title: Mil Mi-17

text: During the Hydroaviasalon-2018 exhibition, subsidiaries of Rostec State Corporation – Russian Helicopters, National Service of Medical Aviation and Avia Capital Services LLC – signed a contract to supply 104 Ansat and 46 Mi-8AMT medically equipped helicopters.


title: Mil Mi-17

text: A contract was signed on 18 January 2019 between Russian Helicopters, Kazakhstan Engineering, and Kazakh firm Aircraft Repair Plant No 405 (ARP 405) that will see 45 kit versions of the Mil Mi-8AMT and Mi-171 helicopters delivered to Kazakhstan until 2025 for local assembly.


title: Mil Mi-17

text: In 2019, China ordered 100 Mi-171 (including 18 combat-transport Mi-171Sh) and 21 Kazan Ansat helicopters. A contract with the civilian airline QINGDAO for the supply of six Mi-171 helicopters with VK-2500-03 engines was signed in December 2019.


title: Mil Mi-17

text: In 2019, the Philippine Air Force reportedly expressed its interest in acquiring 16 Mi-171 helicopters for its heavy-lift helicopter requirement, with a possible option to add one Mi-171 that is fitted for VVIP transport, in a deal worth P12.5 billion to be signed during the official visit to Russia by President Rodrigo Duterte. From March 3 to 7, 2020, a delegation composed of Philippine military and defense officials, and officials of the Embassy of the Philippines in Russia met with representatives from Sovtechnoexport and visited the Ulan-Ude Aviation Plant. Local defense blog Maxdefense Philippines reported that a Notice of Award (NOA) was issued during the third quarter of 2020, although it was unclear if it was awarded to Sovtechnoexport or Rosoboronexport. On 27 July 2022, the Philippine government canceled the deal to purchase 16 Mi-17 helicopters due to fears of possible U.S. sanctions.


title: Mil Mi-17

text: Russia in October 2021 delivered to Mali 2 Mi-171Sh and 2 Mi-17V-5 helicopters in the framework of a contract signed in December 2020.


title: Mil Mi-17

text: On 13 April 2022, US President Joe Biden announced that the US Government would supply 11 Mi-17 helicopters to the Ukrainian government to aid in its defense during the Russian invasion of Ukraine. The U.S. supplied 16 Mi-17s including five that were undergoing maintenance in Ukraine when Russia invaded. These helicopters belonged to the former Afghan government, but were paid for by the US under the Afghanistan Security Forces Fund. Therefore the Pentagon treated these aircraft as its property after that government fell in September 2021.


title: Man on Fire (2004 film)

text: Man on Fire is a 2004 action thriller film directed by Tony Scott from a screenplay by Brian Helgeland, and based on the 1980 novel of the same name by A. J. Quinnell. The novel had previously been adapted into a feature film in 1987. In this film, Denzel Washington portrays John Creasy, a despondent, alcoholic former CIA SAD/SOG officer (and U.S. Marine Corps Force Reconnaissance captain) turned bodyguard, who goes on a revenge rampage after his charge, nine-year-old Lupita "Pita" Ramos (Dakota Fanning), is abducted in Mexico City. The supporting cast includes Christopher Walken, Radha Mitchell, Giancarlo Giannini, Marc Anthony, Rachel Ticotin and Mickey Rourke.


title: Man on Fire (2004 film)

text: The film received mixed reviews. It grossed $130 million worldwide against a production budget of $60–70 million.


title: Man on Fire (2004 film)

text: Former CIA SAD/SOG officer John W. Creasy visits his old friend Paul Rayburn in Mexico. Rayburn convinces him to take a bodyguard position with Samuel Ramos, a Mexico City automaker whose young daughter Lupita "Pita" requires a bodyguard for her kidnapping insurance policy to take effect. Struggling with alcoholism, burnout, and guilt over his actions with the CIA, Creasy tries to commit suicide but the bullet in his gun misfires. While coaching Pita to become a more confident swimmer, he bonds with her.


title: Man on Fire (2004 film)

text: Waiting outside Pita’s piano lesson, Creasy recognizes a car that followed them earlier, as two Federal Police officers block the street. Realizing that Pita is about to be abducted, Creasy kills four of the attackers including the officers but is critically wounded and Pita is taken. Recovering from his injuries, Creasy is declared a suspect, but reporter Mariana Garcia Guerrero questions the story. AFI agent Miguel Manzano relocates Creasy to a veterinary clinic to protect him from corrupt police.


title: Man on Fire (2004 film)

text: "The Voice," leader of the kidnapping ring, demands a $10 million ransom and Samuel complies, with the help of police Lt. Victor Fuentes. However, the kidnappers are ambushed at the ransom drop and the Voice’s nephew is killed. Holding the Ramoses responsible, the Voice informs them that Pita will be lost to them forever as retribution. Guerrero warns Creasy that the kidnappers belong to a powerful “brotherhood” called La Hermandad that consists of corrupt officials, police, and criminals, but Creasy promises Pita’s mother, Lisa, that he will kill everyone responsible for Pita's death.


title: Man on Fire (2004 film)

text: With Rayburn’s help, Creasy obtains a small arsenal of weapons and equipment. He brutally interrogates and kills the getaway driver, who points him to a club where he confronts three of the kidnappers. Killing middleman "Jersey Boy" and another criminal, Creasy recovers an incriminating ATM card and another kidnapped girl. He turns both over to Guerrero, who reveals that Fuentes is part of the brotherhood. Manzano interviews Rayburn, who describes Creasy as an “artist of death.”


title: Man on Fire (2004 film)

text: Guerrero convinces Manzano to help Creasy wage war against the kidnappers. Waylaying Fuentes’ motorcade with a rocket-propelled grenade, Creasy abducts the officer, who admits that he had his officers ambush the ransom drop, but discovered afterward most of the ransom money had already been stolen by Jordan Kalfus, Samuel's lawyer. After killing Fuentes with a bomb in his rectum, Creasy searches Kalfus’ home and finds his decapitated body and a fax with bank account information leading to Samuel.


title: Man on Fire (2004 film)

text: Creasy confronts the Ramoses, and Samuel confesses that Kalfus suggested they arrange Pita’s kidnapping to claim the insurance payout to pay his father's debts, though they were promised Pita would be unharmed. When Fuentes interfered with the drop, Samuel blamed Kalfus for Pita's death and killed him. Lisa, unaware of Samuel's involvement, angrily tells Creasy to kill her husband or she’ll do it herself; he leaves Samuel a gun and the bullet he attempted suicide with, and a remorseful Samuel shoots himself.


title: Man on Fire (2004 film)

text: Guerrero and Manzano trace the ATM card to the Voice’s wife and find her address, allowing Manzano's officers to infiltrate her home and obtain a photo of the Voice. Despite the brotherhood’s threats, Guerrero publishes the photo and passes the address on to Creasy. Taking the Voice’s wife and his brother Aurelio prisoner, Creasy is shot in the chest but learns the ringleader’s real name is Daniel Sanchez. He calls Daniel and threatens his family, but Daniel reveals that Pita is alive, offering to trade her for his brother and for Creasy, which Creasy accepts.


title: Man on Fire (2004 film)

text: Instructing Lisa to join him at the exchange, Creasy reunites with Pita in the middle of an overpass, assuring her that he loves her before sending her to her mother. He and Aurelio are taken by Daniel’s men, but Creasy succumbs to his wounds. Manzano tracks Daniel down later that day and shoots him dead “during the course of arrest.”


