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9.20: Precipitation

Difficulty Level: At Grade Created by: CK-12
Practice Precipitation
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Do you live in a place that gets lots of rain?

In some places it rains so much that people barely notice it. In others it rains so little that a rainy day is revered. Rain is not the only type of precipitation; see a few below.


Precipitation ( Figure below ) is an extremely important part of weather. Water vapor condenses and usually falls to create precipitation.

Dew and Frost

Some precipitation forms in place. Dew forms when moist air cools below its dew point on a cold surface. Frost is dew that forms when the air temperature is below freezing.

Dew and hoar frost on a flower

(a) Dew on a flower. (b) Hoar frost.

Precipitation From Clouds

The most common precipitation comes from clouds. Rain or snow droplets grow as they ride air currents in a cloud and collect other droplets ( Figure below ). They fall when they become heavy enough to escape from the rising air currents that hold them up in the cloud. One million cloud droplets will combine to make only one rain drop! If temperatures are cold, the droplet will hit the ground as snow .

Picture of a rain and snow storm

(a) Rain falls from clouds when the temperature is fairly warm. (b) Snow storm in Helsinki, Finland.

Other less common types of precipitation are sleet ( Figure below ). Sleet is rain that becomes ice as it hits a layer of freezing air near the ground. If a frigid raindrop freezes on the frigid ground, it forms glaze . Hail forms in cumulonimbus clouds with strong updrafts. An ice particle travels until it finally becomes too heavy and it drops.

Picture of sleet, glaze, and hail

(a) Sleet. (b) Glaze. (c) Hail. This large hail stone is about 6 cm (2.5 inches) in diameter.

An online guide from the University of Illinois to different types of precipitation is seen here: http://ww2010.atmos.uiuc.edu/%28Gh%29/guides/mtr/cld/prcp/home.rxml .


  • A surface can be colder than the surrounding air, causing the air to cool below its dew point.
  • Rain droplets caught up in air currents within a cloud get larger by the addition of condensed droplets until they are too heavy and they fall.
  • If the ground is very cold, rain can freeze to become sleet or glaze.

Explore More

Use this resource to answer the questions that follow.


  1. What is precipitation? What are the four main types?
  2. What determines whether a bit of precipitation starts as water or ice? What determines what form it is in when it reaches the ground?
  3. What does rain start out as and what does it end up as? Why?
  4. Why is freezing rain solid when it reaches the surface?
  5. What happens to sleet as it falls through the atmosphere?
  6. How is hail different from sleet?
  7. What does snow start as and what does it end as? What is the air temperature as it falls?


  1. Describe how raindrops form.
  2. Why does hail only come from cumulonimbus clouds?
  3. How does sleet form?




Moisture from water vapor that condenses on an object that is below the dewpoint temperature.


Ice crystals that form on an exposed surface when the temperature of the surface is below freezing.


A coating of ice due to freezing of liquid raindrops.


Balls of ice formed as water freezes and in layers in the strong updrafts of thunderstorms.


Water that falls from the sky to Earth's surface; may take the form of rain, snow, sleet, hail, or freezing rain.


Liquid water that falls from the atmosphere.


Raindrops that freeze to ice before striking the ground.


Frozen precipitation in patterns.

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Difficulty Level:

At Grade


Date Created:

Feb 24, 2012

Last Modified:

Jan 05, 2015
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