Stratus may form from the transformation of Stratocumulus (St stratocumulomutatus) when it thickens and lowers, forming a fairly uniform base; or the base stays at the same height, but loses its relief or its subdivisions.
Precipitating Stratocumulus can thicken and lower and lose its relief or subdivisions. This is not transformation to Stratus. If the precipitating Stratocumulus is devoid of relief or subdivisions for an extended period of time, consideration must be given to the cloud having transformed to Nimbostratus.
Stratus often forms from the slow lifting of a fog layer, due to warming of the Earth’s surface or an increase in wind speed.
Stratus fractus of wet weather (the conditions that generally exist during precipitation and a short time before and after) is often produced by Altostratus, Nimbostratus or Cumulonimbus (St fra altostratogenitus, St fra nimbostratogenitus or St fra cumulonimbogenitus); it may also result from precipitating Cumulus (St fra cumulogenitus).