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04/07/2008 - Updated 02:49 PM ET
Today's Top Weather Stories

Answers archive: Thunderstorms

Q: Why is it that radar shows rain over my house, but I'm not seeing any raindrops? What is the name for that? What causes it?

A: On the radar, you’re seeing virga -- rain that evaporates before reaching the ground. While in the clouds, raindrops form in air that is saturated. However, they usually fall through unsaturated air as they descend. If the air below the cloud is very dry and thick, raindrops can evaporate completely. Radars are able to pick up the rain within the clouds, but give no information about what happens to that rain on its way down since they can’t scan low levels near the ground. Virga can be more common in winter, when low levels of the atmosphere tend to be drier.

There's more about virga on this USA TODAY resource page.

(Answered by meteorologist Bob Swanson, USA TODAY's assistant weather editor, August 29, 2007)

Q: How fast does a typical thunderstorm travel?

A: Some storms can be nearly stationary, while others can clip along at the rate of 50 mph or more. The steering winds, generally located between 10,000 and 20,000 feet above the surface, tend to determine the speed and direction of most thunderstorms. Multicell thunderstorms occur when a new thunderstorm arises from the outflow of a dissipating thunderstorm. When a multicell cluster forms a line, it’s called a squall line. A squall line can race along at 50 mph or more and can produce damaging winds in excess of 100 mph.

Learn more about severe thunderstorms on this USA TODAY resource page.

(Answered by meteorologist Bob Swanson, USA TODAY's assistant weather editor, August 15, 2007)

Q: Is there a specific time of day that most thunderstorms occur? If so, why?

A thunderstorm is formed when a combination of moisture and warm air rise in the atmosphere and condense. Thunderstorms are most likely to occur at the warmest, most humid part of the day -- usually the afternoon or evening. Thunderstorms happen all over the world at all times of the day. At any given moment there are 1,800 thunderstorms occurring worldwide.

This USA TODAY resource page shows the time of day that tornadic thunderstorms are most likely to form across different regions of the USA.

(Answered by Elizabeth Caldwell of USA TODAY's weather staff, July 9, 2007)

Q: I often hear the term "lift" or "lifted index" that's used to determine if there is actually a possibility for severe weather. What is the "lifted index" and how does it impact thunderstorm development?

A: Imagine that a bubble of air, with a given temperature and humidity, is lifted high in the atmosphere to about 18,000 feet. The parcel will cool as it rises, depending on the amount of water vapor in the bubble. Subtracting the temperature of this theoretical bubble from the actual or computer model forecast of the temperature of the atmosphere at 18,000 feet gives the lifted index. Negative values of the lifted index indicate an unstable atmosphere, which is conducive for thunderstorms. Positive values indicate stability and thunderstorms are not as likely.

There's plenty more about the stability of the atmosphere on this USA TODAY resource page.

(Answered by meteorologist Bob Swanson, USA TODAY's assistant weather editor, June 20, 2007)

Q: I’ve noticed that farm animal behavior changes as severe storms approach. What is it they’re sensing?

A: Most likely, they’re sensing a change in barometric pressure and other environmental conditions that would accompany a storm. While there is documented evidence of animals and insects reacting to approaching rain showers, severe storms, hurricanes and even earthquakes—sometimes in advance of humans—it is unlikely that they possess a “sixth sense” about the weather. Rather, they are simply more sensitive to subtle changes in the environment than we are.

Whether or not animals can predict disaster was the subject of a recent episode of the PBS series Nature. One researcher interviewed for the program believes that animals have the ability to detect “infrasonic” sound pulses created by storms and earthquakes that would allow them to flee to safety. Such sounds, according to researchers, occur at frequencies below the range of human hearing.

Animal behavior has long been the source of proverbs relating to weather. In a 1903 book published by the U.S. Weather Bureau titled Weather Folklore and Local Weather Signs, Weather Bureau Professor of Meteorology Edward B. Garriott suggested the following:

“The observations of naturalists, shepherds, herdsmen, and others who have been brought much into contact with animals, have proved most clearly that these creatures are cognizant of approaching changes in the state of the air long before we know of their coming by other signs.”

