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How precipitation forms
Frictional force (Stokes’ drag force)
 : viscosity of the air
6 rV
Frictional force = Gravitational force
Terminal velocity
V  c r 2 ; c  1.27  10 4 m 1s 1
Gravitational force
4
3
 r3L
1 1
V  c r ; c  1.27  10 m s
2
4
Condensation
nucleus
r  2  10-7 m; V  5  10-10 ms -1 ;
Typical cloud
droplet
r  2  10-5 m; V  5  10-6 ms -1 ;
Large cloud
droplet
r  5  10-5 m; V  3.2  10-5 ms -1 ;
Rain
droplet
D  100m; t  VD  2  1011 s  5.6  107 h
D  100m; t  VD  2  107 s  5.6  103 h
D  100m; t  VD  3.1 106 s  8.8  102 h
r  2  10-3 m; V  5  10-2 ms -1 ;
D  100m; t  VD  2  103 s  0.55h
Bergeron Process
When vapor condenses into water, the molecules spend part of their time as
members of clusters which are continually forming and breaking up. To make
the final transition from a liquid to the lattice structure of ice, however, some
kind of foreign particle--an ice nucleus--is needed to initiate freezing. Until this
nucleus forms, liquid water can exist far below the freezing point. Such water
is called supercooled. In fact, pure water droplets can remain in liquid form
down to temperatures near -40F.
Ice crystal: molecules more
organized, difficult to escape
Isupercooled water droplet: molecules
less organized, easy to escape
The saturated vapor pressure above ice crystals is some what
lower than above supercooled water droplets. Es < E
Es  e  E
condensate
evaporate
Mixed phase clouds
1. Supercooled water droplets are readily to freeze if they impact an object.
2. Enlarged crystals are easy to break up into fragments serving as freezing nuclei.
Precipitation from warm clouds
Collision-Coalescence Process
a.Collision
-Larger drops fall faster than smaller drops,
so as the drops fall, the larger drops
overtake the smaller drops to form larger
drops until rain drops are formed.
-Cloud droplets collide and are either:
1.Collected on the larger drop's forward
edge as it falls.
2.Captured in the wake of a larger drop
that is falling.
-In a cloud with cloud droplets that are tiny
and uniform in size:
-The droplets fall at a similar speed and do
not Collide.
-The droplets have a strong surface
tension and never combine even if they
collide.
b.Coalescence
-The merging or "sticking together" of cloud droplets as they collide.
Droplets may:
1.Stick together/Merge
2.Bounce off one another
3. Airstream repelling
4. Coalescence is enhanced when droplets
have opposite electrical charges.
The collision-coalescence mechanism is more efficient
in environments where large cloud droplets are plentiful.
Where?
tropics
The Bergeron process is more efficient for mixed clouds with
plentiful ice crystals and supercooled water droplets.
Where?
Mid-latitudes
Forms of precipitation
Hail
Heaviest hailstone fell in
Coffeeville, Kansas on
September3, 1970 and
weighed 1.67 lbs.
Some hails clear show
ring structure.
Inside of a thunderstorm are strong updrafts
of warm air and downdrafts of cold air.
If a water droplet will freeze if it is picked up
by the updrafts well above the freezing level.
As the frozen droplet begins to fall...carried
by cold downdrafts...it may thaw as it moves
into warmer air.
The half-frozen droplet may get picked up
again by another updraft...carrying it back
into very cold air and re-freezing it.
Finally...the frozen water droplet...with many
layers of ice (like rings)...falls to the ground.
Precipitation measurement
Rain gauge
Measuring snowfall: depth and water equivalent
Measurement error
Weather radar
Rain gauge
with
metal slats
to
minimize
“under-catch”
in
windy
conditions
Weather modification
The first employs energy to forcefully alter the weather
(e.g., disperse fog at airports).
The second involves modifying land and water surface to change their natural
interaction with the lower atmosphere (e.g., greenhouse)
The third involves triggering, intensifying, or redirecting atmospheric process
(e.g., cloud seeding)