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CLOUDS AND PRECIPITATION
I.
Clouds
A.
Suspended water droplets and ice crystals in troposphere.
B.
Cloud formation
1.
Liquid water condenses collects around suspended particles of dust
2.
Avg duration of cloud: 10 minutes
C.
Cloud classification
1.
Degree of vertical development – reflects relative atmospheric instability
a.
Stratus-type clouds
(1)
Horizontal sheets, often featureless, can cover much/all of the sky. Low
to moderate instability.
b.
Cumulus-type clouds
(1)
Vertically developed, billowy clouds. Look like ever-changing cotton balls.
Potentially high instability.
2.
Height of the cloud base
D.
II.
Fog-a stratus cloud at surface. Lots of airports, highways, and aspects of our economy are
impacted by fogs.
1.
Three types
a.
Radiation fog-created by radiation cooling at night (also known as an inland fog)
(1)
High RH during day
(2)
Clear, night with radiation cooling
(3)
Temp drops and water condenses out
(4)
When sun rises, air is heated from below to increase max amount of
water vapor the air can hold
(5)
Water evaporates (warmer air holds more water vapor) and fog "lifts"
b.
Advection fog-created by warm air chilled to dew point over cold surface
(1)
Moist air chilled to saturation to form fog
(2)
Example: San Francisco, where fogs form offshore but are pulled in by
sea breezes during the day.
c.
Evaporation (or steam) fog-created when cool air moves over warm water
(1)
Evaporated water vapor mixes with overlying cold air
(2)
Cold air gains more water vapor and reaches saturation
(3)
Example: Maine, where the warm Gulf Steam is adjacent to shore and
brings warmer waters to a high latitude
Precipitation
A.
Any form of water falling from the sky
B.
All forms of precipitation are also known as hydrometeors.
C.
Must collect immense number of small cloud droplets to form a mass of water (snowflake,
raindrop) heavy enough to fall rapidly to the earth’s surface without being evaporated along the
way.
D.
Two precipitation formation mechanisms recognized
1.
Collision-coalescence
a.
Liquid water droplets collide and coalesce around condensation nuclei until
droplet is of sufficient mass for gravity to pull downward
b.
Occurs in tropics
2.
Bergeron process
a.
Occurs in cold clouds (temps between -10 and -40°C). Many cloud droplets at
these temperatures resist freezing by their strong cohesive forces (hydrogen
bonds). They remain supercooled liquid droplets.
b.
In the presence of dust particles that serve as freezing nuclei, supercooled
droplets evaporate water vapor, which then is deposited as ice crystals on
c.
E.
freezing nuclei. What forms is a snowflake, which eventually becomes large
enough to fall out of the cloud due to gravity.
All precipitation in the midlatitudes and polar regions forms in this manner. Even
on a hot summer day at these locations, temperatures in the cloud tops are well
below freezing,
Precipitation that reaches the ground may be snow, sleet, freezing rain, or rain, depending upon
the temp of the intervening layers of the atmosphere.
1.
Rain
a.
Ice crystals melt in layer of warm air below cloud
2.
Snow
a.
Ice crystals fall from directly from cloud
3.
Sleet
a.
Layer of warm air below cloud melts ice crystals to form rain
b.
Layer of cold air near ground refreezes rain to make sleet
c.
This layer of cold air underneath a warm layer comprises a temperature
inversion.
d.
Hail is very different from sleet. See the lecture on thunderstorms to understand
how hail forms and why it is different from sleet. Hail is more of a summertime
thunderstorm phenomena
4.
Freezing rain
a.
Same as sleet, except warm layer below cloud is thicker and the cold layer is
thinner
b.
Freezes on impact