In this 1903 book, Garriott lists the following weather proverbs associated with animal behavior:

• "When cattle go out to pasture and lie down early in the day it indicates early rain."
• "Dogs making holes in the ground, eating grass in the morning, or refusing meat are said to indicate coming rain."—Colonel Dunwoody [a forecaster for the U.S. Army Signal Service, the predecessor of the Weather Bureau].
• "All shepherds agree in saying that before a storm comes sheep become frisky, leap, and butt or 'box' each other."—Folklore Journal.
• "When horses and cattle stretch out their necks and sniff the air it will rain."
• "Horses, as well as other domestic animals, foretell the coming of rain by starting more than ordinary and appearing in other respects restless and uneasy."
• "Hogs crying and running unquietly [sic] up and down with hay or litter in their mouths foreshadow a storm to be near at hand."—Thomas Willsford.
• "Kine [an archaic term for cattle], when they assemble at one end of a field with their tails to windward, often indicate rain or wind."
• "When oxen or sheep collect together as if they were seeking shelter a storm may be expected."—Apache Indians.

(Answered by Sean Potter, a certified consulting meteorologist and science writer in New York City, June 6, 2007)

Q: What is a "wet microburst?"

A: First of all, a microburst refers to a blast of downbursting winds within a thunderstorm. These winds are concentrated in an area less than 2.5 square miles and can sometimes exceed 150 mph, resulting in damage similar to a tornado. Unlike tornadic winds, which tend to swirl debris in all directions, objects toppled by microburst winds tend to fall in a straight line.

Dry microbursts occur when the air beneath the thunderstorm is very dry. Much of the precipitation associated with the storm evaporates before reaching the ground. The only evidence of the microburst might be a cloud of dust kicked up by the winds. In contrast, wet microbursts occur in more humid locations when the downbursting winds are accompanied by heavy rain.

In the humid East, wet microbursts are most likely, while dry microbursts occur more often in the West.

Learn more about microbursts on this USA TODAY resource page.

(Answered by meteorologist Bob Swanson, USA TODAY's assistant weather editor, June 3, 2007)

Q: Why do severe thunderstorms tend to develop in the spring?

A: First of all, severe thunderstorms are those that produce 3/4" hail or larger, winds 58 mph or greater or tornadoes. Severe thunderstorms require warm, moist air at the Earth's surface as well as favorable atmospheric conditions to increase instability and shear in storms as they develop.

While such storms are possible year-round, the required ingredients tend to come together at different times of year for different geographic regions. In the Southeast, severe thunderstorm activity tends to spike in the early spring, with storm season typically peaking in late spring for the southern Plains and in summer in the northern Plains.

Learn more about severe thunderstorms on this USA TODAY resource page.

(Answered by meteorologist Bob Swanson, USA TODAY's assistant weather editor, April 11, 2007)

Q: How long does a typical thunderstorm last?

A: A typical thunderstorm has a lifespan of about 30 minutes, while more mature supercell thunderstorms can last for many hours. How long a thunderstorm affects a given area depends on the steering currents aloft that guide them. Fast winds can move a thunderstorm at speeds of 50 mph or greater. Light winds can bring storms to a crawl (5 mph or less), increasing the chance of flash flooding.

This USA TODAY resource page explains how thunderstorms form.

(Answered by Chris Vaccaro of the National Oceanic and Atmospheric Administration, February 20, 2007)

Q: What is convection?

A: Convection in the atmosphere occurs when energy is transferred by the movement of air, typically upward. When air warms near the ground, it becomes less dense and rises, taking the energy with it up into the atmosphere. If the air contains water vapor, the water vapor cools and condenses into cloud droplets as it rises. The process of condensation releases energy that can further enhance the convection within the cloud, sometimes producing tall thunderstorms in the process.

Learn more about convection in the atmosphere on this USA TODAY resource page.

(Answered by meteorologist Bob Swanson, USA TODAY's assistant weather editor, December 25, 2006)

Q: Why are some cold fronts “dry,” while others come through with big thunderstorms?

A: In order for thunderstorms to occur, three ingredients must be present: lift, instability and moisture. A cold front by itself is a form of lift, but if the other two ingredients aren’t present, thunderstorms won’t develop. Instability is simply a parcel of air that's warmer than the air surrounding it. Although this warm air parcel has the potential to rise on its own, a lifting mechanism like a cold front helps bump it up. Once it rises, the air must also be moist in order for a thunderstorm to develop.

This USA TODAY resource page has lots of links to resources about thunderstorms.

(Answered by meteorologist Julya Johnson of WBIR-TV in Knoxville, Tenn., November 8, 2006)

Q: Does hail only appear with thunderstorms, or can it also show up in winter storms, along with sleet and snow?

