Clouds and Precipitation

Clouds are visible evidence of atmospheric moisture and atmospheric motion. Those that indicate instability may serve as a warning to the fire-control managers. Some produce precipitation and become an ally to the firefighter. Some clouds develop into full-blown thunderstorms with firestarting potential and often disastrous effects on fire behavior. The amount of precipitation and its seasonal distribution are important factors in controlling the beginning, ending, and severity of local fire seasons. Prolonged periods with lack of clouds and precipitation set the stage for severe burning conditions by increasing the availability of dead fuel.

Clouds consist of minute water droplets, ice crystals, or a mixture of the two in sufficient quantities to make the mass discernible. Air becomes saturated either by the addition of moisture, or, more commonly, by cooling to the dew point. In saturated air, clouds form by the condensation of water vapor, which takes place on fine particles called condensation or sublimation nuclei. Cloud droplets grow to sizes large enough to precipitate by the ice-crystal process, in which water vapor is transferred from evaporating, supercooled liquid droplets to ice crystals where sublimation takes place, or by coalescence of droplets or ice crystals into raindrops or clumps of snowflakes. Precipitation falls in the form of liquid rain or drizzle, freezing rain or drizzle, or frozen snow, sleet, or hail.

Clouds are classified according to their structure as stratus or cumulus, and according to their altitude as high, middle, low clouds, and those with large vertical development. In the last group are cumulonimbus or thunderstorm clouds.

• Saturation Processes
• Condensation, Sublimation, and Precipitation
• Kinds of Precipitation
• Kinds of Clouds