Stratus clouds are low-level, grey, fog-like clouds that often encompass the entire sky. They are uniform, often forming low-hanging shelves which lead to overcast days with little to no precipitation. Stratus clouds do not occur above 6,000 feet, and though they often resemble fog, these clouds do not reach all the way to the ground.
Stratus clouds form when wind pushes warm, humid air higher into the atmosphere where temperatures are cooler. As moist air cools, it expands and becomes even more humid. The increase in humidity causes water vapor to condense around debris in the atmosphere to form visible clouds. Stratus clouds are often visible after ground-level fog lifts.
Stratus clouds also form hybrids with other types of clouds. Nimbostratus clouds are very similar in appearance to stratus but produce more constant precipitation. When humid air lifts very high into the atmosphere, cirrostratus clouds form. Cirrostratus clouds occur so high that they are associated with ice crystals rather than liquid precipitation. Cirrostratus clouds appear as a slight, white veil in the sky or may be entirely invisible from the ground, with only a halo or spectrum indicating their presence.
Stratocumulus clouds form like cumulus clouds but flatten out like stratus. Stratocumulus are one of the most common clouds in temperate regions of the world.