Out of the Blue and Into the Black

New Views of the Earth at Night


By Michael Carlowicz Design by Paul Przyborski December 5, 2012










Night Vision for Weather Forecasters

The original motivation for making low-light Earth observations was practical: military meteorologists wanted to track clouds and storms, smoke plumes, and dust storms at all hours. The Air Force, in particular, needed to provide more accurate and timely forecasts for pilots working at night both over land and at sea.

If combined with thermal (infrared) observations, visible images of moonlit skies can be extremely useful in characterizing the nighttime environment. Clouds are always changing, and there are certain kinds that are more prominent during the night. For instance, fog is more likely to form at night, when the air and surface are cooling and moisture condenses. Convective clouds and thunderstorms tend to form during the day as the Sun heats the surface and destabilizes the lower atmosphere. The properties and distributions of many clouds change rapidly in the transition from sunlight to darkness.


Lit by moonlight, high clouds cast shadows on lower, marine layer clouds off the coast of California in this visible light image (top). Only the high clouds show up in infrared channels (lower) traditionally used for meteorology at night. Low clouds pose serious hazards for air and ship traffic. (View Large Image - NASA Earth Observatory and NOAA National Geophysical Data Center)

Clouds are critical for understanding both daily weather and the long-term climate. As Miller points out, measuring cloud properties — such as water content and particle size — is relatively easy to do during the daytime, but not so much at night.

Since the 1960s, the U.S. Air Force has operated the Defense Meteorological Satellite Program (DMSP), a series of 18 polar-orbiting satellites that observe clouds and other weather variables in key wavelengths of infrared and visible light. Since 1972, the DMSP satellites have included the Operational Linescan System (OLS), which gives weather forecasters some ability to see in the dark. It has been a highly successful sensor, but it is dependent on older technology with lower resolution than most scientists would like. And for many years, DMSP data were classified.

Since just a few civilian scientists have had access to nighttime data over the years, the atmospheric science community was eager to have a night-vision tool of its own. The VIIRS instrument and its day-night band was the answer.


In April 2012, VIIRS spotted waves rippling through the faint airglow in the upper atmosphere over Texas. The waves were provoked by a massive thunderstorm. (View Large Image - NASA Earth Observatory and NOAA National Geophysical Data Center)

“For all the reasons that we need to see the Earth during the day, we also need to see the Earth at night,” says Miller, who started working with DMSP data for the Naval Research Laboratory during the wars in Afghanistan and Iraq. “Visible light information provides a unique ability to detect and characterize atmospheric and surface features that confound thermal emission-based measurements. Unlike humans, the Earth never sleeps; it’s constantly moving, evolving, building up here, and tearing down there. There are many things happening on the nighttime side that we don’t understand because of limited observations.”

“The use of the day-night band by the National Weather Service is growing,” says Mitch Goldberg, NOAA’s program scientist for the Joint Polar Satellite System (including Suomi NPP). “For example, the NWS forecast office in Monterey, California, is now using the day-night band’s exceptional spatial resolution and contiguous coverage to provide higher confidence in issuing marine dense fog advisories.”


The day-night band uses moonlight, airglow, zodiacal light, and starlight to detect changes in clouds, snow cover, and sea ice in the Arctic. This is an important tool for meteorologists who do not have access to daylight imagery during the long polar night. (View Large Image - NASA Earth Observatory and NOAA National Geophysical Data Center)

Nighttime observations are particularly valuable to people living at high latitudes. Many weather satellites are parked in geostationary orbit over the equator, and the polar regions are far off on the fringes of what they can see. Most other earth-observing satellites only provide visible observations during the daylight portions of their orbits. But the ground tracks of the polar-orbiting Suomi NPP overlap at high latitudes, providing multiple observations and measurements per day of moonlit clouds, sea ice, snow cover, and open waterways for people whose winter nights can last for months.

“As a new capability, I thought everyone would be impressed with the VIIRS day-night band,” says James Gleason, NASA’s project scientist for Suomi NPP. “But the remarkable images and the community response to them has exceeded my pre-launch expectations.”

Putting on Glasses for the First Time













The Night Electric

The nighttime views collected by the day-night band are already proving useful to meteorologists and atmospheric scientists, who have been observing the dynamics of storms such as hurricanes Isaac and Sandy.

