Characteristics and Types of Precipitation

CHAPTER TWO

AVIATION WEATHER

atmosphere. Saturation occurs when the dew point and the air temperature are equal. This can

occur either by raising the dew point (evaporation) or lowering the air temperature (cooling).

Another measure of atmospheric moisture is the relative humidity (RH), which is the percent of

saturation of the air. The air can become saturated (RH = 100%) by one of two ways. If the air is

cooled, the falling air temperature decreases the dew point spread closer to zero, while the RH

rises closer to 100%. If evaporation occurs, this adds moisture to the atmosphere, increasing the

dew point, which again lowers the dew point spread and increases the RH. Once the dew point

spread reaches 4F, the RH will be 90%, and the water vapor will begin to condense into fog or

clouds. Any further cooling or evaporation will produce precipitation, as there will be more

water present in the air than it can hold.

207.

CHARACTERISTICS AND TYPES OF PRECIPITATION

The characteristics and types of precipitation reveal information about various atmospheric

processes. The nature of precipitation may give a clue about a cloud's vertical and horizontal

structure or indicate the presence of another cloud deck aloft. The three characteristics of

precipitation are:

Showers - Characterized by a sudden beginning and ending, and abruptly changing

intensity and/or sky conditions. Showers are associated with cumuliform clouds.

Continuous - Also known as steady (not showery). Intensity changes gradually, if at all.

Continuous or steady precipitation is associated with stratiform clouds.

Intermittent - Stops and restarts at least once during the hour. Intermittent precipitation

may be showery or steady, and therefore may be associated with cumuliform or stratiform

clouds.

Precipitation takes many forms. A few of the more common types of precipitation are mentioned

here.

Drizzle Very small droplets of water that appear to float in the atmosphere.

Freezing drizzle Drizzle that freezes on impact with objects.

Rain Precipitation in the form of water droplets that are larger than drizzle and fall to the

ground.

4. Freezing rain Rain that freezes on impact with objects.

Hail or graupel A form of precipitation composed of irregular lumps of ice that develop

in severe thunderstorms, consisting of alternate opaque and clear layers of ice in most cases.

Water drops, which are carried upward by vertical currents, freeze into ice pellets, start falling,

accumulate a coating of water, and are carried upward again, causing the water to freeze. A

2-14 Atmospheric Mechanics of Winds, Clouds and Moisture, and Atmospheric Stability