Stratus clouds are low-lying uniform clouds which blanket the sky. They are often a dullish white to gray in color, and they are generally the sort of clouds which appear when people talk about a “cloudy day.” Stratus clouds are not generally associated with poor weather, although they can be accompanied by drizzle, and at higher altitudes, they can be an indicator that rain is on the way. The next time you look out the window onto an oppressively cloudy day with low-lying clouds, you'll know that you are looking at stratus clouds.

These clouds are among the lowest of cloud formations, and in fact sometimes they creep so low that they turn into ground-based fog. Stratus clouds are characterized by having horizontal layers with an extremely even, uniform bottom. In Latin, “stratus” refers to a layer or blanket, and these terms are very apt descriptions of classical stratus clouds. Sometimes, stratus clouds will appear in a more fragmented way, as is the case with stratus fractus.

Altostratus and cirrostratus are two forms of stratus clouds which appear higher up in the troposphere, while nimbostratus are associated with rain and oncoming storms. Stratus clouds can also create a halo effect around the moon and stars when they are thinner and more hazy, and on a very cloudy day, the sun may be totally obscured by stratus clouds, or it may appear in the form of a blurry bright patch in the cloud cover.

Sometimes, lighting conditions under stratus clouds can be very interesting. These clouds have a tendency to dull the light, creating a flattened look which filmmakers sometimes use to set a specific mood or tone. Many people find stratus clouds rather boring, because of their uniform bottom, although they can get more interesting when they are fragmented beneath other clouds in the atmosphere. When stratus becomes extremely fragmented under higher clouds, it can be a warning that poor weather is on the way.

Like other clouds, stratus clouds are formed when water vapor condenses. This is typically caused by a flow of warm, moist air which flows into a block of cooler air. As a result, stratus clouds often appear on very muggy days, where the air is high in moisture. In urban areas, these low-lying clouds can also trap pollution, making people feel very dirty and sticky by the end of the day as a combined result of humidity and pollutants.