When the temperature and dew point are close together (within 5 degrees), visible moisture in the form of clouds, dew or fog is likely. The stability of the atmosphere determines the type and vertical extent of clouds. A stable atmosphere resists any upward or downward displacement; an unstable atmosphere allows an upward or downward disturbance to grow into a vertical or convective current. When temperature decreases uniformly and rapidly with altitude (approaching 3 degrees C per 1000 feet), it is an indication of unstable air. If the temperature remains unchanged or decreases only slightly with altitude, the air tends to be stable. When air near the surface is warm and moist, instability is likely. Stable air is characterized by stratiform clouds, smooth air, steady precipitation, and fair to poor visibility. Unstable air is characterized by cumuliform clouds, turbulence, showery precipitation, and good visibility. Clouds are named according to the following characteristics:

cirroform (or fibrous)—consisting mainly of ice crystals

cumuliform (or heaped)—formed by unstable air rising and cooling

stratiform (or layered)—formed by the cooling of a stable layer

nimbus (or rain-bearing)

fractus (fragmented)

castellanus (common base with separate vertical development, often in lines)

lenticularis (lens-shaped, often formed in strong winds over mountains)