Stratus clouds are uniformed layered clouds that are below 6,000 feet. They are formed in sheets and are usually associated with overcast weather. Fog or mist is the result of very low stratus clouds. They can form only a few hundred feet above ground. They are shallow but cover a large area, and they can bring precipitation. Stratus clouds are more known for drizzle than for precipitation, however. When heavier rain falls from them, their title is changed to nimbostratus clouds. Stratus clouds are formed when a weak upward air current lifts a thin layer of air high enough to start condensation of the excess water vapor if the air temperature falls below the dew point.
Altostratus clouds appear in altitudes of 6,000 to 20,000 feet. They are very thin and uniform, and are gray or blue-gray, creating overcast. They are translucent enough to see the sun or moon through them, however they do not allow enough light to make shadows on the Earth’s surface. Altostratus clouds are associated with coming rain, and they usually cover most, if not all, of the sky. Altostratus clouds are incapable of producing heavy precipitation, but they are often the cause of a light drizzle. Following altostratus clouds are nimbostratus clouds, which are the source of heavier precipitation.
Nimbostratus clouds form at or below 6,000 feet. They are dark, low level clouds that bring light to moderately heavy prolonged precipitation, such as snow or rain. These clouds, although low in the atmosphere, may contain ice crystals when the temperatures get below the freezing point. Below nimbostratus clouds can lie broken up clouds that are called fractostratus clouds.