A: Hail is typically only a warm-weather phenomenon, because its formation requires the strong updrafts of a thunderstorm. However, on rare occasions, sufficient updrafts can develop in a winter storm to produce hail as well as thundersnow.

It is important to note that hail forms differently than sleet. While hailstones are chunks of ice that form in the storm cloud and fall to Earth before melting, sleet starts as snow or ice in the cloud, melts during its descent, then refreezes as an ice pellet before hitting the ground. It is important to note that hail forms differently than sleet.

There's plenty more about hail formation on this USA TODAY resource page.

(Answered by meteorologist Bob Swanson, USA TODAY's assistant weather editor, October 31, 2006)

Q: What causes some thunderstorms to produce hail?

A: Hail forms when thunderstorm updrafts are strong enough to carry water droplets well above the freezing level. (The temperature of the atmosphere typically decreases with increasing altitude.) This freezing process forms a hailstone, which can grow as additional water freezes onto it. Eventually, the hailstone becomes too heavy for the updrafts to support it. The hailstone will melt as it falls through air that is above freezing. Some small hailstones will melt entirely and fall as rain, while others will remain chunks of ice as they hit the ground.

Typically, the stronger the updraft in the storm, the larger the hailstone that can be produced.

Learn more about hail formation on this USA TODAY resource page.

(Answered by meteorologist Bob Swanson, USA TODAY's assistant weather editor, July 26, 2006)

Q: Which major U.S. city averages the most thunderstorms per year?

A: When referring to the average number of thunderstorms a U.S. city receives per year, thunderstorm days can be a useful guide. A thunderstorm day is simply a day during which thunder is heard; precipitation is not necessary. A map from the National Weather Service shows the average number of thunderstorm days each year throughout the USA.

Tampa is the leader in major U.S. cities that experience the most thunderstorms per year. The city averages over 100 thunderstorm days per year, while other cities in Florida, including Orlando and Tallahassee, average at least 80 per year. Thunderstorm development is very favorable throughout Florida because of the available warmth and moisture from the Gulf of Mexico and the Atlantic Ocean.

Areas of northeastern New Mexico and southeastern Colorado experience at least 70 thunderstorm days per year. Thunderstorms often develop in that area during the monsoon season months of July, August and September. During the monsoon, prevailing winds shift from west to south, bringing in moisture from the Pacific Ocean and Gulf of Mexico. Orographic lifting -- air being forced upwards by hills and mountains -- from the nearby Rocky Mountains, along with the abundance of moist, warm air, enhances development of thunderstorms in that area.

Worldwide, parts of central Africa and South America near the equator have the most thunderstorm days per year, with some spots recording close to 200 thunderstorm days annually.

Check out FAQ's about thunderstorms on this USA TODAY resource page. Find out how thunderstorms form on this USA TODAY resource page.

(Answered by Greg Seroka of USA TODAY’s weather staff, July 17, 2006)

Q: What is a "supercell" thunderstorm? How does it form?

A: A supercell thunderstorm is the largest, longest-lasting and most severe thunderstorm on Earth. Supercells have a deep rotating updraft called a mesocyclone that enables the storms to produce tornadoes, large hail, downbursts of wind, heavy rain and lightning. These types of thunderstorms form when horizontal spin in the atmosphere tilts into the vertical, creating the rotation that sometimes results in tornadoes.

Supercell mesocyclones can be detected by Doppler weather radar, which can notice any tornadoes that develop within the storm by their unique signature. Supercells are most common in the Great Plains and desert states of the USA, but they can form anywhere in the world.

Check out this USA TODAY resource page about supercell thunderstorms.

(Answered by Greg Seroka of USA TODAY’s weather staff, June 19, 2006)

Q: What does "CAPE" stand for?



CAPE can sometimes increase when the atmosphere is "capped." Learn more about how convective caps form on this USA TODAY resource page.

(Answered by meteorologist Bob Swanson, USA TODAY's assistant weather editor, June 9, 2006)

Q: Can the sun actually "aggravate" the atmosphere and cause showers as a result?

A: Yes, the heating of the sun can make the atmosphere unstable and cause showers and thunderstorms. As the sun heats the Earth, air near the surface also warms. When this air becomes less dense than the air above it, it rises into the atmosphere. As the air rises and cools, water vapor in the air condenses into cloud droplets.

Sometimes, on a sunny summer day, this is as far as the process goes and only fair-weather cumulus clouds form. However, given the right atmospheric conditions, this process of surface heating can provide enough instability to generate heavy rainfall and strong thunderstorms.