title: Man on Fire (2004 film)

text: Tony Scott, the film's director, had tried to adapt the 1980 source novel, by A. J. Quinnell, into a film in 1983. Journalist Paul Davies theorized that movie producers likely believed that Scott, whose only directorial work as of the time was 1983's "The Hunger", lacked the experience to direct this as his second film.


title: Man on Fire (2004 film)

text: The novel was first adapted into the 1987 film "Man on Fire", starring Scott Glenn as Creasy. This movie, like the novel, was set in Italy, then a major center of kidnapping.


title: Man on Fire (2004 film)

text: When a remake was first under consideration, producer Arnon Milchan (who also produced the 1987 version) looked at Michael Bay and Antoine Fuqua to direct, before asking Scott if he was still interested.


title: Man on Fire (2004 film)

text: 20th Century Fox wanted the film to still be set in Italy. An early draft of the script was set in Naples, with early reporting suggesting that the Mexico City filming was an odd stand in for Naples. Scott argued that if the setting would be Italy, then the film would have to be a period piece, since by the 2000s kidnappings had become a rare occurrence in Italy. Mexico City became the setting of the 2004 film because Mexico City had a high kidnapping rate, and for other reasons unspecified. As a result, the character Rika Balletto was renamed Lisa Martin Ramos, and Pinta Balletto was renamed Lupita "Pita" Ramos. Ettore Balletto became Samuel Ramos. Robert De Niro was originally offered the role of Creasy. Marlon Brando was the original choice to play Rayburn.


title: Man on Fire (2004 film)

text: "Man on Fire" opened in the U.S. on April 23, 2004, in 2,980 theaters and grossed $22,751,490 with an average of $7,634 and ranking No. 1 at the box office. The film's widest release was 2,986 theaters and it ended up earning $77,911,774 in North America and $52,381,940 internationally for a total of $130,293,714 worldwide, against its $60–70 million production budget. The film was successful in the U.S. home video market, grossing more than $123 million in DVD and VHS rentals and sales in U.S.


title: Man on Fire (2004 film)

text: On Rotten Tomatoes the film has a rating of 38% based on 169 reviews, with an average rating of 5.30/10. The consensus states, ""Man on Fire" starts out well, but goes over the top in the violent second half." On Metacritic the film has a score of 47 out of 100 based on 36 reviews, indicating "mixed or average reviews". Audiences polled by CinemaScore gave the film a grade "A-" on scale of A to F.


title: Man on Fire (2004 film)

text: Paul Davies, a journal article author, said that the critical reception to "Man on Fire" in the United States was "somewhat less than kind" because critics did not like the vigilantism that Creasy uses. Davies argues that "most critics missed" Creasy not taking "sadistic pleasure" in the killings since he kills to get information to get to all of the people involved in the kidnapping of Pita Ramos, and does not like harming innocent parties.


title: Man on Fire (2004 film)

text: A. J. Quinnell had a favorable reception to this adaptation, mainly because the film used many of the book's lines. Quinnell said that usually screenwriters "like to leave their mark on the product." Quinnell added that even though he usually dislikes film adaptations of books, the writers "did a good job with "Man On Fire" and I loved the chemistry between Creasy and the girl" and "When I first heard Denzel was playing the part of Creasy I missed a couple of heartbeats but he played the part brilliantly. The film is violent and if the anger is not portrayed properly, the result can be awful." Kevin Freese of the Foreign Military Studies Office stated that "it appears that the allusion" of the fictional Sánchez brothers with the real Arizmendi brothers "escaped the comprehension of much of the audience."


title: Man on Fire (2004 film)

text: The cut "Smiling", from the soundtrack composed by Harry Gregson-Williams, has been adopted as the theme of a number of television commercials for Omega Watches in 2012 to 2013. The soundtrack containing 20 tracks, was composed by Harry Gregson-Williams, and was released on July 27, 2004.


title: Man on Fire (2004 film)

text: The film heavily features the work of Nine Inch Nails. Trent Reznor is credited as "Musical Consultant". The movie features six Nine Inch Nails songs.


title: Man on Fire (2004 film)

text: In 2005, a Hindi remake of the film by director Apoorva Lakhia, called "Ek Ajnabee", was released. It starred Amitabh Bachchan as the lead (named Suryaveer "Surya" Singh). The same year, it was also remade in Tamil language as "Aanai" starring Arjun Sarja.


title: Tunguska event

text: The Tunguska event (occasionally also called the Tunguska incident) was an approximately 12-megaton explosion that occurred near the Podkamennaya Tunguska River in Yeniseysk Governorate (now Krasnoyarsk Krai), Russia, on the morning of June 30, 1908. The explosion over the sparsely populated Eastern Siberian Taiga flattened an estimated 80 million trees over an area of of forest, and eyewitness reports suggest that at least three people may have died in the event.<ref name="SAO/NASA - ADS"></ref> The explosion is generally attributed to a meteor air burst: the atmospheric explosion of a stony asteroid about in size. The asteroid approached from the east-southeast, and likely with a relatively high speed of about (~Ma 80). It is classified as an impact event, even though no impact crater has been found; the object is thought to have disintegrated at an altitude of rather than having hit the surface of the Earth.


title: Tunguska event

text: The Tunguska event is the largest impact event on Earth in recorded history, though much larger impacts have occurred in prehistoric times. An explosion of this magnitude would be capable of destroying a large metropolitan area. It has been mentioned numerous times in popular culture, and has also inspired real-world discussion of asteroid impact avoidance.


title: Tunguska event

text: On 30 June 1908 (N. S.) (cited in Russia as 17 June 1908, O. S., before the implementation of the Soviet calendar in 1918), at around 07:17 local time, Evenki natives and Russian settlers in the hills northwest of Lake Baikal observed a bluish light, nearly as bright as the Sun, moving across the sky and leaving a thin trail. Closer to the horizon, there was a flash producing a billowing cloud, followed by a pillar of fire that cast a red light on the landscape. The pillar split in two and faded, turning to black. About ten minutes later, there was a sound similar to artillery fire. Eyewitnesses closer to the explosion reported that the source of the sound moved from the east to the north of them. The sounds were accompanied by a shock wave that knocked people off their feet and broke windows hundreds of kilometres away.


title: Tunguska event

text: The explosion registered at seismic stations across Eurasia, and air waves from the blast were detected in Germany, Denmark, Croatia, and the United Kingdom—and as far away as Batavia, Dutch East Indies, and Washington, D.C. It is estimated that, in some places, the resulting shock wave was equivalent to an earthquake measuring 5.0 on the Richter magnitude scale. Over the next few days, night skies in Asia and Europe were aglow. There are contemporaneous reports of brightly lit photographs being successfully taken at midnight (without the aid of flashbulbs) in Sweden and Scotland. It has been theorized that this sustained glowing effect was due to light passing through high-altitude ice particles that had formed at extremely low temperatures as a result of the explosion—a phenomenon that decades later was reproduced by Space Shuttles. In the United States, a Smithsonian Astrophysical Observatory program at the Mount Wilson Observatory in California observed a months-long decrease in atmospheric transparency consistent with an increase in suspended dust particles.


title: Tunguska event

text: Though the region of Siberia in which the explosion occurred was very sparsely populated in 1908, there are accounts of the event from eyewitnesses who were in the surrounding area at the time, and regional newspapers reported the event shortly after it occurred.