“Having the ability to see at night also helps identify features that could be missed by the other satellite sensors,” says Straka. “We can bring out things like thin plumes of smoke, low-level volcanic ash, areas of fog and other features that otherwise would blend in with the background.”

Chris Elvidge is particularly interested in the view of combustion sources — such as wildfires and gas flares — which glow in different wavelengths than manmade lights. His team can distinguish between the two by simultaneously using visible light and short-wave infrared bands. “We’re detecting much smaller gas flares with VIIRS than we could with DMSP, and we’re able to estimate the temperature and the total radiant output.” Elvidge hopes the measurements could eventually lead to better estimates of carbon dioxide emissions.


Northwestern North Dakota, one of the least-densely populated parts of the U.S., has been aglow at night in recent years. The light comes from oil drilling and gas flaring in the Bakken shale formation. (View Large Image - NASA Earth Observatory and NOAA National Geophysical Data Center)

But the most popular and poignant use of night-lights imagery remains the study — scientific and aesthetic — of city lights. “Nighttime imagery provides an intuitively graspable view of our planet,” says William Stefanov, senior remote sensing scientist for the International Space Station program office.

“City lights provide a fairly straightforward means to map urban versus rural areas, and to show where the major population centers are and where they are not,” Stefanov notes. “They are also an excellent means to track urban and suburban growth, which feeds into planning for energy use and urban hazards, for studying urban heat islands, and for initializing climate models.”

Elvidge has seen dozens of different uses for city lights imagery. “Artificial lighting is a excellent remote sensing observable and proxy for human activity.”


City lights are usually an indicator of where people live, but not on the Korean Peninsula. South Korea’s population is roughly 49 million and the land teems with light. North Korea has 24 million people and hardly any lights beyond Pyongyang. Offshore, the seas glow with the light of hundreds of fishing boats lining up along invisible borders. (View Large Image - NASA Earth Observatory and NOAA National Geophysical Data Center)


Elvidge’s research group has been approached by groups seeking to model the distribution of carbon dioxide emissions from fossil fuels and to monitor the activity of commercial fishing fleets, who light up the seas to increase their catch. Biologists have examined how urban growth has fragmented animal habitat. He even learned once of a study of dictatorships in various parts of the world and how nighttime lights had a tendency to expand in the dictator’s hometown or province.

In recent years, Miller, Elvidge, and colleagues have come across unexpected applications for these “dark side” measurements, including the first satellite observations of the mysterious milky seas — the bioluminescence of maritime lore, which causes vast expanses of ocean to glow. And the unanticipated capability of the day-night band to detect reflected airglow and starlight suggests that scientists may never have to be without a visible light measurement, even on nights without moonlight.


City light maps have been used to model the distribution of economic activity and populations, to monitor human development around parks and wildlife refuges, and to observe blackouts. (View Large Image - NASA Earth Observatory and NOAA National Geophysical Data Center)

“Nighttime light is the most interesting data that I’ve had a chance to work with in my career,” says Elvidge, who has been working with nighttime imagery since 1992. “Even after 20 years, I’m always amazed at what city light images show us about human activity.”

“Even as we look toward other planets and the very corners of the universe for the next great discovery,” Miller adds, “these nighttime observations are a humbling reminder that that there are new frontiers of discovery that still await us in our own backyard.”

  1. Related Reading

  2. NASA Earth Observatory (2005) Blue Marble: Next Generation.
  3. NASA (n.d.) Suomi NPP.
  4. NOAA National Geophysical Data Center (n.d.) Earth Observations Group–DMSP.
  5. Lee, T. et al (2006, Feb) The NPOESS VIIRS Day/Night Visible Sensor. Bulletin of the American Meteorological Society, Volume 87, Number 2.
  6. Miller, Steven D. et al (2012) Suomi satellite brings to light a unique frontier of nighttime environmental sensing capabilities. Proceedings of the National Academy of Sciences, Volume 109, Number 39.
  7. NASA Earth Observatory (2012, February 4) Earth, Behind the Scenes.
  8. Colorado State University Cooperative Institute for Research in the Atmosphere.
  9. Cooperative Institute for Meteorological Satellite Studies, University of Wisconsin–Madison (n.d.) CIMSS Satellite Blog.
  10. NASA Earth Observatory (2008) Cities at Night: The View from Space.
  11. NASA Earth Observatory (2000) Bright Lights, Big City.