Learn more about atmospheric stability on this USA TODAY resource page.

(Answered by meteorologist Bob Swanson, USA TODAY's assistant weather editor, June 7, 2006)

Q: What is convection?

A: It’s one of the three main processes in which energy is transferred, along with radiation and conduction. In the atmosphere, convection occurs when the addition of heat changes the density of the air. As air warms near the ground, it becomes less dense and rises, taking the energy with it up into the atmosphere. Convection helps to distribute energy and moisture through the atmosphere.


(Answered by meteorologist Bob Swanson, USA TODAY's assistant weather editor, May 11, 2006)

Q: What makes a thunderstorm severe?

A: The National Weather Service (PDF) classifies a thunderstorm as severe if wind speeds reach 58 mph or faster, if hail is three quarters of an inch in diameter or larger, or if the thunderstorm produces a tornado or tornadoes. A thunderstorm only needs to meet one of these three criteria to be severe. Whether a thunderstorm is severe or not, lightning and heavy rain are a danger with any thunderstorm.

This USA TODAY resource page has more about severe thunderstorms.

(Answered by Doyle Rice, USA TODAY's weather editor, May 9, 2006)

Q: Is there any part of the USA that never (or very seldom) has thunderstorms?

A: Every part of the USA experiences a thunderstorm at some time. However, the western third of Alaska averages only one day a year with a thunderstorm. In the lower 48 states, coastal California, from San Francisco to San Diego, averages less than five days a year with a thunderstorm.

Check this USATODAY.com thunderstorm resource page for links to much more about thunderstorms.

(Answered by meteorologist Dennis Feltgen, National Weather Service, Silver Spring, Md., April 17, 2006)

Q: Do supercell thunderstorms only occur in the Midwest?

A: While supercell thunderstorms are more frequent in the Midwest, as well as in the southern and central Plains, they can develop in many other locations. In fact, any area that typically has thunderstorms can potentially see a supercell thunderstorm. Unlike pulse thunderstorms that typically form on a summer afternoon and dissipate within an hour, supercell thunderstorms can last for hours, causing more severe weather damage and producing more tornadoes.

Learn more about supercells on this USA TODAY resource page.

(Answered by meteorologist Bob Swanson, USA TODAY's assistant weather editor, April 9, 2006)

Q: Why do some cold fronts produce severe storms, and others no rain at all?

A: The rain associated with a cold front is typically the result of cold air undercutting warm, moist air and forcing it up, with the warm air typically ascending over the cold air. This type of cold front is called an anafront. Because of the forceful lifting of the warm air, thunderstorms are often the result of an advancing cold front.

Sometimes, the front becomes so steep that warm air ascends on a forward tilt, with any precipitation occurring well ahead of the front. In this instance, the front would pass with little or no rain produced. This type of cold front is called a katafront. Learn more about katafronts on this USA TODAY resource page.

(Answered by meteorologist Bob Swanson, USA TODAY's assistant weather editor, February 15, 2006)

Q: What was the most damaging thunderstorm in U.S. history?

A: In terms of monetary damage, the worst thunderstorm in U.S. history was likely the one that battered Fort Worth, Texas, in May 1995. Total damages were $2 billion, with an estimated $1.1 billion of that attributed to hail. Thirteen people were killed and more than 100 injured in flash floods and lightning in the storm.

These pages by Tim Marshall and Jennifer Miller have more about this devastating hailstorm.

Learn more about thunderstorms on this USA TODAY resource page.

(Answered by Doyle Rice, USA TODAY’s weather editor, February 6, 2006)

Q: What is a dryline and how is it important for storm formation?

A: As the name suggests, a dryline is a boundary between a hot, dry air mass and a warm, moist air mass. Quite often drylines form in the central and southern High Plains as dry desert air from the Southwest encounters moist air from the Gulf of Mexico. This contrast in air masses often triggers severe thunderstorms and tornadoes along and east of the dryline.

Learn more about drylines on this USA TODAY resource page.

(Answered by Bob Swanson, USA TODAY's assistant weather editor, November 8, 2005)

Q: What happens if two thunderstorms are on the same path and collide with each other?

A: The rain produced by each thunderstorm creates a downdraft. When this downdraft hits the earth's surface, it forms an outflow boundary. If two storms were on the same path, it is the outflow boundaries that would collide first. The collision of these boundaries could give rise to an entirely new thunderstorm, and could even produce non-supercell tornadoes.

