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Collector drops collide with smaller drops. |
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Due to compressed air beneath falling drop,
there is an inverse relationship between collector drop size and collision
efficiency. |
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Collisions typically occur between a collector
and fairly large cloud drops. |
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Smaller drops are pushed aside. |
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Collision is more effective for the droplets
that are not very much smaller than the collect droplet. |
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When collisions occur, drops either bounce apart
or coalesce into one larger drop. |
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Coalescence efficiency is very high indicating that most
collisions result in coalescence. |
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Collision and Coalescence together form the primary mechanism
for precipitation in the tropics, where warm clouds dominate. |
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Snow results from the growth of ice crystals
through deposition, riming, and aggregation. |
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Snowflakes have a wide assortment of shapes and
sizes depending on moisture content and temperature of the air. |
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Snowfall distribution in North America is
related to north-south alignment of mountain ranges and the presence of the
Great Lakes. |
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Lake effect snows develop as the warm lake
waters evaporate into cold air. |
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Rain is associated with warm clouds exclusively
and cool clouds when surface temperatures are above freezing |
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Rainshowers are episodic precipitation events
associated with convective activity and cumulus clouds |
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Drops tend to be large and widely spaced to
begin, then smaller drops become more prolific |
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Raindrop Shape begins as spherical |
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As frictional drag increases, changes to a
mushroom shape |
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Drops eventually flatten |
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Drops split when frictional drag overcomes the
surface tension of water |
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Splitting ensures a maximum drop size of about 5
mm and the continuation of the collision-coalescence process |
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Graupel are ice crystals that undergo extensive
riming |
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Lose six sided shape and smooth out |
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Either falls to the ground or provides a nucleus
for hail |
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Hail forms as concentric layers of ice build
around graupel |
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Formed as graupel is carried aloft in updrafts |
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At high altitudes, water accreting to graupel
freezes, forming a layer |
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Hail falls but is eventually carried aloft again
by an updraft where the process
repeats |
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The ultimate size of the hailstone is determined
by the intensity of the updraft. |
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Great Plains = highest frequency of hail events |
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Sleet begins as ice crystals which melt into
rain through a mid-level inversion before solidifying in colder near
surface air |
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Freezing Rain forms similarly to sleet, however,
the drop does not completely solidify before striking the surface |
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Two primary methods are used to trigger the
precipitation process. |
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Dry ice is used to lower cloud drops to a
freezing point in order to stimulate ice crystal production leading to the
Bergeron process. |
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Silver iodide initiates the Bergeron process by
directly acting as freezing nuclei. |
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Under ideal conditions, seeding may enhance
precipitation by about 10%. |
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Terminal Velocity (drag force = gravity force) |
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The processes to form precipitation |
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Condensation; Collision-Coalescence (in warm
cloud); Bergeron (in cool and cold cloud) & Riming and Aggregation |
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Forms of Precipitation |
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Snow, Rain, Graupel, Hail, Sleet, Freezing Rain |
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Cloud Seeding |
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Dry ice; Silver iodide |
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