title: Tunguska event

text: According to the testimony of S. Semenov, as recorded by Russian mineralogist Leonid Kulik's expedition in 1930:


title: Tunguska event

text: Since the 1908 event, there have been an estimated 1,000 scholarly papers (most in Russian) published about the Tunguska explosion. Owing to the remoteness of the site and the limited instrumentation available at the time of the event, modern scientific interpretations of its cause and magnitude have relied chiefly on damage assessments and geological studies conducted many years after the event. Estimates of its energy have ranged from .


title: Tunguska event

text: It was not until more than a decade after the event that any scientific analysis of the region took place, in part due to the isolation of the area and significant political upheaval affecting Russia in the 1910s. In 1921, the Russian mineralogist Leonid Kulik led a team to the Podkamennaya Tunguska River basin to conduct a survey for the Soviet Academy of Sciences. Although they never visited the central blast area, the many local accounts of the event led Kulik to believe that the explosion had been caused by a giant meteorite impact. Upon returning, he persuaded the Soviet government to fund an expedition to the suspected impact zone, based on the prospect of salvaging meteoric iron.


title: Tunguska event

text: Kulik led a scientific expedition to the Tunguska blast site in 1927. He hired local Evenki hunters to guide his team to the centre of the blast area, where they expected to find an impact crater. To their surprise, there was no crater to be found at ground zero. Instead they found a zone, roughly across, where the trees were scorched and devoid of branches, but still standing upright. Trees more distant from the centre had been partly scorched and knocked down in a direction away from the centre, creating a large radial pattern of downed trees.


title: Tunguska event

text: In the 1960s, it was established that the zone of levelled forest occupied an area of , its shape resembling a gigantic spread-eagled butterfly with a "wingspan" of and a "body length" of . Upon closer examination, Kulik located holes that he erroneously concluded were meteorite holes; he did not have the means at that time to excavate the holes.


title: Tunguska event

text: During the following 10 years, there were three more expeditions to the area. Kulik found several dozens of little "pothole" bogs, each in diameter, that he thought might be meteoric craters. After a laborious exercise in draining one of these bogs (the so-called "Suslov's crater", in diameter), he found an old tree stump on the bottom, ruling out the possibility that it was a meteoric crater. In 1938, Kulik arranged for an aerial photographic survey of the area covering the central part of the leveled forest (). The original negatives of these aerial photographs (1,500 negatives, each ) were burned in 1975 by order of Yevgeny Krinov, then Chairman of the Committee on Meteorites of the USSR Academy of Sciences, as part of an initiative to dispose of flammable nitrate film. Positive prints were preserved for further study in the Russian city of Tomsk.


title: Tunguska event

text: Expeditions sent to the area in the 1950s and 1960s found microscopic silicate and magnetite spheres in siftings of the soil. Similar spheres were predicted to exist in the felled trees, although they could not be detected by contemporary means. Later expeditions did identify such spheres in the resin of the trees. Chemical analysis showed that the spheres contained high proportions of nickel relative to iron, which is also found in meteorites, leading to the conclusion they were of extraterrestrial origin. The concentration of the spheres in different regions of the soil was also found to be consistent with the expected distribution of debris from a meteoroid air burst. Later studies of the spheres found unusual ratios of numerous other metals relative to the surrounding environment, which was taken as further evidence of their extraterrestrial origin.


title: Tunguska event

text: Chemical analysis of peat bogs from the area also revealed numerous anomalies considered consistent with an impact event. The isotopic signatures of carbon, hydrogen, and nitrogen at the layer of the bogs corresponding to 1908 were found to be inconsistent with the isotopic ratios measured in the adjacent layers, and this abnormality was not found in bogs located outside the area. The region of the bogs showing these anomalous signatures also contains an unusually high proportion of iridium, similar to the iridium layer found in the Cretaceous–Paleogene boundary. These unusual proportions are believed to result from debris from the falling body that deposited in the bogs. The nitrogen is believed to have been deposited as acid rain, a suspected fallout from the explosion.


title: Tunguska event

text: However other scientists disagree: "Some papers report that hydrogen, carbon and nitrogen isotopic compositions with signatures similar to those of CI and CM carbonaceous chondrites were found in Tunguska peat layers dating from the TE (Kolesnikov et al. 1999, 2003) and that iridium anomalies were also observed (Hou et al. 1998, 2004). Measurements performed in other laboratories have not confirmed these results (Rocchia et al. 1990; Tositti et al. 2006).".


title: Tunguska event

text: Researcher John Anfinogenov has suggested that a boulder found at the event site, known as John's stone, is a remnant of the meteorite, but oxygen isotope analysis of the quartzite suggests that it is of hydrothermal origin, and probably related to Permian-Triassic Siberian Traps magmatism.


title: Tunguska event

text: In 2013, a team of researchers published the results of an analysis of micro-samples from a peat bog near the centre of the affected area, which show fragments that may be of extraterrestrial origin.


title: Tunguska event

text: The leading scientific explanation for the explosion is a meteor air burst by an asteroid above the Earth's surface.


title: Tunguska event

text: Meteoroids enter Earth's atmosphere from outer space every day, travelling at a speed of at least . The heat generated by compression of air in front of the body (ram pressure) as it travels through the atmosphere is immense and most meteoroids burn up or explode before they reach the ground. Early estimates of the energy of the Tunguska air burst ranged from to 30 megatons of TNT (130 PJ), depending on the exact height of the burst as estimated when the scaling laws from the effects of nuclear weapons are employed. More recent calculations that include the effect of the object's momentum find that more of the energy was focused downward than would be the case from a nuclear explosion and estimate that the air burst had an energy range from 3 to 5 megatons of TNT (13 to 21 PJ). The 15-megaton (Mt) estimate represents an energy about 1,000 times greater than that of Trinity, and roughly equal to that of the United States' Castle Bravo nuclear test in 1954 (which measured 15.2 Mt) and one-third that of the Soviet Union's Tsar Bomba test in 1961. A 2019 paper suggests the explosive power of the Tunguska event may have been around 20–30 megatons.


title: Tunguska event

text: Since the second half of the 20th century, close monitoring of Earth's atmosphere through infrasound and satellite observation has shown that asteroid air bursts with energies comparable to those of nuclear weapons routinely occur, although Tunguska-sized events, on the order of 5–15 megatons, are much rarer. Eugene Shoemaker estimated that 20-kiloton events occur annually and that Tunguska-sized events occur about once every 300 years. More recent estimates place Tunguska-sized events at about once every thousand years, with 5-kiloton air bursts averaging about once per year. Most of these air bursts are thought to be caused by asteroid impactors, as opposed to mechanically weaker cometary materials, based on their typical penetration depths into the Earth's atmosphere. The largest asteroid air burst to be observed with modern instrumentation was the 500-kiloton Chelyabinsk meteor in 2013, which shattered windows and produced meteorites.


title: Tunguska event

text: In 2020, a group of Russian scientists used a range of computer models to calculate the passage of asteroids with diameters of 200, 100, and 50 metres at oblique angles across Earth's atmosphere. They used a range of assumptions about the object's composition as if it was made of iron, rock or ice. The model which most closely matched the observed event was an iron asteroid up to 200 metres in diameter, travelling at 11.2 km per second which glanced off the Earth's atmosphere and returned into solar orbit.


title: Tunguska event

text: The explosion's effect on the trees near the hypocentre of the explosion was similar to the effects of the conventional Operation Blowdown. These effects are caused by the blast wave produced by large air-burst explosions. The trees directly below the explosion are stripped as the blast wave moves vertically downward, but remain standing upright, while trees farther away are knocked over because the blast wave is travelling closer to horizontal when it reaches them.


