Precipitation
As you know from your daily life, precipitation is an extremely important part of weather.
Precipitation most commonly falls as rain or snow, but can also be sleet, hail, dew or frost.
Sleet is a mixture of rain and snow, and often forms when snow partially melts as it falls.
Dew forms when moist air comes into contact with a cold surface, like the ground or a car
windshield, and cools below its dew point. Frost forms under similar conditions, but when
the air cools to below freezing (Figure16.11).
Figure 16.11: Hoar frost. ( 43 )The other types of precipitation come from clouds. Rain or snow droplets fall when they
become heavy enough to escape from the rising air currents that hold them up in the cloud.
The most common way for rain or snow to droplets to grow, occurs in cold clouds, where
the temperature is -10oC(14oF) or less (Figure16.12). Here the water vapor freezes directly
into ice crystals, which continue to grow as more water vapor freezes onto them. When
the ice crystals become heavy enough, they fall. Even as they fall, the ice crystals collect
more moisture. If temperatures are cold, the ice will hit the ground as a snowflake. If
temperatures near the ground are greater than 4oC(39oF), the ice crystal may melt and
become a raindrop. One million cloud droplets will combine to make only one rain drop!
Water may also precipitate from warm clouds. Here too, water droplets get trapped in rising
and falling air currents. As the droplet travels around the convection cell, it collides with
other small droplets. At some point, the droplet is large enough to escape the convecting air
currents and it falls to the ground as rain. If the air currents are very strong, the droplets
must be very large before they fall.
Ifaraindropfallsthroughwarmairbuthitsalayeroffreezingairneartheground, itbecomes
frozen into a small clear ice pellet known assleet. Sleet usually is mixed with liquid water
drops that did not freeze as they descended from the cloud. If the layer of frigid air near the