Precipitation I
RECAP
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• Condensation and evaporation in the air (dew point).
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Moisture in the air (different types of humidity).
Stability of the atmosphere: determines the type of
clouds
•
Cloud formation: moist air rises up in the atmosphere
where it expands adiabatically, cools down, saturates
and forms clouds.
Chapter 7: precipitation. Any form of water (liquid or
solid) that falls from a cloud and reaches the ground.
From droplets to raindrops
• Typical
sizes (diameter)
♦ Condensation nuclei: 0.2 mm
♦ Cloud droplet: 20 mm
♦ Raindrop: 2000 mm
• The
cloud droplets need to grow
in order to become raindrops!
• Growth
is determined by the
balance of condensation (C) and
evaporation (E)
♦ C>E the droplet grows
♦ C<E the droplet gets smaller
♦ C=E the droplet stays the same
(in equilibrium), hence:
• Saturation
(equilibrium) vapor
pressure
The growth of cloud droplets
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• molecules to leave the surface of the liquid water.
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Curvature effect: The saturation water vapor pressure
depends on the curvature of the water surface.
The larger the curvature the easier it is for the water
The saturation vapor pressure for small droplets is higher
therefore they require more vapor to keep their size
Bottom line: the smaller the droplet, the more difficult it
is to grow. Small droplets don’t make it as raindrops.
The growth of cloud droplets
• Ifsupersaturation
small droplets were to survive and grow, they require
of the air and the help of condensation nuclei
• Condensation nuclei: jump start the formation of the cloud droplets.
♦ Hygroscopic nuclei: condensation begins at RH<100%.
♦ Decrease the starting curvature of the initial droplet.
♦ Solute effect: salt particles partially dissolve in the water and
decrease the saturation vapor pressure.
From droplets to raindrops
• produce raindrops, only cloud droplets (~20 mm).
•
• evaporate and don’t make it to the ground
•
Condensation by itself is a very slow process, cannot
The cloud droplets may stay suspended in the air by
air currents for a long time
Those which happen to descend below the cloud,
There must be other ways to grow raindrops:
♦ Collision-coalescence process: first must discuss
how droplets fall.
♦ Ice-crystal (Bergeron) process
Terminal Velocity
•
• Air-drag force increases with
•
•
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Gravity makes things fall,
constant force -> acceleration
the speed of the falling object
Eventually gravity and air-drag
become equal -> no net force ->
the body falls at constant speed.
This velocity is called terminal
velocity.
The terminal velocity depends on
♦ Shape of the body;
♦ Size of the body;
♦ Mass of the body;
♦ Air properties (density).
Terminal Velocity-Examples
Fdrag
Falling object
Mass
Area
Terminal velocity
Skydiver
Baseball (3.66 cm)
Golf ball (2 cm)
Hail stone (0.5 cm radius)
Raindrop (0.2 cm radius)
75 kg
145 gm
46 gm
.48 gm
.034 gm
0.7 m2
42 cm2
14 cm2
.79 cm2
.13 cm2
60 m/s
33 m/s
32 m/s
14 m/s
9 m/s
134 mi/hr
74 mi/hr
72 mi/hr
31 mi/hr
20 mi/hr
1
C air A 2  mg
2
1
 C air A 2
2
G  Fdrag
G  mg
2mg

C air A
m – mass, g – gravity acceleration, C-shape coefficient of proportionality, -air density, A
- area of the cross section of the body, v- velocity
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•
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Collision and Coalescence Processes
Larger drops fall faster,
overtake and absorb
smaller drops:
♦ coalescence
Larger drops grow faster
than smaller drops.
Larger drops are the
first to hit the ground,
the first raindrops are
very large and heavy.
•
•
Smaller drops evaporate before they reach the ground.
Smaller drops are absorbed on the way down
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Droplet growth in warm clouds
Warm clouds: above
freezing temperature
Thick clouds are associated
with strong updraft winds.
This maximizes the time
the droplet spends in the
cloud -> more time for
coalescence to take place
♦ Thick clouds produce
large rain drops.
♦ Thin clouds produce at
most a drizzle.
“Cold” clouds
• The
temperature of a “cold”
cloud drops below the water
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•
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freezing point.
Below 0 deg C the cloud water
droplets are supercooled.
The smaller the droplet, the
lower the temperature at
which it will freeze.
Below -40 deg C almost all
droplets freeze and form ice
crystals.
Small particles in the air
serve as ice nuclei:
♦ deposition nuclei
♦ freezing nuclei
♦ contact nuclei
Saturation Vapor
Pressure
saturated
• The saturation vapor
•
pressure Ps, depends on
the temperature. It
increases with
temperature.
Ps over water is larger
than it is over an ice
surface at the same
temperature.
Supersaturated
unsaturated
Ice Crystal Particles
• There
are many more condensation nuclei than ice nuclei -> there are
many more water droplets than ice crystals in the cloud
•
•
How does snow form?
The saturation vapor
pressure above a water
surface is larger than the
saturation vapor pressure
above an ice surface.
• Water molecules
evaporate more easily
than ice molecules
Ice-crystal (Bergeron) process
• Water vapor molecules
migrate towards the
ice crystals.
• Cloud ice crystals grow
at the expense of the
water droplets.
Ice crystals in the clouds
Accretion
Fracture
Aggregation
Skip the sections “Cloud seeding and precipitation” and “Precipitation in clouds”