AOSC 200 Lesson 8

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AOSC 200 Lesson 8
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Precipitation Growth in Warm Clouds

• Cloud droplets are typically 10 microns in size.
  small raindrops are typically 1000 microns
  (almost one million droplets)
• How does one go from one to the other?
• Simplest explanation is that the droplet grows by
  condensation of water vapor on its surface. But
  this would take several days.
• One way for raindrops to form is by a processes
  known as collision-coalescence.
• This process requires lots of droplets - high
  absolute humidity - seen only in the tropics.

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Collision-Coalescence

• Warm cloud because process because it only occurs
  in the tropics.
• As the cloud droplet is formed it is pulled down
  by gravity but then is moved upward by the rising
  air within the cloud. Hence within a cloud one
  has droplets moving in all directions and they
  can collide and form larger droplets by
  coalescing.
• Once a drop grows to a size when the force of
  gravity exceeds the uplift from the rising air,
  the drop moves downward through the cloud picking
  up other droplets as it falls.

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Collision Coalescence
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Collision-coalescence process
Fig. 4-31, p. 114
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Precipitation Growth in Cold Clouds

• Outside of the tropics there are just not enough
  cloud droplets to form rain drops by the
  collision-coalescence process.
• A Swedish atmospheric physicist, Dr. Bergeron,
  would vacation in the fall in the mountains. He
  often took an early morning walk along a path
  that led through a pine forest and encountered
  uplift fogs.
• He noted that on the days when the temperature
  was above 0 C, the fog went all the way to the
  ground, whereas if the temperature fell below -10
  C, the fog lay above the tops of the trees.

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Bergeron walk
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Why?

• He further noted when the temperature fell below
  -10 C, that the pine needles were covered with
  ice.
• The reason is that the vapor pressure of water
  above ice is less than the vapor above water.

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Attraction of water vapor to ice versus water
Fig. 4-33, p. 116
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Saturation vapor pressure over ice and over water
Fig. 4-35, p. 117
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Fig. 4-34, p. 116
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Accretion and Aggregation

• It should be noted that the ice crystal growth
  gives a snow flake which eventually will also
  begin to fall and then rise in the updraft.
• These ice crystals then begin to aggregate into
  larger snowflakes.
• The ice crystal can also collide with a
  supercooled water droplet which instantly freezes
  - accretion.
• As the ice crystals descend at some point the
  temperature can rise above 0 C, and the crystal
  melts to form a raindrop.

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Process of aggregation
Fig. 4-32, p. 115
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Aggregation
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Fig. 4.27
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These picture shows fall-streaks. They consist of
ice particles that have fallen out of a cloud and
evaporate before they reach the ground.
Fig. 4.33
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Steps in the formation of the precipitation types
Fig. 4.38
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Warm Front
Fig. 9.13
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Fig. 4-37, p. 118
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FORMS OF PRECIPITATION

• RAIN - DROPLETS OF WATER GREATER THAN 0.5 MM IN
  DIAMETER. WHEN DROPLETS SMALLER THAN 0.5 MM
  CALLED DRIZZLE.
• MUCH RAIN STARTS OUT ALOFT AS ICE CRYSTALS
• SNOW - ICE CRYSTALS. IF AIR IS COLD (LOW
  HUMIDITY), WE GET LIGHT AND FLUFFY SNOW (POWDER).
  IF AIR IS WARM THAN ABOUT -5 CELSIUS, THEN WE GET
  WET SNOW (GOOD FOR SNOWBALLS).
• SLEET - SMALL PARTICLES OF ICE. RAINDROPS
  ENCOUNTER FREEZING AIR ON DESCENT. IF FREEZING
  NOT COMPLETE - FREEZING RAIN.
• HAIL - LAYERS OF ICE FORM AS THE HAILSTORM
  TRAVELS YUP NAND DOWN IN A STRONG CONVECTIVE
  CLOUD
• RIME - FORMED BY FREEZING OF SUPERCOOLED FOG ON
  OBJECTS.

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The effects of airflow over a mountain
Fig. 4-42, p. 122
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Clouds and Precipitation near Mountains

• As air ascends mountain it cools adiabatically,
  clouds form, and precipitation occurs.
• Above this altitude the relative humidity stays
  at 100
• At the peak of the mountain the absolute humidity
  is determined by the saturation vapor pressure at
  -12C.
• As the air descends its absolute humidity remains
  the same as at the peak

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Clouds and Precipitation near Mountains

• As the air descends it is compressed, so it warms
• Hence the saturation vapor pressure will
  increase, and the relative humidity will decrease
• The net effect of the air ascending and
  descending the mountain is that the air becomes
  drier and warmer.
• On the island of Hawaii, the west side of the
  coast (westerly winds) has rain forests, the
  eastern side has deserts.

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Table 4-4, p. 123