(Answered by Bob Swanson, USA TODAY's assistant weather editor, August 30, 2005)

Q: How many gallons of water are stored in an average thunderstorm cloud?

A: According to calculations from Peggy LeMone, senior scientist at the National Center for Atmospheric Research, a thunderstorm cloud contains approximately 275 million gallons of water. With 750,000 gallons of water going over Niagara Falls each second, it would take six minutes for an equal amount of water to go over the Falls.

So how much does a thunderstorm cloud weigh? With one gallon of water weighing 8.33 pounds, the weight of the cloud is 2.3 billion pounds, or 1.1 million tons.

Learn more about thunderstorms by vistiting our thunderstorm resource page.

(Answered by Bob Swanson, USA TODAY's assistant weather editor, August 9, 2005)

Q: What percentage of rain is accompanied by thunder and lightning?

A: Because weather stations are scattered so thinly across the globe, there are vast areas that lack routine data on rainfall and thunder, so it’s hard to give a single answer to this question. Overall, the risk of thunder and lightning decreases as you move poleward and toward the west coasts of major continents, where cool maritime currents prevail.

(Answered by meteorologist Bob Henson, a writer at the University Corporation for Atmospheric Research and the author of The Rough Guide to Weather, July 25, 2005)

Q: How can there be hail during the heat of summer, since hail is made of ice?

A: While it may be hot here at the Earth's surface, temperatures are well below freezing high above in summer's thunderstorm clouds. Hail develops in a storm's updrafts, until the stones become too heavy and fall out of the storm. While many small hailstones melt into raindrops on the way down, the larger ones reach the Earth's surface.

While crops are the major victims, hail is also a hazard to vehicles and windows. Deaths are rare in the USA. The last known U.S. hail fatality was a person killed by a baseball-size hailstone in Fort Worth, Texas, in March 2000.

(Answered by Bob Swanson, USA TODAY's assistant weather editor, July 7, 2005)

Q: What is the tallest thunderstorm on record?

A: There is no standard method for measuring thunderstorm or cloud top heights. In the tropics or subtropics, strong thunderstorms can be more than 10 miles high, which is the top of the tropopause -- the layer at the base of the stratosphere. Even stronger thunderstorms can penetrate an additional mile into the stratosphere. Meteorologists can use satellite images of these “overshooting tops” to pinpoint vigorous thunderstorms.

In the April/May 1997 issue of Weatherwise magazine, Thomas Schlatter addressed a similar question. Schlatter noted that the heights of tornadic thunderstorms and severe squall lines have been measured accurately over the USA based on a stereoscopic view from two geosynchronous satellites. The most lofty cloud summits ranged from 9.3 to 10.6 miles above the surface. These are indeed tall clouds for a mid-latitude location.

However, Schlatter notes that a truly amazing low-latitude measurement comes from well-calibrated infrared radiometers aboard two different satellites. The temperature at the top of thunderstorms embedded in Tropical Cyclone Hilda in March 1990 in the Coral Sea off eastern Australia was measured at -100°C. From two nearby balloon soundings, the cloud height was estimated to be almost 12 miles. In this case, the tropopause was very high (near 100 mb), very cold (-85°C or -86°C) and atmospheric instability was sufficient for substantial penetration of the tropopause.

The region of tropical northern Australia and Indonesia, sometimes called the "Maritime Continent," supports the deepest convection in the world. Thunderstorm tops as high as 12 to 13.6 miles have been reported there.

(Answered by Bob Swanson, USA Today's assistant weather editor, July 5, 2005)

Q: What’s the difference between isolated and scattered thunderstorms?


For more, see our FAQ page about understanding weather forecasts.

(Answered by Chris Vaccaro, public affairs specialist at the National Weather Service in Silver Spring, Md., May 31, 2005)

Q: What's the difference between thundershowers and thunderstorms?

A: Thundershower is not a term used by the National Weather Service and does not have an official definition. The term "shower" is used to describe precipitation with a short duration and varying intensity. The suffix "storm" highlights the potentially deadly nature of lightning that accompanies rainfall.

See the USATODAY.com Thunderstorm resources page for more about these storms.

