Researchers have traditionally estimated the net cloud effect by measuring a broad spectrum of sunlight that makes it to the earth's surface, from ultraviolet to infrared. But clouds are white that's because the large water droplets within them scatter light of all colors almost equally in the visible spectrum, the part of the electromagnetic spectrum that includes the colors of the rainbow.
On the other hand, aerosols both within clouds and in the open sky bounce different-colored light unequally. Broadband measurements that fail to distinguish color differences might be covering up important details, the researchers thought.
Instead of taking one broadband measurement that covers everything from ultraviolet to infrared, Kassianov and crew wanted to determine how individual wavelengths contribute to the net cloud effect. To do so, the team used an instrument that can measure brightness at four different wavelengths of color violet, green, orange, red and two of infrared.
In addition, this instrument, a spectral radiometer at DOE's Atmospheric Radiation Measurement Climate Research Facility located on the southern Great Plains in Oklahoma, allowed the team to calculate what the brightness would be if the day sported a cloudless, blue sky. The spectral measurements taken by the radiometer can be converted into the amount and properties of aerosols. Then aerosol properties can be used to calculate clear blue sky brightness.
Clouds Gone Wild
Comparing measured values for cloudy sky to the calculated values for clear sky, the researchers found that,
|Contact: Mary Beckman|
DOE/Pacific Northwest National Laboratory