Re: Why rain cloud is sometimes black in colour, esp. before thunderstorm?

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.

The second effect which enhances our perception of a dark cloud has to do with how the human eye and brain perceive light and color. We don't judge brightenss and color very accurately in an absolute sense; rather, our perception of one part of a scene is affected by what else is going on in the scene. For example, think about how a television screen looks like when the set is turned off: the screen is a medium dark gray. Yet, when the set is turned on, parts of the picture can look quite black. But the reality is that what we perceive as "black" is really this medium dark grey color. (The picture tube can only add light, it can't "suck out" light!) This phenomenon of color perception is called "chromatic induction" and is the basis of some optical illusions. Here's an example that shows how a dark a grey square looks when surrounded by white, and how light the same square looks when surrounded by black.

A similar thing is going on with the clouds: a region of the sky is darker because less light is passing through the cloud, but we perceive the cloud to be even darker and blacker than it is because it's surrounded by brighter clouds.

A number of times I've seen storms or squall lines approach that off in the distance appear very dark, menacing color. Yet, as the storm clouds draw near, they invariably seem to lose some of their dark, ominous coloring, and by the time they're upon us, don't seem to be anywhere near as dark (even though the storm may be raging over our heads).

The reason is the trick of color perception... once the storm clouds are overhead, we can no longer compare their brightness to the brighter clouds surrounding them, and it no longer seems as dark.

Hmmm... a good science project would be to use a light meter to measure the relative brightness of clouds when they are far off and overhead to show that while it is indeed darker underneath storm clouds, that our perception is affected by the brightness of other nearby clouds.

Steve Czarnecki