"Stratus" is the Latin word for layer or blanket. Stratus clouds form a low layer that can cover the entire sky like a blanket, bringing generally gray and dull weather. Stratus clouds may form only a few hundred feet above the ground. Over hills and mountains they can reach ground level when they may be termed fog. Conversely, fog that "lifts" off the ground forms a layer of low stratus clouds.
Stratus clouds can form when very weak, upward vertical air currents lift a thin layer of air high enough to initiate condensation of the excess water vapour, if air temperature falls below the dew point. Precipitation rarely falls from true stratus clouds since the upward vertical motion needed for precipitation is very weak, but light mist and drizzle can sometimes accompany stratus clouds. When heavier rain falls from them, they are called nimbostratus clouds.
The word "stratus" is combined with other cloud types. Other cloud combinations which use "stratus" include high level cirrostratus (cirrus + stratus), medium level altostratus, and the low level stratocumulus (stratus + cumulus).