title: Tunguska event

text: Soviet experiments performed in the mid-1960s, with model forests (made of matches on wire stakes) and small explosive charges slid downward on wires, produced butterfly-shaped blast patterns similar to the pattern found at the Tunguska site. The experiments suggested that the object had approached at an angle of roughly 30 degrees from the ground and 115 degrees from north and had exploded in mid-air.


title: Tunguska event

text: In 1930, the British meteorologist and mathematician F. J. W. Whipple suggested that the Tunguska body was a small comet. A comet is composed of dust and volatiles, such as water ice and frozen gases, and could have been completely vaporised by the impact with Earth's atmosphere, leaving no obvious traces. The comet hypothesis was further supported by the glowing skies (or "skyglows" or "bright nights") observed across Eurasia for several evenings after the impact, which are possibly explained by dust and ice that had been dispersed from the comet's tail across the upper atmosphere. The cometary hypothesis gained a general acceptance among Soviet Tunguska investigators by the 1960s.


title: Tunguska event

text: In 1978, Slovak astronomer Ľubor Kresák suggested that the body was a fragment of Comet Encke. This is a periodic comet with an extremely short period of just over three years that stays entirely within the orbit of Jupiter. It is also responsible for the Beta Taurids, an annual meteor shower with a maximum activity around 28–29 June. The Tunguska event coincided with the peak activity of that shower, and the approximate trajectory of the Tunguska object is consistent with what would be expected from a fragment of Comet Encke. It is now known that bodies of this kind explode at frequent intervals tens to hundreds of kilometres above the ground. Military satellites have been observing these explosions for decades. During 2019 astronomers searched for hypothesized asteroids ~100 metres in diameter from the Taurid swarm between 5–11 July, and 21 July – 10 August. , there have been no reports of discoveries of any such objects.


title: Tunguska event

text: In 1983, astronomer Zdeněk Sekanina published a paper criticising the comet hypothesis. He pointed out that a body composed of cometary material, travelling through the atmosphere along such a shallow trajectory, ought to have disintegrated, whereas the Tunguska body apparently remained intact into the lower atmosphere. Sekanina also argued that the evidence pointed to a dense rocky object, probably of asteroidal origin. This hypothesis was further boosted in 2001, when Farinella, Foschini, "et al." released a study calculating the probabilities based on orbital modelling extracted from the atmospheric trajectories of the Tunguska object. They concluded with a probability of 83% that the object moved on an asteroidal path originating from the asteroid belt, rather than on a cometary one (probability of 17%). Proponents of the comet hypothesis have suggested that the object was an extinct comet with a stony mantle that allowed it to penetrate the atmosphere.


title: Tunguska event

text: The chief difficulty in the asteroid hypothesis is that a stony object should have produced a large crater where it struck the ground, but no such crater has been found. It has been hypothesised that the passage of the asteroid through the atmosphere caused pressures and temperatures to build up to a point where the asteroid abruptly disintegrated in a huge explosion. The destruction would have to have been so complete that no remnants of substantial size survived, and the material scattered into the upper atmosphere during the explosion would have caused the skyglows. Models published in 1993 suggested that the stony body would have been about across, with physical properties somewhere between an ordinary chondrite and a carbonaceous chondrite. Typical carbonaceous chondrite substance tends to be dissolved with water rather quickly unless it is frozen.


title: Tunguska event

text: Christopher Chyba and others have proposed a process whereby a stony meteorite could have exhibited the behaviour of the Tunguska impactor. Their models show that when the forces opposing a body's descent become greater than the cohesive force holding it together, it blows apart, releasing nearly all of its energy at once. The result is no crater, with damage distributed over a fairly wide radius, and all of the damage resulting from the thermal energy released in the blast.


title: Tunguska event

text: Three-dimensional numerical modelling of the Tunguska impact done by Utyuzhnikov and Rudenko in 2008 supports the comet hypothesis. According to their results, the comet matter dispersed in the atmosphere, while the destruction of the forest was caused by the shock wave.


title: Tunguska event

text: During the 1990s, Italian researchers, coordinated by the physicist Giuseppe Longo from the University of Bologna, extracted resin from the core of the trees in the area of impact to examine trapped particles that were present during the 1908 event. They found high levels of material commonly found in rocky asteroids and rarely found in comets.


title: Tunguska event

text: Kelly "et al." (2009) contend that the impact was caused by a comet because of the sightings of noctilucent clouds following the impact, a phenomenon caused by massive amounts of water vapour in the upper atmosphere. They compared the noctilucent cloud phenomenon to the exhaust plume from NASA's "Endeavour" Space Shuttle. A team of Russian researchers led by Edward Drobyshevski in 2009 have suggested that the near-Earth asteroid may be a possible candidate for the parent body of the Tunguska object as the asteroid has made a close approach of from Earth on 27 June 1908, three days before the Tunguska impact. The team suspected that 's orbit likely fits with the modelled orbit of the Tunguska object, even with the effects of weak non-gravitational forces. In 2013, analysis of fragments from the Tunguska site by a joint US-European team was consistent with an iron meteorite.


title: Tunguska event

text: The February 2013 Chelyabinsk bolide event provided ample data for scientists to create new models for the Tunguska event. Researchers used data from both Tunguska and Chelyabinsk to perform a statistical study of over 50 million combinations of bolide and entry properties that could produce Tunguska-scale damage when breaking apart or exploding at similar altitudes. Some models focused on combinations of properties which created scenarios with similar effects to the tree-fall pattern as well as the atmospheric and seismic pressure waves of Tunguska. Four different computer models produced similar results; they concluded that the likeliest candidate for the Tunguska impactor was a stony body between in diameter, entering the atmosphere at roughly , exploding at altitude, and releasing explosive energy equivalent to between 10 and 30 megatons. This is similar to the blast energy equivalent of the 1980 volcanic eruption of Mount St. Helens. The researchers also concluded impactors of this size hit the Earth only at an average interval scale of millennia.


title: Tunguska event

text: In June 2007, scientists from the University of Bologna identified a lake in the Tunguska region as a possible impact crater from the event. They do not dispute that the Tunguska body exploded in mid-air, but believe that a fragment survived the explosion and struck the ground. Lake Cheko is a small bowl-shaped lake approximately north-northwest of the hypocentre.


title: Tunguska event

text: The hypothesis has been disputed by other impact crater specialists. A 1961 investigation had dismissed a modern origin of Lake Cheko, saying that the presence of metres-thick silt deposits at the lake's bed suggests an age of at least 5,000 years, but more recent research suggests that only a metre or so of the sediment layer on the lake bed is "normal lacustrine sedimentation", a depth consistent with an age of about 100 years. Acoustic-echo soundings of the lake floor provide support for the hypothesis that the lake was formed by the Tunguska event. The soundings revealed a conical shape for the lake bed, which is consistent with an impact crater. Magnetic readings indicate a possible metre-sized chunk of rock below the lake's deepest point that may be a fragment of the colliding body. Finally, the lake's long axis points to the hypocentre of the Tunguska explosion, about away. Work is still being done at Lake Cheko to determine its origins.


