If the droplets grow large enough they form raindrops and fall to the ground. The atmosphere consists of air masses which are different notably in temperature and moisture content. In general the air masses do not mix but are separated by relatively narrow transition zones called fronts. The fronts are frequently characterized by clouds and produce some of the stormiest weather. The type and extent of the clouds depend on the air mass characteristics.
Some fronts have few clouds when the air is very dry.
Clouds frequently rise to an altitude of 6 km, less frequently to about 10 km, but in the case of very severe storms may extend upwards at least 18 km. The first sign of an approaching warm front is usually a layer of cirrus that begins to cover the western or south-western sky. This layer thickens to a cirrostratus and perhaps, also altocumulus begin to form. Light rain may begin at this point becoming more intense in the dark grey nimbostratus with fog often developing. Stratus cloud and some thick stratocumulus may also form.