Stratus and cumulus clouds are the two primary types of cloud structure, with each producing different types of weather. Stratus clouds themselves come in four varieties: cirrostratus, altostratus, stratus and nimbostratus. Some of these stratus clouds provide a strong indication of approaching precipitation, while others produce precipitation. Knowing how to read these cloud types can help identify the weather you will encounter.
Cirrostratus clouds are a type of high-level clouds, occurring above 18,000 feet. This type of stratus cloud consists of thin, sheetlike layers of white clouds. These clouds are comprised of ice crystals and produce no precipitation. However, cirrostratus clouds are a key meteorological predictor of the weather to come. Widespread layers of cirrostratus clouds are typically the first visual indicator of an approaching front. As such, they can signal the possibility of rain or snow within 24 hours when followed by mid-level cloud formation.
Altostratus clouds are a type of mid-level cloud, occurring between 6,000 and 18,000 feet. These clouds typically develop behind cirrostratus clouds, indicating the approach of a storm front and the potential for widespread, continuous rain. Altostratus clouds typically cover the entire sky, extending over large areas. These types of stratus clouds rarely produce any precipitation.
Stratus clouds are a type of low-level cloud, occurring below 6,000 feet. They are characterized by uniform layers of grayish clouds. Stratus cloud layers are generally thin but cover the entire sky, known as overcast. Stratus clouds are formed by weak, gentle upward air currents that lift large layers of air high enough to produce condensation. However, the overall atmosphere remains too stable to produce cumulus clouds, which require dynamic vertical instability. Stratus clouds appear like fog that does not reach the ground. Stratus clouds are capable of producing light mist or an occasional light drizzle. If the stratus clouds begin to produce consistent precipitation, they are reclassified as nimbostratus.
Nimbostratus clouds are a type of low-level cloud, occurring below 6,000 feet. They are similar to stratus clouds, except they are actively producing precipitation. This precipitation is typically consists of light to moderate rain or snow, which is continuous in nature. This low-intensity precipitation can last for several hours or several days. Because they form in conditions unfavorable to vertical development, nimbostratus clouds never produce heavy rains or thunderstorms. Nimbostratus clouds are associated with warm fronts, where warm, moist air gradually overrides colder air at the surface. Visibility is very low beneath nimbostratus clouds. One of the reasons for this lack of visibility is steady precipitation. However, a secondary cause is the formation of fog and scud, resulting from precipitation-cooled air below the cloud layer. Scud, also known as stratus fractus or fractostratus, is low, fast-moving fragments of clouds below the main cloud deck.