First, it is necessary to clarify that they are not entirely black. It’s people who perceive them that way, because the base of storm clouds really is darker. The main reason for this pitch is that rain clouds are quite thick. When they reach several kilometers in height, the sun’s rays cannot reach the droplets that remain in the lower part. As they do not receive light, their color ends up being black. Just to compare, it’s the same thing that happens in the ocean: the bottom gets darker and darker because the light doesn’t reach more than a few hundred meters deep. On the other hand, less thick clouds are white because the water vapor droplets that compose them disperse all the colors of the light spectrum. The sum of this rainbow, from our point of view, is white. Another important point to clarify is that a black cloud is not always synonymous with a storm. “Even so, we can estimate that in 90% of cases this relationship is true, as thick clouds are more loaded with water droplets and are better able to generate rain”, says meteorologist Marcelo Seluchi, from the Space Research Institute (Inpe ), from Cachoeira Paulista (SP). The curious thing is that certain white clouds can also bring thunderstorms. This is especially true at the coast, where the water vapor that forms raindrops gathers around sea salts. “They are the best nuclei to create droplets and generate rain”, says Marcelo. When the downpour comes, the heaviest raindrops drag what comes ahead, causing air currents within the cloud itself. Because these winds come from high altitudes, they are cold. This explains the feeling of freshness that accompanies a downpour, softening the heat.
1. Usually, the clouds that we see as black are the ones that generate storms, called cumulonimbus clouds. They are gigantic towers of water vapor droplets that can extend over 15 kilometers in height. With all this size, the cloud ends up blocking the Sun’s luminosity, preventing it from reaching the surface.
2. Of the sun’s rays reaching the cloud, about 20% is reflected by the vapor droplets back into the atmosphere. Another 3% is absorbed and most ends up going through the gout. But if the cloud is very thick, the amount of light passing through the drop decreases in the lower layers of the atmosphere. When there is no light left, the impression for those observing from the earth is that the cloud is black
3. Because they are almost always full of moisture, black clouds end up generating heavy rain as soon as the drops gain size to overcome the rising wind that throws them upwards. After the downpour, the clouds become less dense and turn white again. When they pass through a hot or dry area, the remaining moisture evaporates and they disappear from the sky.
KILOMETRIC SHADOW
Underneath a storm megacloud small black clouds may appear that do not have enough moisture to generate rain. The explanation for this apparent contradiction is simple: the “dwarf” clouds are black because the huge storm cloud blocks the Sun’s luminosity.