Question
Wed February 29, 2012 By: Akankshya Sahu
 

Why do clouds appear black in rainy days?

Expert Reply
Thu March 01, 2012
There are two things going on to make some rain clouds to appear very dark. The first is that some clouds, especially thunderstorm clouds such as nimbocumulus clouds can tower to great heights with tops 15 to 20 km (50-60 thousand feet) above the earth. Plus these clouds, which tend to produce very heavy rains and hail are very dense, being full of water vapor and raindrops at lower altitudes and ice crystals at high altitude. 

As a result, it is more difficult for sunlight to pass through these clouds than it is for light to pass through neighboring clouds which aren't as high and dense. More of the light is reflected back into space or sideways because there's more obstacles. Plus, there's more absorption of the light on its way through the cloud, because the light will bounce around very much and take a very long total path before finally coming out the bottom of the cloud.
Ask the Expert