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Chapter 7

1 Concordia University Geog/Sci-381 Chapter 7

Precipitation Processes �  Precipitation is any form of water that falls from a cloud and

reaches the ground. �  How do cloud drops grow?

�  When air is saturated with respect to a flat surface it is unsaturated with respect to a curved droplet of water.

�  Air must be super-saturated to keep growing if water is pure

�  The solute effect lowers vapor pressure and lets clouds grow at lower RH

�  Over land: more condensation nuclei ○  More, smaller droplets than over water

�  Normal condensation processes do not produce rain ○  Would take several days to produce

drops large enough to fall as rain


Fig. 7-3, p. 167 3 Concordia University Geog/Sci-381 Chapter 7

Precipitation Processes

� Two Mechanisms that produce raindrops: � Collision & Coalescence �  Ice-Crystal (Bergeron) Process

� Collision & Coalescence � Droplets of different sizes collide and

coalesce into larger droplets; warm cloud process

�  Largest drops reach ground first from the warm cloud process


5 Concordia University Geog/Sci-381 Chapter 7 6 Concordia University Geog/Sci-381 Chapter 7

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Table 7-1, p. 168

Precipitation Processes �  Ice-Crystal (Bergeron) Process

�  More common in middle or upper latitudes �  Cold clouds are a mixture of ice & water �  Ice crystals grow at the expense of surrounding water

droplets �  Supercooled water drops can exist at temperatures below

0oC ○  At -10oC, there are a million liquid droplets for every ice

crystal ○  Freezing is enhanced if there are ice nuclei

�  Ice nuclei are more rare than condensation nuclei: clay minerals, bacteria

○  Below -40oC, the cloud is glaciated, only ice �  Cirrus clouds are generally high enough to be glaciated

�  Saturation vapor pressure is greater over water as compared to ice.



Precipitation Processes �  Ice-Crystal (Bergeron) Process

�  Saturation vapor pressure is greater over water as compared to ice.

�  The water vapor supplied by the liquid droplets diffuse toward the ice crystals and make them grow.

�  Accretion �  Ice crystals collide with supercooled droplets and freeze

them, making the crystal grow, forming graupel.

�  Aggregation �  Ice crystals collide and stick together, forming

snowflakes


Fig. 7-10, p. 173 11 Concordia University Geog/Sci-381 Chapter 7

Precipitation Processes

� Artificial Cloud Seeding �  Inject cloud with small particles that act as

condensation nuclei, starting the precipitation process.

�  There must already be clouds: seeding does not generate clouds

� Dry ice, silver iodide � Natural Cloud Seeding

� Cirriform clouds can drop ice onto lower cumulus clouds and seed the precipitation


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Precipitation in Clouds �  In cold, strongly convective clouds, precipitation

may form quickly �  Most Precipitation is formed through accretion �  Rain usually starts as ice: it is therefore rare in

stratus clouds which are too warm. �  Rimed crystals (ice crystals with attached frozen droplets)

and graupel:


Precipitation Types

� Rain: falling drop of liquid water � Drizzle: less than 0.5 mm (not rain) ○  Most common from stratus clouds ○  Can also be drops that fall through dry air and

evaporate while falling �  Virga: Streaks of falling precipitation that

evaporate before reaching the ground (see next picture)


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Precipitation Types �  Showers occur when downdrafts suddenly

allow suspended rain to fall �  Excessively heavy showers: Cloudbursts

�  Nimbostratus: continuous rain �  Cumulonimbus: lots of updrafts and

downdrafts �  So there can be rain on only one side of a

street. �  Raindrops are seldom larger than 6 mm

because they tend to collide and break up. �  What conditions produce huge raindrops?


Precipitation Types

� Topic: Tear Drops � Raindrops not tear shaped �  Shape is size dependent ○  Less than 2 mm = sphere ○  Greater than 2 mm = flattened sphere


Table 7-2, p. 176 21 Concordia University Geog/Sci-381 Chapter 7

Precipitation Types �  Snow: frozen water falling from sky (crystal

or flake) �  Most precipitation starts as snow �  The freezing level is generally about 3600 m

(12,000 ft) in the summer. �  You can sometimes see the melting level.


Precipitation Types �  Snow & Cloud Appearance

�  Looking up into precipitation, the snow looks more white than rain, so the shade of the region beneath the cloud lets you predict whether snow or rain will fall.

�  Fall Streaks can be seen below cirriform clouds �  Similar to virga, they sublimate in the dryer air.


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Precipitation Types �  Flurries

�  Light, intermittent snow from developing cumulus clouds �  Snow Squall

�  Brief heavy snow showers from cumiliform clouds �  Ground blizzards are characterized by blowing snow and

drifts on the ground �  True blizzards have low temperatures and strong winds

bearing large amounts of snow.

�  A blanket of snow is a good insulator



Precipitation Types �  Sleet: air below freezing, then travels

through a layer of air above freezing, begins to melt and then falls through a layer of air below freezing just above the ground surface.

�  Freezing Rain (glaze): ground surface is freezing. As rain hits the surface it freezes. �  What’s black about black ice? �  Small drops that freeze onto cold surfaces

produce rime.


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Precipitation Types �  Snow Grains (<1 mm): solid equivalent of drizzle, no bounce

or shatter �  Snow Pellets (<5 mm): bounce, break, crunch underfoot,

tapioca snow �  Graupel: ice particle with a heavy coating of rime �  Hail: graupel acts as embryo in intense thunderstorm, grow

through aggregation as pushed up by updraft. �  Killed two in the US in 20th century �  Golf ball-sized hail takes 5 to 10 minutes to grow


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Fig. 7-30a, p. 186 39 Concordia University Geog/Sci-381 Chapter 7 Fig. 7-30b, p. 186 40 Concordia University Geog/Sci-381 Chapter 7

Table 7-5, p. 187 41 Concordia University Geog/Sci-381 Chapter 7 42 Concordia University Geog/Sci-381 Chapter 7

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Measuring Precipitation �  Instruments

�  Rain gauge: standard, tipping bucket (can under-measure during heavy rain), weighing ○  Rain less than 0.01” = a trace ○  Snow: average depth at 3 locations, 10:1 water equivalent

�  Can vary from 6:1 to 30:1

�  Doppler Radar �  Transmitter sends microwaves toward target, the returned

energy is measured and displayed ○  Brightness of echo = amount/intensity of rain

�  Doppler: measures speed of horizontal rain �  Radar sometimes produces images of rain that doesn’t

reach the ground.




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Measuring Precipitation �  Measuring from space

�  Specific satellites designed to assess clouds, atmospheric moisture, and rain ○  TRMM

�  Tropical Rainfall Measuring Mission �  Infrared, microwave, visible �  400 km high, 91 minute orbital period, Cyclone Susan

○  CloudSat �  700 km high




Homework for Chapter 7 �  Chapter 7 Questions for Review, p. 193

�  #2, 7, 9, 12, 14, 18, 19

�  Chapter 7 Questions for Thought, p. 193 �  #3, 5

�  Chapter 7 Problems and Exercises, p. 194 �  #1, 4


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Project for Chapter 7 �  None


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