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Cloud Classification
Clouds are typically classified by the height of their bases. High-level clouds occur above altitudes of 18,000 feet. Mid-level clouds occur between 6,500 and 18,000 feet, and low-level clouds occur between the ground and 6,500 feet. Stratus clouds are low-level clouds. Furthermore, clouds that are actively producing precipitation have their names modified to identify this feature. In the case of stratus clouds, they become known as nimbostratus clouds.
Stratus Description
Stratus clouds are uniformly gray in color, with nimbostratus clouds appearing darker. "Stratus" comes from Latin, meaning layer. These clouds appear as a layer of featureless clouds that typically blanket the entire sky. This condition is known as overcast skies. They are characterized by horizontal development, rather than vertical development. Their ragged bases are usually only a couple of hundred feet above the ground and frequently cover the tops of hills, mountains and tall structures. When these clouds come in contact with the ground, they are often referred to as fog.
Stratus Formation
Stratus clouds are primarily associated with low-pressure storm systems. They form along the warm front that extends from these systems to the east. These warm fronts provide very weak vertical air currents that slowly lift a wide, thin layer of warm, moist air. As the air rises, it cools and condenses, forming a cloud layer. This same lifting can also occur when winds drive warm, moist air over rising terrain. Finally, stratus clouds can be formed when layers of moist air are cooled from below. If they are cooled to their dew point temperature, condensation occurs and a layer of stratus clouds is formed. This typically produces fog, such as over lakes.
Stratus Effects
Stratus clouds normally begin as high-level cirrus clouds in advance of a storm system. This layer drops as the front approaches, becoming mid-level altostratus clouds. Finally, they thicken and drop to become a stratus layer. Stratus clouds can produce a light mist or occasional drizzle, while nimbostratus clouds produce continuous light-to-moderate rain or snow. Nimbostratus clouds are typically only associated with warm front formation. The rainy or snowy weather that comes along with nimbostratus clouds can last for hours or days because warm fronts move much more slowly than cold fronts. Furthermore, it is not uncommon for the fronts to stall and remain over a location for long periods of time before moving on.