standard atmosphere 300mb ? 30,000 ft

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standard atmosphere 300mb ? 30,000 ft
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standard atmosphere 500mb ? 18,000 ft
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standard atmosphere 700mb ? 10,000 ft
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standard atmosphere 850mb ? 5,000 ft
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Important Facts -cloud droplets are tiny (20
micrometers). -many condensation nuclei are
present. -tiny particles fall more slowly than
large ones. -a cloud droplets diameter must
grow 200 times to reach a raindrops
diameter. -to attain the volume of a rain
droplet, the cloud droplet increases a million
times in volume.
How Precipitation Forms
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Precipitation Formation Mechanisms

• The Bergeron Process (Cold Clouds)
• The Collision-Coalescence Process (Warm Clouds)

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The Bergeron process describes how rain or snow
forms when the cloud temperature is below
freezing.
This process where ice crystals grow at the
expense of cloud droplets is called the Ice
Crystal Process. It is also named after the
Norwegian researcher who discovered it (Tor
Bergeron, there were others).
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• Three important properties of water droplets
• 1. Cloud droplets do not freeze at 0.C
• Supercooled (water in the liquid state below 0C)
  
• water droplets will freeze immediately if
• agitated sufficiently or when they come in
  contact with
• freezing nuclei (a crystalline structure
  similar to ice)
• 3. The saturation vapor pressure with respect to
  ice is
• lower than the saturation vapor pressure
  with respect
• to liquid water.

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Saturation with Respect to Ice and Water
Liquid Water
Ice

• Vapor pressure is the pressure due to water vapor
  molecules when the
• evaporation rate is equal to the condensation
  rate.
• Because of the crystalline structure of ice,
  water molecules are not able to break
• free from an ice surface as easily than from
  a water surface.
• Therefore, the saturation vapor pressure with
  respect to an ice surface would be
• less than the saturation vapor pressure with
  respect to a liquid water surface at a
• given temperature.

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Growth Process by Bergeron Process
Coexistence of supercooled water droplets and ice
crystals is essential to precipitation process in
the Mid latitudes.
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Terminal Velocity occurs when ?F 0 (when
Fgravity Ffriction )
Gravitational Force Frictional
Force
mass gravity area velocity
?
? 4/3? r3 g ? r2 v
?
4/3 ? g r v
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From Table 5-2
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From Table 5-A
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The Process from Warm Clouds The
Collision-Coalescence Process

• As droplets fall they collide with
• smaller droplets and coalesce.
• after collecting 1 million cloud
• droplets the particle is large enough
• to fall without evaporating.
• Because there are a large number of
• collisions needed, clouds with great
• vertical extent are typically produce
• precipitation by this process.

