Stratus clouds are dull, flat, heavy horizontal clouds that lay low in the sky. They are commonly gray but can be white.
Stratus clouds often bring a drizzle or they can bring snow.
The altitude of stratus clouds indicate how fast a storm is approaching. Low stratus clouds usually mean rain will begin soon.
Stratus clouds appear as a hazy veil. They are featureless and depressing. Sometimes they are called “high fog.” They always obscure the sun.
The word “stratus” comes from the Latin prefix “strato-“, meaning “layer”.
Stratus clouds form when a uniform sheet of relatively warm, moist air rises up and comes to rest against a higher, uniform sheet of heavier cool air. Stratus clouds indicate that the atmosphere is organized into stable horizontal layers. This commonly happens when a front of warm air is advancing against a mass of cold air.
Stratus clouds often linger for days as prolonged cloudy weather.
When a stratus cloud produces precipitation it is called a nimbostratus cloud. Dark, heavy nimbostratus clouds can drop steady rain for hours.
Heavy, horizontal stratus clouds can be contrasted to cumulus clouds which are formed by accumulations of warm air rising in columns.
When an accumulation of warm air breaks through a higher mass of cool air it can billow up to form a stratocumulus cloud. When stratus clouds form higher than 7,000 feet, they are called altostratus clouds.