A combination of starlight and the upper atmosphere’s own subtle glow can help satellites see Earth’s
clouds on moonless nights, according to Colorado State University researchers.
During the daytime, ultraviolet light from the sun bombards the Earth’s upper atmosphere and breaks apart gaseous
molecules and atoms.
During the nighttime, these molecules and atoms recombine, emitting faint visible light in the process.
Now, researchers know that airglow and starlight illumination holds important and immediate practical implications
for weather and hazards monitoring, climate assessment, and our ability to observe interactions between the Earth's
lower and upper atmosphere.
The green airglow light shows up beautifully from orbit in this time exposure photograph.
The blurred lights in the foreground are cities. Credit: NASA
Click on image to enlarge
Low-light imagery from a series of adjacent Suomi NPP VIIRS/DNB nighttime passes over the Pacific Ocean on the night of February 22, 2012. The
coverage domain spans 20,000 km east-to-west and 12,500 km north-to-south, with geopolitical boundaries drawn in green. The data were collected during
new moon conditions (no sunlight or moonlight present). In addition to city light emissions (e.g., L), the observations capture clouds (e.g., C) illuminated by
reflected airglow, starlight, and zodiacal light. Also apparent are broad, diffuse regions of primary airglow emission. Credits: NASA/CSU
This “air glow” combined with starlight illuminates clouds at night, and by using a new and improved satellite
instrument, scientists can take advantage of this signal for the first time from space, according to
a new study published in the
Proceedings of the National Academy of Sciences by Steve Miller,
a research scientist at CSU’s Cooperative Institute for Research
in the Atmosphere (CIRA), along with colleagues from National Oceanic Atmospheric Administration (NOAA),
Northrop Grumman and the U.S. Department of Defense (DoD).
The team captured the data from a new advanced weather-and-climate monitoring satellite.
The satellite, a joint venture between NASA and NOAA, is called the Suomi National Polar-orbiting Partnership,
or Suomi NPP, and carries five advanced instruments at an orbit approximately 512 miles above the earth.
The scientists believe that this new ability to see clouds at night could have significant implications for weather
and climate observations for forecasters and other research scientists.
"This development is exciting and impressive," said Mary Kicza, assistant administrator for NOAA’s Satellite
and Information Service. "This could be especially useful to our meteorologists in areas like Alaska, where
the winter months have long periods of darkness."
Among these sensors is the Visible/Infrared Imager/Radiometer Suite (VIIRS), which includes a “Day/Night Band”
that is sensitive to extremely low levels of light. Researchers at CIRA, collaboration between CSU and NOAA,
perform many instrument check-out activities for the NPP mission.
“The Day/Night Band is a new capability for NOAA users,” said Mitch Goldberg, program scientist at NOAA Joint
Polar Satellite System (JPSS) Office.
“We are very encouraged by this remarkable discovery by the CIRA scientists.”
Click on image to enlarge
During the daytime, ultraviolet light from the sun bombards the Earth’s upper atmosphere and breaks
apart gaseous molecules and atoms. During the nighttime, these molecules and atoms recombine, emitting
faint visible light in the process.
This 'air glow' combined with starlight illuminates clouds at night, and by using a new and improved satellite
instrument, scientists can take advantage of this signal for the first time from space. Credit: Colorado State University
The scientists were applying methods to reduce “noise” in the Day/Night Band measurements, when they found that
the instrument was sensitive enough to see clouds and other objects in what would appear to the human eye as
The new capability will be useful for improving our views of very low clouds and features such as sea ice at
night, potentially benefiting travel and commerce.
Streaks of glowing air, caused by light emitted from molecules in Earth’s
upper atmosphere, stripe the sky near the horizon. Credit: Space Physics Research Group, Univ. of California, Berkeley.
“Most weather satellites aren’t even sensitive enough to see the lights from a large city like Denver, much less
the reflected moonlight, which is nearly a million times fainter than sunlight. These air glow/starlight sources
are 100-1000 times fainter still,” Miller said.
“Instead of using visible light, nighttime observations are typically relegated to infrared (heat) measurements,
where near-surface features (such as fog) can blend into their surroundings because they have nearly the same temperature.”
The Day/Night Band was intended to advance the low light-sensor technology pioneered in the 1960’s on the DoD’s
meteorological satellite program, but no one expected it to see clouds on moonless nights, Miller said.
“In some ways, the day just got twice as long and that’s pretty exciting for scientists,” he added.
In addition to the clouds, Miller said that sensitivity of the Day/Night Band to direct emissions from air glow
allows the sensor to see waves moving through the upper atmosphere, forced by thunderstorms below – which appear
like ripples in a pond atop some of the stronger storms.
Goldberg added that the NOAA JPSS Proving Ground supports activities promoting the use of the Day/Night Band
for our National Weather Service.
“We are very fortunate to have Dr. Miller as part of our team,” Goldberg said.
“To most of us, it’s a small revelation in itself that the night really isn’t as dark as we might think,”
“We’re literally seeing our world in a ‘new light.’
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