title: Tunguska event

text: In 2017, new research by Russian scientists pointed to a rejection of the theory that Lake Cheko was created by the Tunguska event. They used soil research to determine that the lake is 280 years old or even much older; in any case clearly older than the Tunguska event. In analyzing soils from the bottom of Lake Cheko, they identified a layer of radionuclide contamination from mid-20th century nuclear testing at Novaya Zemlya. The depth of this layer gave an average annual sedimentation rate of between 3.6 and 4.6 mm a year. These sedimentation values are less than half of the 1 cm/year calculated by Gasperini "et al." in their 2009 publication on their analysis of the core they took from Lake Cheko in 1999. The Russian scientists in 2017, counted at least 280 such annual varves in the 1260 mm long core sample pulled from the bottom of the lake, representing an age of the lake that would be older than the Tunguska event.


title: Tunguska event

text: Additionally, there are problems with impact physics: It is unlikely that a stony meteorite in the right size range would have the mechanical strength necessary to survive atmospheric passage intact, and yet still retain a velocity large enough to excavate a crater that size on reaching the ground.


title: Tunguska event

text: Though scientific consensus is that the Tunguska explosion was caused by the impact of a small asteroid, there are some dissenters. Astrophysicist Wolfgang Kundt has proposed that the Tunguska event was caused by the release and subsequent explosion of 10 million tons of natural gas from within the Earth's crust. The basic idea is that natural gas leaked out of the crust and then rose to its equal-density height in the atmosphere; from there, it drifted downwind, in a sort of wick, which eventually found an ignition source such as lightning. Once the gas was ignited, the fire streaked along the wick, and then down to the source of the leak in the ground, whereupon there was an explosion.


title: Tunguska event

text: The similar verneshot hypothesis has also been proposed as a possible cause of the Tunguska event. Other research has proposed a geophysical mechanism for the event.


title: Tunguska event

text: A smaller air burst occurred over a populated area on 15 February 2013, at Chelyabinsk in the Ural district of Russia. The exploding meteoroid was determined to have been an asteroid that measured about across. It had an estimated initial mass of 11,000 tonnes and exploded with an energy release of approximately 500 kilotons. The air burst inflicted over 1,200 injuries, mainly from broken glass falling from windows shattered by its shock wave.


title: Tropical cyclone

text: A tropical cyclone is a rapidly rotating storm system characterized by a low-pressure center, a closed low-level atmospheric circulation, strong winds, and a spiral arrangement of thunderstorms that produce heavy rain and squalls. Depending on its location and strength, a tropical cyclone is referred to by different names, including hurricane (), typhoon (), tropical storm, cyclonic storm, tropical depression, or simply cyclone. A hurricane is a strong tropical cyclone that occurs in the Atlantic Ocean or northeastern Pacific Ocean, and a typhoon occurs in the northwestern Pacific Ocean. In the Indian Ocean, South Pacific, or (rarely) South Atlantic, comparable storms are referred to simply as "tropical cyclones", and such storms in the Indian Ocean can also be called "severe cyclonic storms".


title: Tropical cyclone

text: "Tropical" refers to the geographical origin of these systems, which form almost exclusively over tropical seas. "Cyclone" refers to their winds moving in a circle, whirling round their central clear eye, with their surface winds blowing counterclockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere. The opposite direction of circulation is due to the Coriolis effect. Tropical cyclones typically form over large bodies of relatively warm water. They derive their energy through the evaporation of water from the ocean surface, which ultimately condenses into clouds and rain when moist air rises and cools to saturation. This energy source differs from that of mid-latitude cyclonic storms, such as nor'easters and European windstorms, which are powered primarily by horizontal temperature contrasts. Tropical cyclones are typically between in diameter. Every year tropical cyclones impact various regions of the globe including the Gulf Coast of North America, Australia, India, and Bangladesh.


title: Tropical cyclone

text: The strong rotating winds of a tropical cyclone are a result of the conservation of angular momentum imparted by the Earth's rotation as air flows inwards toward the axis of rotation. As a result, they rarely form within 5° of the equator. Tropical cyclones are very rare in the South Atlantic (although occasional examples do occur) due to consistently strong wind shear and a weak Intertropical Convergence Zone. Conversely, the African easterly jet and areas of atmospheric instability give rise to cyclones in the Atlantic Ocean and Caribbean Sea, while cyclones near Australia owe their genesis to the Asian monsoon and Western Pacific Warm Pool.


title: Tropical cyclone

text: The primary energy source for these storms is warm ocean waters. These storms are therefore typically strongest when over or near water, and they weaken quite rapidly over land. This causes coastal regions to be particularly vulnerable to tropical cyclones, compared to inland regions. Coastal damage may be caused by strong winds and rain, high waves (due to winds), storm surges (due to wind and severe pressure changes), and the potential of spawning tornadoes. Tropical cyclones draw in air from a large area and concentrate the water content of that air (from atmospheric moisture and moisture evaporated from water) into precipitation over a much smaller area. This replenishing of moisture-bearing air after rain may cause multi-hour or multi-day extremely heavy rain up to from the coastline, far beyond the amount of water that the local atmosphere holds at any one time. This in turn can lead to river flooding, overland flooding, and a general overwhelming of local water control structures across a large area. Although their effects on human populations can be devastating, tropical cyclones may play a role in relieving drought conditions, though this claim is disputed. They also carry heat and energy away from the tropics and transport it towards temperate latitudes, which plays an important role in regulating global climate.


title: Tropical cyclone

text: A tropical cyclone is the generic term for a warm-cored, non-frontal synoptic-scale low-pressure system over tropical or subtropical waters around the world. The systems generally have a well-defined center which is surrounded by deep atmospheric convection and a closed wind circulation at the surface.


title: Tropical cyclone

text: Historically, tropical cyclones have occurred around the world for thousands of years, with one of the earliest tropical cyclones on record estimated to have occurred in Western Australia in around 4000 BC. However, before satellite imagery became available during the 20th century, there was no way to detect a tropical cyclone unless it impacted land or a ship encountered it by chance.


title: Tropical cyclone

text: These days, on average around 80 to 90 named tropical cyclones form each year around the world, over half of which develop hurricane-force winds of or more. Around the world, a tropical cyclone is generally deemed to have formed once mean surface winds in excess of are observed. It is assumed at this stage that a tropical cyclone has become self-sustaining and can continue to intensify without any help from its environment.


title: Tropical cyclone

text: A study review article published in 2021 in "Nature Geoscience" concluded that the geographic range of tropical cyclones will probably expand poleward in response to climate warming of the Hadley circulation.


title: Tropical cyclone

text: Tropical cyclone intensity is based on wind speeds and pressure; relationships between winds and pressure are often used in determining the intensity of a storm. Tropical cyclone scales such as the Saffir-Simpson Hurricane Wind Scale and Australia's scale (Bureau of Meteorology) only use wind speed for determining the category of a storm. The most intense storm on record is Typhoon Tip in the northwestern Pacific Ocean in 1979, which reached a minimum pressure of and maximum sustained wind speeds of . The highest maximum sustained wind speed ever recorded was in Hurricane Patricia in 2015—the most intense cyclone ever recorded in the Western Hemisphere.


title: Tropical cyclone

text: Warm sea surface temperatures are required in order for tropical cyclones to form and strengthen. The commonly-accepted minimum temperature range for this to occur is , however, multiple studies have proposed a lower minimum of . Higher sea surface temperatures result in faster intensification rates and sometimes even rapid intensification. High ocean heat content, also known as Tropical Cyclone Heat Potential, allows storms to achieve a higher intensity. Most tropical cyclones that experience rapid intensification are traversing regions of high ocean heat content rather than lower values. High ocean heat content values can help to offset the oceanic cooling caused by the passage of a tropical cyclone, limiting the effect this cooling has on the storm. Faster-moving systems are able to intensify to higher intensities with lower ocean heat content values. Slower-moving systems require higher values of ocean heat content to achieve the same intensity.


