A stratus cloud is a cloud

Cloud

A cloud is a visible mass of liquid droplets or frozen crystals made of water and/or various chemicals suspended in the atmosphere above the surface of a planetary body. They are also known as aerosols. Clouds in Earth's atmosphere are studied in the cloud physics branch of meteorology...

 belonging to a class characterized by horizontal layering with a uniform base, as opposed to convective clouds that are as tall or taller than wide (these are termed cumulus cloud

Cumulus cloud

Cumulus clouds are a type of cloud with noticeable vertical development and clearly defined edges. Cumulus means "heap" or "pile" in Latin. They are often described as "puffy" or "cotton-like" in appearance. Cumulus clouds may appear alone, in lines, or in clusters...

s). More specifically, the term stratus is used to describe flat, hazy, featureless clouds of low altitude varying in color from dark gray to nearly white. Stratus clouds may produce a light drizzle or snow. A "cloudy day" usually features a sky filled with stratus clouds obscuring the disk of the sun

Sun

The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...

. These clouds are essentially above-ground fog

Fog

Fog is a collection of water droplets or ice crystals suspended in the air at or near the Earth's surface. While fog is a type of stratus cloud, the term "fog" is typically distinguished from the more generic term "cloud" in that fog is low-lying, and the moisture in the fog is often generated...

 formed either through the lifting of morning fog or when cold air moves at low altitudes over a region.

Forecast

Can form from stratocumulus spreading out under an inversion and means a continuation of prolonged cloudy weather with drizzle for several hours and then an improvement as it breaks into stratocumulus. Can last for days in anticyclones. Often forms instead of nimbostratus on a weak warm front. If it is seen after rain it should clear (warm front succession) but if it doesn't it is cloud from the warm sector of a frontal system and heavy rain may start again (cold front) after several hours of stratus.