(Answered by John Jensenius, warning coordination meteorologist at the National Weather Service in Gray, Maine, May 24, 2005)

Q: On the afternoon of Thursday, April 2, 2003, Dover, Del., was reporting a temperature of 49 degrees while Washington, D.C., was reporting 81 degrees. Only 75 miles separate the two cities. I often hear weathermen talking about strong cold fronts spawning severe weather, and they often point to the big difference in temperatures on either side of the front as an indication of just how strong a cold front is. But when the front nudged down past Washington by the next morning, dropping the temperature some 30 degrees, the frontal passage was heralded by almost nothing at all, except a wind direction change, increased clouds, and a drop in temperature. Why doesn't this seemingly "strong" frontal boundary present the danger of spawning severe weather like other strong cold fronts?

A: Simply put, TV weather people often oversimplify the weather so they won't confuse viewers. It takes more than a strong cold front to trigger severe weather.

Severe thunderstorms require four ingredients:

  • moisture (humidity);
  • instability (cold air over warm air that gives the rising warm air the buoyancy needed to continue rising on its own);
  • wind shear (changing wind speed or direction with increasing altitude);
  • lift, in the form of an upward force.

Back door cold fronts, which move more or less from nrotheast to suothwest on the East Coast, are shallow masses of cool air. They provide very little lift — the boost or force needed to trigger thunderstorms — and create almost no instability to help the rising warm air become buoyant and grow into towering clouds and eventually thunderstorms.

Even regular cold fronts are not enough to trigger severe thunderstorms. Usually an upper-air disturbance moving in tandem with the cold front is the impetus for severe weather.

Upper-air disturbances almost always bring in cool to cold air well above the ground, which increases instability. They also always enhance the winds aloft, contributing to wind shear. And they add a kick to the lift generated by the surface cold front, forcing the warm, humid air near the ground upward high enough so that it rises on its own.

To learn more about fronts and upper-air disturbances, visit USATODAY.com's Understanding storms and fronts page. Learn all about thunderstorms with USATODAY.com's Resources: Thunderstorms page.

(Answered by Chris Cappella, USATODAY.com Weather Team, June 1, 2003)

Q: What is the difference between a strong and severe thunderstorm?

A: The National Weather Service defines a "severe" thunderstorm as having winds of 58 mph or faster or hail more than three-quarters of an inch in diameter, or both. The NWS has no formal definition of a "strong" thunderstorm. The term is the subjective opinion of a forecaster or someone experiencing the storm. You'll find more information by going to the USATODAY.com Resources: Thunderstorms page.

(Answered by Jack Williams, USATODAY.com weather editor, May 5, 2003)

Q: Under what conditions will thunderstorms train?

A: When winds in the middle atmosphere that steer thunderstorms blow parallel to a warm or cold front — or any other surface boundary that marks a change in temperature, humidity, or wind speed and direction — the storms that form ahead of these boundaries will move along it, like the cars of a train. The individual thunderstorms in the spiraling bands of a hurricane also parade one after the other. Such "training" thunderstorms, as they are known, often unleash their downpours on the same areas, triggering flooding.

An interactive USATODAY.com graphic, Floods arrive on storm train, shows how this occurs.

(Answered by Chris Cappella, USATODAY.com Weather Team, May 4, 2003)

Q: Do large tornadic thunderstorms develop over the ocean? Could violent F-4 or F-5 tornadoes develop far out on the ocean and we would never hear about it?

A: When it comes to weather, you generally never want to say "never." But, thunderstorms fierce enough to spin up the biggest tornadoes are certainly thought to be rare over the ocean because one of the conditions such storms need is a big temperature contrast between the hot ground and cold air aloft. Since water absorbs heat, the air over the ocean doesn't generally warm enough.

Hurricanes and other tyopical cyclones such as typhoons (in the western Pacific) are the strongest storms found over the oceans.. They are made of spiral bands of thunderstorms, and some of the individual storms in a hurricane can grow fairly strong. But, the flight crews and scientists who routinely fly hurricane hunter airplanes into the strongest storms will not fly into Great Plains supercell thunderstorms; the kind that can breed strong tornadoes. The supercell thunderstorms are more violent that those in hurricanes.

That said, tornadoes are expected wherever and whenever the right combination of ingredients, such as sufficient instability and wind shear — a change in wind speed or direction with increasing altitude — occur.

Occasionally, large and intense winter storms moving over the warm Gulf Stream waters off the Southeast Coast of the USA bring the right conditions together. A paper put together by a National Weather Service meteorologist in Melbourne, Fla., discusses observations using Doppler radar of a Midwestern-type thunderstorm over the Gulf Stream in 1991 that was certainly capable of producing a tornado. Since three-fouths of the world is covered with water, however, observations of actual tornadoes are extraordinarily rare. So you are correct surmising that they would most likely be missed.