title: Tropical cyclone

text: The passage of a tropical cyclone over the ocean causes the upper layers of the ocean to cool substantially, a process known as upwelling, which can negatively influence subsequent cyclone development. This cooling is primarily caused by wind-driven mixing of cold water from deeper in the ocean with the warm surface waters. This effect results in a negative feedback process that can inhibit further development or lead to weakening. Additional cooling may come in the form of cold water from falling raindrops (this is because the atmosphere is cooler at higher altitudes). Cloud cover may also play a role in cooling the ocean, by shielding the ocean surface from direct sunlight before and slightly after the storm passage. All these effects can combine to produce a dramatic drop in sea surface temperature over a large area in just a few days. Conversely, the mixing of the sea can result in heat being inserted in deeper waters, with potential effects on global climate.


title: Tropical cyclone

text: Vertical wind shear negatively impacts tropical cyclone intensification by displacing moisture and heat from a system's center. Low levels of vertical wind shear are most optimal for strengthening, while stronger wind shear induces weakening. Dry air entraining into a tropical cyclone's core has a negative effect on its development and intensity by diminishing atmospheric convection and introducing asymmetries in the storm's structure. Symmetric, strong outflow leads to a faster rate of intensification than observed in other systems by mitigating local wind shear. Weakening outflow is associated with the weakening of rainbands within a tropical cyclone.


title: Tropical cyclone

text: The size of tropical cyclones plays a role in how quickly they intensify. Smaller tropical cyclones are more prone to rapid intensification than larger ones. The Fujiwhara effect, which involves interaction between two tropical cyclones, can weaken and ultimately result in the dissipation of the weaker of two tropical cyclones by reducing the organization of the system's convection and imparting horizontal wind shear. Tropical cyclones typically weaken while situated over a landmass because conditions are often unfavorable as a result of the lack of oceanic forcing. The Brown ocean effect can allow a tropical cyclone to maintain or increase its intensity following landfall, in cases where there has been copious rainfall, through the release of latent heat from the saturated soil. Orographic lift can cause an significant increase in the intensity of the convection of a tropical cyclone when its eye moves over a mountain, breaking the capped boundary layer that had been restraining it. Jet streams can both enhance and inhibit tropical cyclone intensity by influencing the storm's outflow as well as vertical wind shear.


title: Tropical cyclone

text: Tropical cyclones tend to develop during the summer, but have been noted in nearly every month in most tropical cyclone basins. Tropical cyclones on either side of the Equator generally have their origins in the Intertropical Convergence Zone, where winds blow from either the northeast or southeast. Within this broad area of low-pressure, air is heated over the warm tropical ocean and rises in discrete parcels, which causes thundery showers to form. These showers dissipate quite quickly; however, they can group together into large clusters of thunderstorms. This creates a flow of warm, moist, rapidly rising air, which starts to rotate cyclonically as it interacts with the rotation of the earth.


title: Tropical cyclone

text: Several factors are required for these thunderstorms to develop further, including sea surface temperatures of around and low vertical wind shear surrounding the system, atmospheric instability, high humidity in the lower to middle levels of the troposphere, enough Coriolis force to develop a low-pressure center, a pre-existing low-level focus or disturbance,


title: Tropical cyclone

text: There is a limit on tropical cyclone intensity which is strongly related to the water temperatures along its path. and upper-level divergence.


title: Tropical cyclone

text: An average of 86 tropical cyclones of tropical storm intensity form annually worldwide. Of those, 47 reach strength higher than , and 20 become intense tropical cyclones (at least Category 3 intensity on the Saffir–Simpson scale).


title: Tropical cyclone

text: Climate cycles such as ENSO and the Madden–Julian oscillation modulate the timing and frequency of tropical cyclone development. Rossby waves can aid in the formation of a new tropical cyclone by disseminating the energy of an existing, mature storm. Kelvin waves can contribute to tropical cyclone formation by regulating the development of the westerlies. Cyclone formation is usually reduced 3 days prior to the wave's crest and increased during the 3 days after.


title: Tropical cyclone

text: On occasion, tropical cyclones may undergo a process known as rapid intensification, a period in which the maximum sustained winds of a tropical cyclone increase by or more within 24 hours. Similarly, rapid deepening in tropical cyclones is defined as a minimum sea surface pressure decrease of per hour or within a 24-hour period; explosive deepening occurs when the surface pressure decreases by per hour for at least 12 hours or per hour for at least 6 hours. For rapid intensification to occur, several conditions must be in place. Water temperatures must be extremely high (near or above ), and water of this temperature must be sufficiently deep such that waves do not upwell cooler waters to the surface. On the other hand, Tropical Cyclone Heat Potential is one of such non-conventional subsurface oceanographic parameters influencing the cyclone intensity. Wind shear must be low; when wind shear is high, the convection and circulation in the cyclone will be disrupted. Usually, an anticyclone in the upper layers of the troposphere above the storm must be present as well—for extremely low surface pressures to develop, air must be rising very rapidly in the eyewall of the storm, and an upper-level anticyclone helps channel this air away from the cyclone efficiently. However, some cyclones such as Hurricane Epsilon have rapidly intensified despite relatively unfavorable conditions.


title: Tropical cyclone

text: There are a number of ways a tropical cyclone can weaken, dissipate, or lose its tropical characteristics. These include making landfall, moving over cooler water, encountering dry air, or interacting with other weather systems; however, once a system has dissipated or lost its tropical characteristics, its remnants could regenerate a tropical cyclone if environmental conditions become favorable.


title: Tropical cyclone

text: A tropical cyclone can dissipate when it moves over waters significantly cooler than . This will deprive the storm of such tropical characteristics as a warm core with thunderstorms near the center, so that it becomes a remnant low-pressure area. Remnant systems may persist for several days before losing their identity. This dissipation mechanism is most common in the eastern North Pacific. Weakening or dissipation can also occur if a storm experiences vertical wind shear which causes the convection and heat engine to move away from the center; this normally ceases the development of a tropical cyclone. In addition, its interaction with the main belt of the Westerlies, by means of merging with a nearby frontal zone, can cause tropical cyclones to evolve into extratropical cyclones. This transition can take 1–3 days.


title: Tropical cyclone

text: Should a tropical cyclone make landfall or pass over an island, its circulation could start to break down, especially if it encounters mountainous terrain. When a system makes landfall on a large landmass, it is cut off from its supply of warm moist maritime air and starts to draw in dry continental air. This, combined with the increased friction over land areas, leads to the weakening and dissipation of the tropical cyclone. Over a mountainous terrain, a system can quickly weaken; however, over flat areas, it may endure for two to three days before circulation breaks down and dissipates.


title: Tropical cyclone

text: Over the years, there have been a number of techniques considered to try to artificially modify tropical cyclones. These techniques have included using nuclear weapons, cooling the ocean with icebergs, blowing the storm away from land with giant fans, and seeding selected storms with dry ice or silver iodide. These techniques, however, fail to appreciate the duration, intensity, power or size of tropical cyclones.


