1. Physical Geography
Class 24
Types and Formation
Of Precipitation
© 2011 Pearson Education, Inc. 1

2. Precipitation
• Cloud droplets or crystals are too small
to fall
• Must combine to form precipitation

3. Warm Clouds: Collision - Coalescence
• Water drops form by condensation
• Grow by colliding with other drops
• Result: Rain

4. 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

5. © 2011 Pearson Education, Inc.
Cold Clouds
5
Figure 6-28

6. 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

7. Types of Precipitation
• Snow (ice crystals)
• Rain and drizzle (liquid)
• Freezing rain or Glaze
• Sleet
• Hail

8. At which surface temperature would it be
hardest to predict type of precipitation?
© 2011 Pearson Education, Inc. 8
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

9. Snow
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10. 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

11. 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

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

14. Freezing Rain
• Melts on its way to Earth, freezes only
when it contacts a solid surface

16. Which is more hazardous?
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0%
100%
0% 1. Sleet
2. Freezing rain
3. Snow

17. Hail
• Forms in cumulonimbus clouds only

18. During which season is hail
most common?
© 2011 Pearson Education, Inc. 18
27%
9%
0%
18%
45% 1. Winter
2. Spring
3. Summer
4. Fall
5. Equally common in all four seasons

19. © 2011 Pearson Education, Inc.
Atmospheric Lifting
• Four types of atmospheric lifting
19
Figure 6-32

20. Orographic Precipitation
• Ask me to tell you about my Glacier
Park camping trip (or stop me from
repeating myself)

21. Hawaii – where do the winds
come from?

22. 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

23. GLOBAL DISTRIBUTION OF
PRECIPITATION
23

24. © 2011 Pearson Education, Inc.
Global Distribution of
Precipitation
• High precipitation regions, tropics
• Low precipitation regions, deserts and poles
24
Figure 6-34

25. © 2011 Pearson Education, Inc.
Global Distribution of Precipitation
25
Figure 6-35

26. © 2011 Pearson Education, Inc.
Acid Rain
• Definition of acid rain
• Sources of acid rain
• Principal acids—
sulfuric and nitric
• Number of hydrogen
ions—pH
26
Figure 6-38

27. © 2011 Pearson Education, Inc.
Distribution of acid rain in US
27

28. © 2011 Pearson Education, Inc. 28

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31. THE END
31

32. © 2011 Pearson Education, Inc.
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
atmosphere 32

33. © 2011 Pearson Education, Inc.
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
33

34. © 2011 Pearson Education, Inc.
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
34

35. © 2011 Pearson Education, Inc.
Global Distribution of Precipitation
35
Figure 6-37