You'll find links to a lot of information about the science of tornadoes by going to the USATODAY.com Understanding tornadoes page.

(Answered by Jack Williams, USATODAY.com weather editor, and Chris Cappella, assistnat weather editor, June 26, 2002)

Q: How tall can a thunderstorm get?

A: Thunderstorms, which are clouds that grow vertically in the atmosphere, have been measured as high as about 70,000 feet. But most top out between 30,000 and 50,000 feet. The tropics consistently have the tallest thunderstorms, though periodic severe and tornadic thunderstorms outside the tropics usually are intense enough to grow taller than 50,000 feet.

A search for information about this turned up a related question and answer on Weatherwise magazine's Web site with some examples. Learn more about thunderstorms in USATODAY's Understanding thunderstorms section.

(Answered by Chris Cappella, USATODAY.com weather team, June 16, 2002)

Q: How would a person calculate the number of calories of energy created by the latent heat of condensation in a thunderstorm? Thank you.

A: This is a good question that shows that meteorology is an excellent application of physical principles. For starters, we need to work in metric units, as scientists do. Any good physics book and many meteorology books will have conversion tables to change to English units if need be. (Since you both are from Canada, this should not be necessary.)

First, it takes 600 calories to convert a gram of liquid water to water vapor. This is evaporation. The process you describe is the exact opposite, so condensing a gram of water vapor into a liquid would give off 600 calories of heat. But this is at average room temperature of about 20 degrees Celsius, and, as you likely know, temperature changes with altitude. Luckily, unless you are doing critical calculations for some kind of sensitive experiment, you can ignore the temperature change as the latent heat of condensation varies by less than 2 percent between 35 degrees Celsius and minus 30 degrees Celsius. With that latent heat value in mind, it's time to do some calculating.

Second thing to note: this is going to be a very complex calculation if you need to find the latent heat of condensation for a specific volume of cloud that you don't already know. Scientists have been studying thunderstorms in detail for a half century and they still are trying to understand their nature.

For our purposes, we are going to assume a certain size cloud. And, we are only going to look at the latent heat of condensation in the updraft. As you might imagine, water vapor in a rising updraft will eventually freeze, or become supercooled. But experiments done inside thunderstorms in the 1970s indicate that the ice content of a thunderstorm updraft is negligible, and can be ignored. (This is not true for the thunderstorm downdraft(s), which often are heavily laden with ice at higher altitudes. But we are going to ignore them , and not get into the latent heat of fusion in this answer.)

An average size thunderstorm is 6 miles wide. Let's assume it's a square. So that's about 10,000 meters wide by 10,000 meters long. Now let's assume that it is also 10,000 meters high. Additional experiments and idealized calculations reveal that a mature cloud like that has anywhere between 1.5 and 3 grams of liquid water in each cubic meter; a lot more than your average puffy cumulus cloud, which has about 0.5 grams in each cubic meter. Let's for round number's sake use 2 grams per cubic meter, because our thunderstorm is not all that strong, but it is big.

So, if we multiply 10,000 x 10,000 x 10,000 x 2, we find that the cloud has about 2 trillion grams of water inside. If the latent heat of condensation for each gram is 600 calories, that's 1,200 trillion calories, which is a heck of a lot of heat energy.

As you probably already know, hurricanes are comprised of bands of thunderstorms. It has been estimated that the heat energy generated by the thunderstorms in a mature hurricane in one day, if converted to electrical energy, could power the United States for about half a year.

I took the value for the latent heat of condensation from C. Donald Ahrens' book Meteorology Today, while the value for the amount of liquid in a typical thunderstorm came from A Short Course in Cloud Physics, R.R. Rogers and M.K. Yau. You can find both in bookstores. Some good libraries in your area might carry them, too.

You'll find more about condensation and evaporation through links from our Water in the atmosphere index.

(Answered by Chris Cappella, USATODAY.com assistant weather editor, 12-14-99)

Q: How is the intensity of a thunderstorm judged? Is it judged by the intensity of the thunder and lightning or by the intensity of the precipitation?

A: Thunderstorms aren't classified in the same way that tornadoes are with the Fujita scale or hurricanes with the Saffir-Simpson scale. The only classification the U.S. National Weather Service uses for thunderstorms is weather they are or not. A USATODAY.com graphic shows the wind and hail size criteria used to classify a storm as severe.