title: Tropical cyclone

text: A variety of methods or techniques, including surface, satellite, and aerial, are used to assess the intensity of a tropical cyclone. Reconnaissance aircraft fly around and through tropical cyclones, outfitted with specialized instruments, to collect information that can be used to ascertain the winds and pressure of a system. Tropical cyclones possess winds of different speeds at different heights. Winds recorded at flight level can be converted to find the wind speeds at the surface. Surface observations, such as ship reports, land stations, mesonets, coastal stations, and buoys, can provide information on a tropical cyclone's intensity or the direction it is traveling. Wind-pressure relationships (WPRs) are used as a way to determine the pressure of a storm based on its wind speed. Several different methods and equations have been proposed to calculate WPRs. Tropical cyclones agencies each use their own, fixed WPR, which can result in inaccuracies between agencies that are issuing estimates on the same system. The ASCAT is a scatterometer used by the MetOp satellites to map the wind field vectors of tropical cyclones. The SMAP uses an L-band radiometer channel to determine the wind speeds of tropical cyclones at the ocean surface, and has been shown to be reliable at higher intensities and under heavy rainfall conditions, unlike scatterometer-based and other radiometer-based instruments.


title: Tropical cyclone

text: The Dvorak technique plays a large role in both the classification of a tropical cyclone and the determination of its intensity. Used in warning centers, the method was developed by Vernon Dvorak in the 1970s, and uses both visible and infrared satellite imagery in the assessment of tropical cyclone intensity. The Dvorak technique uses a scale of "T-numbers", scaling in increments of 0.5 from T1.0 to T8.0. Each T-number has an intensity assigned to it, with larger T-numbers indicating a stronger system. Tropical cyclones are assessed by forecasters according to an array of patterns, including curved banding features, shear, central dense overcast, and eye, in order to determine the T-number and thus assess the intensity of the storm. The Cooperative Institute for Meteorological Satellite Studies works to develop and improve automated satellite methods, such as the Advanced Dvorak Technique (ADT) and SATCON. The ADT, used by a large number of forecasting centers, uses infrared geostationary satellite imagery and an algorithm based upon the Dvorak technique to assess the intensity of tropical cyclones. The ADT has a number of differences from the conventional Dvorak technique, including changes to intensity constraint rules and the usage of microwave imagery to base a system's intensity upon its internal structure, which prevents the intensity from leveling off before an eye emerges in infrared imagery. The SATCON weights estimates from various satellite-based systems and microwave sounders, accounting for the strengths and flaws in each individual estimate, to produce a consensus estimate of a tropical cyclone's intensity which can be more reliable than the Dvorak technique at times.


title: Tropical cyclone

text: Multiple intensity metrics are used, including accumulated cyclone energy (ACE), the Hurricane Surge Index, the Hurricane Severity Index, the Power Dissipation Index (PDI), and integrated kinetic energy (IKE). ACE is a metric of the total energy a system has exerted over its lifespan. ACE is calculated by summing the squares of a cyclone's sustained wind speed, every six hours as long as the system is at or above tropical storm intensity and either tropical or subtropical. The calculation of the PDI is similar in nature to ACE, with the major difference being that wind speeds are cubed rather than squared. The Hurricane Surge Index is a metric of the potential damage a storm may inflict via storm surge. It is calculated by squaring the dividend of the storm's wind speed and a climatological value (), and then multiplying that quantity by the dividend of the radius of hurricane-force winds and its climatological value (). This can be represented in equation form as:


title: Tropical cyclone

text: where v is the storm's wind speed and r is the radius of hurricane-force winds. The Hurricane Severity Index is a scale that can assign up to 50 points to a system; up to 25 points come from intensity, while the other 25 come from the size of the storm's wind field. The IKE model measures the destructive capability of a tropical cyclone via winds, waves, and surge. It is calculated as:


title: Tropical cyclone

text: where p is the density of air, u is a sustained surface wind speed value, and d is the volume element.


title: Tropical cyclone

text: Around the world, tropical cyclones are classified in different ways, based on the location (tropical cyclone basins), the structure of the system and its intensity. For example, within the Northern Atlantic and Eastern Pacific basins, a tropical cyclone with wind speeds of over is called a hurricane, while it is called a typhoon or a severe cyclonic storm within the Western Pacific or North Indian Oceans.<ref name="ATL/EPAC TCOP"></ref> Within the Southern Hemisphere, it is either called a hurricane, tropical cyclone or a severe tropical cyclone, depending on if it is located within the South Atlantic, South-West Indian Ocean, Australian region or the South Pacific Ocean.


title: Tropical cyclone

text: The practice of using names to identify tropical cyclones goes back many years, with systems named after places or things they hit before the formal start of naming. The system currently used provides positive identification of severe weather systems in a brief form, that is readily understood and recognized by the public. The credit for the first usage of personal names for weather systems is generally given to the Queensland Government Meteorologist Clement Wragge who named systems between 1887 and 1907. This system of naming weather systems subsequently fell into disuse for several years after Wragge retired, until it was revived in the latter part of World War II for the Western Pacific. Formal naming schemes have subsequently been introduced for the North and South Atlantic, Eastern, Central, Western and Southern Pacific basins as well as the Australian region and Indian Ocean.


title: Tropical cyclone

text: At present, tropical cyclones are officially named by one of twelve meteorological services and retain their names throughout their lifetimes to provide ease of communication between forecasters and the general public regarding forecasts, watches, and warnings. Since the systems can last a week or longer and more than one can be occurring in the same basin at the same time, the names are thought to reduce the confusion about what storm is being described. Names are assigned in order from predetermined lists with one, three, or ten-minute sustained wind speeds of more than depending on which basin it originates. However, standards vary from basin to basin with some tropical depressions named in the Western Pacific, while tropical cyclones have to have a significant amount of gale-force winds occurring around the center before they are named within the Southern Hemisphere. The names of significant tropical cyclones in the North Atlantic Ocean, Pacific Ocean, and Australian region are retired from the naming lists and replaced with another name. Tropical cyclones that develop around the world are assigned an identification code consisting of a two-digit number and suffix letter by the warning centers that monitor them.


title: Tropical cyclone

text: At the center of a mature tropical cyclone, air sinks rather than rises. For a sufficiently strong storm, air may sink over a layer deep enough to suppress cloud formation, thereby creating a clear "eye". Weather in the eye is normally calm and free of convective clouds, although the sea may be extremely violent. The eye is normally circular and is typically in diameter, though eyes as small as and as large as have been observed.


title: Tropical cyclone

text: The cloudy outer edge of the eye is called the "eyewall". The eyewall typically expands outward with height, resembling an arena football stadium; this phenomenon is sometimes referred to as the "stadium effect". The eyewall is where the greatest wind speeds are found, air rises most rapidly, clouds reach their highest altitude, and precipitation is the heaviest. The heaviest wind damage occurs where a tropical cyclone's eyewall passes over land.


title: Tropical cyclone

text: In a weaker storm, the eye may be obscured by the central dense overcast, which is the upper-level cirrus shield that is associated with a concentrated area of strong thunderstorm activity near the center of a tropical cyclone.


title: Tropical cyclone

text: The eyewall may vary over time in the form of eyewall replacement cycles, particularly in intense tropical cyclones. Outer rainbands can organize into an outer ring of thunderstorms that slowly moves inward, which is believed to rob the primary eyewall of moisture and angular momentum. When the primary eyewall weakens, the tropical cyclone weakens temporarily. The outer eyewall eventually replaces the primary one at the end of the cycle, at which time the storm may return to its original intensity.


