Steady rain that lasts for an entire day or longer is usually produced by the altostratus or nimbostratus variety. In most cases, cumulonimbus clouds are accompanied by showers of rain, snow, or hail, often with a thunderstorm or a tornado. Cirrus, cirrostratus, and cirrocumulus, high clouds composed exclusively of ice crystals, may produce snow, but the snow generally evaporates before reaching the ground if there are no lower clouds present. When thick clouds of waterdrops are present at lower levels, however, the snow from above may become heavier and reach the ground as snow or rain, depending on the temperature. Stratus clouds may produce drizzle, whereas the cumulus type sometimes yields showers, particularly in the tropics.
Clouds are valuable aids in weather forecasting.
If the sky, for example, is becoming overcast with cirrus and cirrostratus clouds, thickening and lowering to altostratus clouds, precipitation is imminent, probably from an approaching warm front in temperate latitudes or perhaps a hurricane in the tropics. New active thunderstorms are indicated by cumulus growing rapidly into cumulonimbus on a humid day, even though the Sun may be shining brightly between the clouds. The presence of cloudiness marks smaller diurnal temperature variations. In the evening a low overcast acts like a blanket, preventing the temperature from dropping much at night. A clear evening sky, on the other hand, indicates rapid cooling, which leads to dew, frost, or even fog the following morning.