The colors you see in images from weather radars represent intensity scales for precipitation, including thunderstorms. These colors are based on how much radio energy is scattered back to the radar antenna by rain, snow, sleet or hail. Before it became relatively cheap and easy to transmit color images, the U.S. National Weather Service came up with a system of classifying radar images as levels 1 through 5 based on the amounts of energy scattered back by the precipitation. These levels were indicated on the old, black and white, national radar summary chart. Level 5 precipitation is an indication that lightning, with its thunder, and extreme turbulence as well as heavy precipitation are probably found inside the rain storm. Levels 3 and 4 also indicate thunderstorms that a prudent pilot would not want to fly into. These thunderstorm levels might be mentioned to a pilot asking for weather information but are rarely used in reports for the general public.

Today's color radar images show more than five levels of intensity, but numbers are not applied to all of these levels. When you see red or magenta on a radar image you can be sure it's showing a pretty strong thunderstorm. Thunderstorms are also informally classified by how high they extend into the sky, but no numbers are assigned. If radar shows that a thunderstorm goes up to 50,000 feet, you can be sure it's a powerful storm. The USATODAY.com Resources: thunderstorms has a lot more information. (5-29-99)

Q: When a microburst occurs, does the rainfall come down in one direction? I'm wondering about a storm during which a microburst was reported. When it hit, the rain and wind were so strong it was like a white-out - that occurs in winter. However this took place two weeks ago. The storm came from the west. The rain came down from the west and then came straight down, then came down from the East. My question is: Was this a 'microburst' or a 'tornado' scenario?

A: It sounds like it must have been a microburst, which is a concentrated form of a downburst. Tornadoes occur where the air is rising into a thunderstorm, which means that rain does not fall when the tornado hits. Heavy rain or hail can fall right before a tornado arrives, in fact tornadoes often form on the boundary between rising air and descending air, which often brings rain down with it. Probably what happened is that the microburst was moving across the ground when it hit where you were. The winds from the west would be where the air hitting the ground was spreading out ahead of the storm. Then you describe the air coming straight down. That would be when the microburst was right overhead. Finally, the wind would have come from the east as the microburst passed and some of the air hitting the ground was spreading out to the east. If a microburst were standing still, you might see air spreading out in all directions when it hits the ground. (9-10-98)

Q: About two years ago my father and I were sitting on the deck watching a thunderstorm rumble over. When it hit the wind obviously increased significantly and the temperature seemed to have dropped. About two minutes later the wind stopped and the temperature became tropical and as I looked to the sky I noticed a spiral was forming in the sky, and it was spinning very fast. Then it stopped, five minutes later it happened again. Nothing ever came of these occurrences, and they lasted about a minute or two each. I was wondering could this have been a tornado forming?

A: It sounds like you saw either a couple of tornadoes that were trying to form and didn't make it, or maybe the mesocyclone, the rotating part of a supercell. Our supercell graphic has more on this. The initial blast of cool air you felt was the downburst that brought cool air down from high in the storm. Then, when you felt the tropical air, the updraft part of the storm had moved over you. As air rose into the thunderstorm it pulled in surrounding, warm, humid air. Tornadoes often form in the boundary between a thunderstorm's downward-moving air and the air moving upward in the updraft. Our Resources: thunderstorms and Resources: Tornadoes pages have links to more information. (8-21-97)

Q: What is a microburst? I have been in one and I don't really know what it is. All I know is that it killed 10 people.

A: A microburst is a concentrated wind that blasts down from a shower or thunderstorm. A USATODAY.com graphic shows how they form and some of their effects. Links from our graphic will lead you to more information.

A microburst can cause the direction and speed of the wind to change quickly and by large amounts. If this happens to hit an airplane that's close to the ground, usually while taking off or landing, it can reduce the speed of the air moving over the airplane's wings. The loss of air speed over the wings reduces the lifting force and the plane will lose altitude. In recent years, after some deadly microburst-caused crashes in the 1980s, new materials for training pilots about microbursts were developed. Radars designed to detect them are being installed at some large airports. (5-26-97)

Q: What is the "gust front" that sometimes appears ahead of a thunderstorm, and why does it occur?

A: When rain begins falling from a shower or thunderstorm it drags down cool air from aloft. When this cool air hits the ground it spreads out and blows along the ground. This wind is the gust front. Often, when a thunderstorm is a few miles away on a hot, sticky day, you'll feel a refreshing, cool breeze when the gust front winds arrive. Our Rersources: Thunderstorms page has links to more on these storms. (3-25-97)

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