title: Tropical cyclone

text: There are a variety of metrics commonly used to measure storm size. The most common metrics include the radius of maximum wind, the radius of wind (i.e. gale force), the radius of outermost closed isobar (ROCI), and the radius of vanishing wind. An additional metric is the radius at which the cyclone's relative vorticity field decreases to 1×10 s.


title: Tropical cyclone

text: On Earth, tropical cyclones span a large range of sizes, from as measured by the radius of vanishing wind. They are largest on average in the northwest Pacific Ocean basin and smallest in the northeastern Pacific Ocean basin. If the radius of outermost closed isobar is less than two degrees of latitude (), then the cyclone is "very small" or a "midget". A radius of 3–6 latitude degrees () is considered "average sized". "Very large" tropical cyclones have a radius of greater than 8 degrees (). Observations indicate that size is only weakly correlated to variables such as storm intensity (i.e. maximum wind speed), radius of maximum wind, latitude, and maximum potential intensity. Typhoon Tip is the largest cyclone on record, with tropical storm-force winds in diameter. The smallest storm on record is Tropical Storm Marco (2008), which generated tropical storm-force winds only in diameter.


title: Tropical cyclone

text: The movement of a tropical cyclone (i.e. its "track") is typically approximated as the sum of two terms: "steering" by the background environmental wind and "beta drift". Some tropical cyclones can move across large distances, such as Hurricane John, the longest-lasting tropical cyclone on record, which traveled , the longest track of any Northern Hemisphere tropical cyclone, over its 31-day lifespan in 1994.


title: Tropical cyclone

text: Environmental steering is the primary influence on the motion of tropical cyclones. It represents the movement of the storm due to prevailing winds and other wider environmental conditions, similar to "leaves carried along by a stream".


title: Tropical cyclone

text: Physically, the winds, or flow field, in the vicinity of a tropical cyclone may be treated as having two parts: the flow associated with the storm itself, and the large-scale background flow of the environment. Tropical cyclones can be treated as local maxima of vorticity suspended within the large-scale background flow of the environment. In this way, tropical cyclone motion may be represented to first-order as advection of the storm by the local environmental flow. This environmental flow is termed the "steering flow" and is the dominant influence on tropical cyclone motion. The strength and direction of the steering flow can be approximated as a vertical integration of the winds blowing horizontally in the cyclone's vicinity, weighted by the altitude at which those winds are occurring. Because winds can vary with height, determining the steering flow precisely can be difficult.


title: Tropical cyclone

text: The pressure altitude at which the background winds are most correlated with a tropical cyclone's motion is known as the "steering level". The motion of stronger tropical cyclones is more correlated with the background flow averaged across a thicker portion of troposphere compared to weaker tropical cyclones whose motion is more correlated with the background flow averaged across a narrower extent of the lower troposphere. When wind shear and latent heat release is present, tropical cyclones tend to move towards regions where potential vorticity is increasing most quickly.


title: Tropical cyclone

text: Climatologically, tropical cyclones are steered primarily westward by the east-to-west trade winds on the equatorial side of the subtropical ridge—a persistent high-pressure area over the world's subtropical oceans. In the tropical North Atlantic and Northeast Pacific oceans, the trade winds steer tropical easterly waves westward from the African coast toward the Caribbean Sea, North America, and ultimately into the central Pacific Ocean before the waves dampen out. These waves are the precursors to many tropical cyclones within this region. In contrast, in the Indian Ocean and Western Pacific in both hemispheres, tropical cyclogenesis is influenced less by tropical easterly waves and more by the seasonal movement of the Intertropical Convergence Zone and the monsoon trough. Other weather systems such as mid-latitude troughs and broad monsoon gyres can also influence tropical cyclone motion by modifying the steering flow.


title: Tropical cyclone

text: In addition to environmental steering, a tropical cyclone will tend to drift poleward and westward, a motion known as "beta drift". This motion is due to the superposition of a vortex, such as a tropical cyclone, onto an environment in which the Coriolis force varies with latitude, such as on a sphere or beta plane. The magnitude of the component of tropical cyclone motion associated with the beta drift ranges between and tends to be larger for more intense tropical cyclones and at higher latitudes. It is induced indirectly by the storm itself as a result of feedback between the cyclonic flow of the storm and its environment.


title: Tropical cyclone

text: Physically, the cyclonic circulation of the storm advects environmental air poleward east of center and equatorial west of center. Because air must conserve its angular momentum, this flow configuration induces a cyclonic gyre equatorward and westward of the storm center and an anticyclonic gyre poleward and eastward of the storm center. The combined flow of these gyres acts to advect the storm slowly poleward and westward. This effect occurs even if there is zero environmental flow. Due to a direct dependence of the beta drift on angular momentum, the size of a tropical cyclone can impact the influence of beta drift on its motion; beta drift imparts a greater influence on the movement of larger tropical cyclones than that of smaller ones.


title: Tropical cyclone

text: A third component of motion that occurs relatively infrequently involves the interaction of multiple tropical cyclones. When two cyclones approach one another, their centers will begin orbiting cyclonically about a point between the two systems. Depending on their separation distance and strength, the two vortices may simply orbit around one another, or else may spiral into the center point and merge. When the two vortices are of unequal size, the larger vortex will tend to dominate the interaction, and the smaller vortex will orbit around it. This phenomenon is called the Fujiwhara effect, after Sakuhei Fujiwhara.


title: Tropical cyclone

text: Though a tropical cyclone typically moves from east to west in the tropics, its track may shift poleward and eastward either as it moves west of the subtropical ridge axis or else if it interacts with the mid-latitude flow, such as the jet stream or an extratropical cyclone. This motion, termed "recurvature", commonly occurs near the western edge of the major ocean basins, where the jet stream typically has a poleward component and extratropical cyclones are common. An example of tropical cyclone recurvature was Typhoon Ioke in 2006.


title: Tropical cyclone

text: The majority of tropical cyclones each year form in one of seven tropical cyclone basins, which are monitored by a variety of meteorological services and warning centres. Ten of these warning centres worldwide are designated as either a Regional Specialized Meteorological Centre or a Tropical Cyclone Warning Centre by the World Meteorological Organisation's (WMO) tropical cyclone programme. These warning centres issue advisories which provide basic information and cover a systems present, forecast position, movement and intensity, in their designated areas of responsibility. Meteorological services around the world are generally responsible for issuing warnings for their own country, however, there are exceptions, as the United States National Hurricane Center and Fiji Meteorological Service issue alerts, watches and warnings for various island nations in their areas of responsibility. The United States Joint Typhoon Warning Center and Fleet Weather Center also publicly issue warnings, about tropical cyclones on behalf of the United States Government. The Brazilian Navy Hydrographic Center names South Atlantic tropical cyclones, however the South Atlantic is not a major basin, and not an official basin according to the WMO.


title: Tropical cyclone

text: Ahead of the formal season starting, people are urged to prepare for the effects of a tropical cyclone by politicians and weather forecasters, amongst others. They prepare by determining their risk to the different types of weather, tropical cyclones cause, checking their insurance coverage and emergency supplies, as well as determining where to evacuate to if needed. When a tropical cyclone develops and is forecast to impact land, each member nation of the World Meteorological Organization issues various watches and warnings to cover the expected impacts. However, there are some exceptions with the United States National Hurricane Center and Fiji Meteorological Service responsible for issuing or recommending warnings for other nations in their area of responsibility.