Physical Geography

Class 24

Types and Formation

Of Precipitation

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Precipitation• Cloud droplets or crystals are too small

to fall

• Must combine to form precipitation

Warm Clouds: Collision - Coalescence

• Water drops form by condensation

• Grow by colliding with other drops

• Result: Rain

Cold Clouds: Bergeron process

• A mix of ice crystals and supercooled water droplets (water droplets below freezing but haven’t frozen yet)

• Deposition (sublimation) causes ice crystals to grow

• Snowflakes grow big enough to fall from cloud

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Cold Clouds

5

Figure 6-28

At which latitude would the Collision-Coalescense process be most

common?

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73%

27%

0% 1. Polar

2. Mid-latitudes

3. Tropical

Types of Precipitation

• Snow (ice crystals)

• Rain and drizzle (liquid)

• Freezing rain or Glaze

• Sleet

• Hail

At which surface temperature would it be hardest to predict type of precipitation?

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20%

50%

30%

0% 1. -10 to -6 degrees C

2. -2 to 2 degrees C

3. 6 to10 degrees C

4. 16 to 20 degrees C

Snow

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What can happen to ice crystals on their way from a cloud to the ground?

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100%

0%

0%

0% 1. It can remain in its original frozen form

2. It can melt and become rain

3. It can melt and then refreeze

4. Any of the above can occur

What can happen to snow on its way to Earth?

• Can melt and form raindrops

• Can stay snowflakes

• Can melt and then refreeze sleet or freezing rain

Sleet = frozen pellets• Snow melts, then refreezes in air

Freezing Rain• Melts on its way to Earth, freezes only

when it contacts a solid surface

Which is more hazardous?

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0%

100%

0% 1. Sleet

2. Freezing rain

3. Snow

Hail

• Forms in cumulonimbus clouds only

During which season is hail most common?

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27%

9%

0%

18%

45% 1. Winter

2. Spring

3. Summer

4. Fall

5. Equally common in all four seasons

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Atmospheric Lifting• Four types of atmospheric lifting

19

Figure 6-32

Orographic Precipitation• Ask me to tell you about my Glacier

Park camping trip (or stop me from repeating myself)

Hawaii – where do the winds come from?

Winds travel from:

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82%

0%

9%

9% 1. Bottom right to upper left

2. Top left to bottom right

3. Bottom left to upper right

4. Top right to bottom left

GLOBAL DISTRIBUTION OF PRECIPITATION

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Global Distribution of Precipitation

• High precipitation regions, tropics• Low precipitation regions, deserts and poles

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Figure 6-34

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Global Distribution of Precipitation

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Figure 6-35

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Acid Rain

• Definition of acid rain• Sources of acid rain• Principal acids—

sulfuric and nitric• Number of hydrogen

ions—pH

26

Figure 6-38

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Distribution of acid rain in US

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THE END

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Summary

• Moisture can impact the landscape in a variety of ways, including fog, haze, and precipitation

• The hydrologic cycle shows the balance between water removed from the oceans and water returned by precipitation

• Water has a number of unique properties• Water vapor is the gas form of water• Evaporation rates change as surrounding

atmospheric conditions change• There are several measures of vapor content in the

atmosphere32

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Summary

• There are several measures of vapor content in the atmosphere, called humidity measurements

• Condensation is the process by which vapor is converted to liquid

• Adiabatic processes explain changes in parcel temperature without the addition or subtraction of heat to the parcel

• Clouds are a visual identification of saturation• Air has buoyancy associated with it that describes

its stability

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Summary

• Many processes are responsible for precipitation• There are five primary types of precipitation• Atmospheric lifting occurs through four primary

mechanisms• The most highly variable rainfall worldwide occurs

over deserts• Tropical regions are generally wet• Acid rain affects the Northeast and results from

compounds released into the air by humans

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Global Distribution of Precipitation

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Figure 6-37