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Approximate size of types of Precipitation
less than 0.5mm
0.5 5 mm
0.005-0.05 mm
Mist
drizzle
rain/sleet
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Forms of Precipitation
(Rain, Snow, Sleet and Glaze, Hail)
Droplet size determines the type of precipitation.
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Rain
Rain is the term for drops of water that fall
from a cloud and have a diameter of 0.5
millimeter (mm). Drizzle and mist have smaller
droplets. Rain mostly occurs in nimbostratus
clouds and cumulonimbus clouds. These clouds are
capable of producing cloudbursts. Most rain
starts as snow or ice crystals as the snow falls
through the cloud it melts. Drizzle is a fine
uniform water droplet with a diameter less than
0.5 mm.
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Temperature Profile for Rain
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Snow
Snow is precipitation in the form of ice crystals
(snowflakes) or more often, aggregates if ice
crystals. The size and structure of the
crystals is a function of the temperature at
which they form. When air temperatures are cold
the moisture content is very small. This results
in the formation of very light fluffy snow made
up of six sided ice crystals. When conditions
are warmer, the ice crystals join together
into larger clumps consisting interlocked
aggregates of crystals.
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Temperature Profile for Snow
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Sleet and Glaze
Sleet is a wintertime phenomenon that refers to
the fall of small particles of ice that are
clear to translucent.
Sleet forms when rain passes through a cold layer
of air and freezes into ice pellets. This occurs
most often in the winter when warm air is forced
over a layer of cold air.
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Temperature Profile for Sleet and Glaze
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Hail
Hail is precipitation in the form of hard,
rounded pellets or irregular lumps of ice. The
layers of ice accumulate as the hailstone
travels up and down in a strong convective cloud.
Hailstones begin as small ice pellets that grow
by adding supercooled water droplets as they move
through the cloud. As the ice crystal cycles up
and down in the cloud the hailstones increase in
size until they are forced out by a downdraft or
become heavy enough to fall out.
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Hail
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This large hailstone fell from a tornadic
supercell northeast of Breckenridge, TX. The
stones left a path miles wide littered with three
to four and one half inch stones. The hail fell
from a dark cloud base to the south of a
developing mesocyclone. No rain or thunder
occurred in the immediate area while the stones
were coming down.
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Rime
Rime is a deposit of ice crystals formed by the
freezing of super cooled fog or cloud droplets on
objects whose surface temperature is below
freezing. When rime forms on trees, it covers
them with ice feathers in windy conditions only
the windward surfaces will accumulate the layer
of rime.
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Table 5-4
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Precipitation Measurements
Standard Instruments Simple rain gauge,
standard rain gauge, tipping bucket gauge,
weighing gauge
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Radar Summary
Good estimates of rainfall amounts can be obtain
from radar summaries. Radar is less reliable
for frozen precipitation. Coverage still sparse
in many locations.
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Tropical Rain Measuring Mission (TRMM)
Idealized Global Precipitation Representation
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Tropical Rainfall Measuring Mission (TRMM)
TRMM will provide the first spaceborne rain radar
and microwave radiometric data that will measure
the vertical distribution of precipitation over
the tropics in a band between 35' in latitude.
Such information will greatly enhance our
understanding of the interactions between the
sea, air and land masses which produce changes
in global rainfall and climate. TRMM
observations will also help improve modeling of
tropical rainfall processes and their influence
on global circulation leading to
better predictions of rainfall and
its variability at various time scales.
As of late 1997, measurements of the global
distribution of rainfall at the Earth's
surface had uncertainties of the order of 50 and
the global distribution of vertical profiles of
precipitation was far less well determined.
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Intentional Weather Modification
The deliberate human intervention to influence
atmospheric processes that constitute the
weather.
Much of the motivation to attempt weather
modification techniques was based on anecdotal
evidence. Results were never conclusive.
Hail cannons where popular in the late 1800s,
with the belief that injection of smoke particles
into developing clouds could increase the ice
nuclei and reduce the size of hailstones or
suppress hail altogether.
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The Moisture Accelerator!
Lots of people in Southern California knew about
Hatfield, who by then rated folk hero status. He
called himself the Moisture Accelerator, but
others knew him best as The Rainmaker. Since 1902
when his first experiments with chemicals and
"evaporating tanks" dampened his father's ranch
near Oceanside, Hatfield had pleased people in
Los Angeles and farmers in the Central and San
Joaquin valleys by fulfilling hundreds of their
rainmaking contracts.
Charles M. Hatfield (1876-1958)
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Cloud Seeding
1946 Vincent J. Schafer discovered that dry ice
dropped into supercooled clouds spurred the
growth of ice crystals which either induced
precipitation or dispersed fog or clouds.
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Cloud Seeding

• In order for cloud seeding to trigger
  precipitation, conditions must
• be just right.
• Clouds must be present seeding cannot create
  clouds.
• A portion of the clouds must contain
  supercooled water.
• One method assumes that the clouds are lacking in
  freezing nuclei
• and adding them will stimulate precipitation by
  the Bergeron
• process. One must be careful not to overseed as
  this will produce
• too many, too small ice crystals.

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Cloud Seeding
Precipitation is a complicated process. After
fifty years of cloud seeding experimentation and
study the scientific community is still lacking a
full understanding of precipitation processes.
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Frost Prevention
Temperature zonation during an inversion.
Temperature increases with height to the top of
the inversion and then decreases. Frost
protection techniques use the warmer air above
the crop as a heat source.
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