Introduction to Precipitation

When considering all the moisture that is in our atmosphere, there is relatively little precipitation that results. That moisture, however, is very important to crops and other vegetation as well as to the survival of mankind.

Precipitation is defined by the Glossary of Meteorology (American Meteorological Society) as "any or all forms of water particles, whether liquid or solid, that fall from the atmosphere and reach the ground. It is a major class of hydrometeor; but it is distinguished from cloud, fog, dew, rime, frost, etc. in that it must fall and it is distinguished from cloud and virga in that it reached the ground."

There are seven types of precipitation; they include rain, drizzle, freezing rain, freezing drizzle, ice pellets (sleet), hail and snow.

Each specific type of winter precipitation requires a rather unique vertical air mass temperature structure.

Precipitation forms as a result of two types of atmospheric processes: the collision-coalescence process and the Bergeron (ice crystal) process.

The collision-coalescence process occurs in clouds that form in temperatures above freezing. In this process, collisions between cloud droplets play an important part in producing precipitation particles that are heavy enough to fall to the earth. When considering this process, there must be condensation nuclei or microscopic airborne particles (preferably hygroscopic or water absorbent), such as dust, that water droplets can condense upon. In the collision-coalescence process, some cloud droplets must be larger than others. Those larger droplets collide with smaller droplets as they fall. Factors that affect this process include how much time the droplets remain in the cloud (the longer they remain, the larger they can become) as well as electrical charges within the cloud (if colliding droplets have opposite electrical charges they attract one another).

The Bergeron (ice crystal) Process assumes that ice crystals and very small pure water cloud droplets (supercooled liquid) coexist in the atmosphere; even at temperatures well below freezing. Tiny ice crystals, in a supercooled cloud, grow larger at the expense of the surrounding liquid cloud droplets. After growing large enough, the crystals fall from the clouds as precipitation. This precipitation formation process is more common at higher altitudes as well as at middle and high latitudes.