Clouds seem to get darker and more ominous as storms approach. Part of this appearance is your perspective, but several factors are also at work when skies darken. Not all clouds become darker before a rain. Light, wispy cirrus and cirrocumulus clouds, for example, form in high altitudes and are not forbearers of stormy conditions.
When clouds become thick and heavy, very little light shines through them and they appear darker to our eyes. Once clouds reach about 3,000 feet in thickness, almost no light penetrates. The cloud appears dark, and ground conditions resemble dusk or evening. Heavy clouds that rise between you and the sun also block light from penetrating, so they appear dark as they approach from the horizon.
Tiny water droplets or ice crystals produce clouds. These droplets and crystals reflect sunlight, scattering the reflections in all different directions. The size of water droplets is just right for scattering all colors in the sunlight’s spectrum. This differs from the relatively smaller size of air molecules, which tend to scatter light in the blue color range more effectively, which is why the sky appears blue. When eyes see all colors scattered, the result is white. Less scattering becomes gray, then nearly black. Storm clouds, laden with water droplets that are ready to fall, have a darker tone.
The shadow of one cloud upon another also makes clouds look dark. As clouds gather in a stormy sky, shadows often occur because clouds are at different distances from the observer. The angle of either a rising or setting sun causes sunlight to shine only on the top layers of clouds. The lower portions do not gather as much light and appear gray to black. Because storms often occur in the morning and evening, the sun’s angle contributes to darkening skies.
Approaching cold fronts often fire up thunderstorms as the cold air mass behind the front lifts warm air ahead of it. This causes tall columns of cumulonimbus clouds, which are capable of reaching heights of 45,000 feet. This type of cloud is an example of the kind of storm cloud that appears to darken as the storm nears, and has dark or gray patterns as viewed from a a distance. Nimbostratus clouds do not rise nearly as high as cumulonimbus, but are thick storm clouds that appear dark. Nimbus is a root word for rain or precipitation in cloud types.