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9.20: Precipitation

Difficulty Level: At Grade Created by: CK-12
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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

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.

(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.

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

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.

(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.

Vocabulary

  • dew: Moisture from water vapor that condenses on an object that is below the dewpoint temperature.
  • frost: Ice crystals that form on an exposed surface when the temperature of the surface is below freezing.
  • glaze: A coating of ice due to freezing of liquid raindrops.
  • hail: Balls of ice formed as water freezes and in layers in the strong updrafts of thunderstorms.
  • precipitation: Water that falls from the sky as rain, snow, sleet, or hail.
  • rain: Liquid water that falls from the atmosphere.
  • sleet: Raindrops that freeze to ice before striking the ground.
  • snow: Frozen precipitation in patterns.

Summary

  • 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.

Practice

Use this resource to answer the questions that follow.

http://www.youtube.com/watch?v=kFFHo4T-g4E

1. What is precipitation?

2. What is the most common form of precipitation?

3. What is hail?

4. What is sleet?

5. What is snow?

Review

1. Describe how raindrops form.

2. Why does hail only come from cumulonimbus clouds?

3. How does sleet form?

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    Vocabulary

    dew

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

    frost

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

    glaze

    A coating of ice due to freezing of liquid raindrops.

    hail

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

    precipitation

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

    rain

    Liquid water that falls from the atmosphere.

    sleet

    Raindrops that freeze to ice before striking the ground.

    snow

    Frozen precipitation in patterns.

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    Date Created:
    Feb 24, 2012
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