Like all clouds, stratus clouds are made of water vapor, water droplets and even ice crystals. Stratus clouds are identified not by what they are composed of, but by their general appearance, which is uniform, wide-ranging and gray.
Clouds that look like fog that's above the ground are stratus clouds. In fact, when patches of fog lift into the sky, they become stratus clouds. According to NASA, stratus clouds resemble bed sheets because they are low to the ground and essentially "blanket" the atmosphere with a layer of cloud below 6,000 feet. These clouds form when air is lifted upward by a gentle current and subsequently falls below the dew point. They can also form when the weak current lifts air just high enough to initiate condensation.
Though stratus clouds can bring precipitation, they are typically not the clouds that are associated with heavy storms; they are much more likely to bring a light drizzle than a torrential downpour. Technically, when stratus clouds develop enough to deliver a hard rain, they are no longer stratus clouds but are considered nimbostratus clouds instead. Stratus clouds can combine with other types of clouds to form cirrostratus, altostratus, nimbostratus and stratocumulus clouds.