The direct component accounts for the shade provided by clouds and cools the earth. The second component accounts for the sunlight scattered between and under clouds, which makes the sky brighter, warming the earth.
"The sunlight scattered by clouds can heat the surface," said Kassianov. "We all know that we can still get sunburned on cloudy days. This explains why."
In the Oklahoma summer, the scattered-light effect measured by the researchers could be quite large. For example, if a cloud passed over the instrument, the measured cloudy sky brightness exceeded calculated clear sky value by up to 30 percent. Kassianov attributes that large difference to scattered sunlight being "caught on tape" by the radiometer.
"Sunlight scattered by three-dimensional, irregular clouds is responsible for the observed large difference. The one-dimensional cloud simulations currently used in large-scale climate models don't capture this diffuse light," said Kassianov.
Aerosols' Day in the Sky
The team also found that the effect changed depending on the measured visible-spectrum wavelength, and whether the light was direct or scattered.
With direct light, the cooling caused by clouds was weakest on the violet end of the spectrum and strongest at infrared. With scattered light, warming caused by clouds was also weakest at violet and the strongest at infrared. Overall, the least cooling and warming occurred at violet, and the most cooling and warming occurred at infrared.
Because large droplets in clouds scatter sunlight almost uniformly across the spectrum, the clouds themselves can't be the reason why different wavelengths contribute differently to the net cloud effect. Co
|Contact: Mary Beckman|
DOE/Pacific Northwest National Laboratory