Starlight and "Air Glow" Reveal the Nighttime Cloudy Sky from Space

Scientists are excited over an inadvertent discovery using instruments aboard the new Suomi National Polar-orbiting Partnership, or Suomi NPP, a joint venture between NASA and NOAA. The satellite is actually sensitive enough to detect clouds and other objects in the nighttime sky from space, what to the human eye would simply be complete darkness.

Such a discovery has immediate implications for weather and climate observations for forecasters and research scientists alike.

“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.”

The Nighttime is just a Little Bit Brighter Now
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.

The discovery was published in a new study published in the journal 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 sensor being used 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 were applying methods to reduce “noise” in the Day/Night Band measurements when they found that the instrument was also picking up clouds and other objects that the human eye simply could not differentiate from the surrounding blackness.

“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.

Another addition to being able to see clouds, according to Miller, is that the sensitivity of the Day/Night Band is able to see waves moving through the upper atmosphere, forced along by thunderstorms below, which appear like ripples in a pond atop some of the stronger storms.

Source: Colorado State University

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