October 3, 2011
Quebec City. Red, orange, and pink clouds occur we can see at sunset are the result of the scattering of sunlight by the atmosphere. When the angle between the sun and the horizon is less than 10 percent, sunlight becomes too red due to refraction for any colors other than those with a reddish hue to be seen. The clouds are reflecting long and unscattered rays of sunlight, which are predominant at those hours. In combination with large, mature thunderheads, this can produce blood-red clouds. Clouds look darker in the near-infrared because water absorbs solar radiation at those wavelengths.