Nimbostratus cloud to a surface observer. Nimbostratus usually has a thickness of about 2000 to 4000 m. Though found worldwide, nimbostratus occurs more commonly in the middle latitudes. It is coded C2 on the SYNOP report. Nimbostratus occurs along a warm front or occluded front where the slowly rising warm air mass creates nimbostratus along with shallower stratus clouds producing less rain, these clouds being preceded by higher-level clouds such as cirrostratus and altostratus. Often, when an altostratus cloud thickens and descends into lower altitudes, it will become nimbostratus. Nimbostratus, unlike cumulonimbus, is not associated with thunderstorms, however at an unusually unstable
Cumulus humilis cloud Cumulus humilis cloud Cumulus humilis are cumuliform clouds with little vertical extent, common in the summer, that are often referred to as "fair weather cumulus". If they develop into cumulus mediocris or cumulus congestus, thunderstorms could form later in the day. They generally form at lower altitudes (500–3000 m (1,500–10,000 ft)), but in hot countries or over mountainous terrain these clouds can occur at an altitude of up to . They show no significant vertical development, indicating that the temperature in the atmosphere above them either drops off very slowly or not at all with altitude; that is, the adiabatic