Chapter 7
Precipitation
Precipitation Processes
An ordinary cloud droplet is extremely small (~20 micrometers)
• 100 times smaller than an average raindrop
• if in equilibrium… the size of the droplet does not change
- # of molecules condensing onto the droplet are exactly
equal to # evaporating
- saturation vapor pressure = equilibrium vapor pressure
Growth of Cloud Drops
Atmospheric
vertical winds
and eddies can
keep small and
light cloud
droplets and
condensation
nuclei aloft.
As the cloud
droplets knock
and join together,
they grow larger
and their weight
increases, causing
them to fall as
precipitation.
To keep a droplet in equilibrium….
• more water vapor molecules are needed around it to
replace those molecules that are constantly evaporating
from the surface
• smaller droplets have greater curvature and a more rapid
rate of evaporation
• this process is called the curvature effect
• smaller droplet require an even greater vapor pressure to
keep them from evaporating away
• therefore, when air is saturated with respect to a flat
surface, it is unsaturated with respect to a curved droplet of
water, which leads to the evaporation of the droplet
• to keep tiny cloud droplets in equilibrium with the
surrounding air, the air must be supersaturated
Vapor Pressure & Saturation
Smaller drops have
greater curvature and
require greater vapor
pressure to keep water
molecules from
evaporating away.
As the drop size
increases, the required
relative humidity
(RH) for equilibrium
decreases.
If the required RH is
exceeded, the drop
will grow.
How do tiny droplets of less than 1 micrometer grow to the size
of an average cloud droplet?
• hygroscopic particles allow for condensation to occur at
relative humidity's less than 100 percent
Example
- when condensation begins on a salt particle, it dissolves
forming a solution
-the salt ions bind closely to the water molecules making
evaporation difficult
- this condition reduces the equilibrium vapor pressure
- this is known as the solute effect
• A droplet containing a CCN (such as salt) can be in
equilibrium with its environment at RH < 100%
Condensation and precipitation
• most clouds cannot produce precipitation (through
condensation)
• this process is too slow to produce precipitation
• it would take several days for this process to produce rain
under ideal conditions
• observations show that clouds can develop and produce
precipitation in under an hour
• it takes about 1 million average size cloud droplets (20
micrometers) to produce one average size rain drop
• must be other processes at work
Collision and coalescence process
• Occurs in clouds with tops warmer than -15ºC
• some cloud droplets must be larger than others in order for
collisions to form a raindrop
• larger drops may form on CCN or through random collision
of droplets
• as cloud droplets fall air slows the drops descent
• the amount of air resistance depends on the size of the drop
and the rate of the fall
• the speed of the falling drop increases until the air resistance
equals the pull of gravity
• at that point the drop reaches its terminal velocity
Collision & Coalescence Process
Both a) collisions
that join together
small cloud droplets
b) coalescence that
attaches faster and
larger droplets with
smaller slower
droplets work
together to
assemble nearly 1
million cloud
droplets into a
raindrop large
enough to fall to
earth.
Collision & Coalescence Process
Both a) collisions
that join together
small cloud droplets
b) coalescence that
attaches faster and
larger droplets with
smaller slower
droplets work
together to
assemble nearly 1
million cloud
droplets into a
raindrop large
enough to fall to
earth.
Warm Clouds
• clouds that have above freezing temperatures at all levels
• precip forms by collision coalescense
• Example: Next Slide
• Rain that falls is called warm rain
• most important factor in prduction of raindrop is the clouds
liquid water content
• Other significant factors:
-range of droplets sizes
-cloud thickness
-updrafts of the cloud
-electric charge of the droplets and electric field of cloud
Warm Cloud Processes
Collision and
coalescence
operates in
warm clouds (>
15° C) to
produce rain,
and is affected
by the clouds
liquid water
content, droplet
sizes, cloud
thickness,
updrafts, and
drop electrical
charges.
Figure 8.5
Ice-crystal Process
• called the Bergeron process
• extremely important in mid and high latitudes
• air below freezing
• cloud are called cold clouds
• Example: next slide
• ice crystals form on ice nuclei
• number of ice nuclei available in the atmosphere is small
• deposition nuclei - allows water vapor to deposit as ice directly
onto their surfaces
• freezing nuclei - promote the freezing of supercooled liquid
water
• contact nuclei - cause freezing after being immersed in liquid
Ice Crystal Process
Cold clouds may drop
below –40° C before
small droplets freeze into
ice embryos that can
serve as condensation
nuclei.
At very low
temperatures, vapor can
also condense as ice onto
nuclei formed by:
a) deposition
b) freezing
c) contact,
primarily in the glaciated
region of the cloud.
Molecules from Water to Ice
Ice crystals have lower saturation
vapor pressures than liquid droplets,
creating a gradient of high to low
water molecules from liquid to ice
that encourages ice growth.
This growth is critical to the icecrystal precipitation process.
Ice Particle Changes
As ice crystals fall and collide with super
cooled drops, they get bigger by accretion.
Falling icy matter is called graupel, and
aggregation describes the joining of two
ice crystals into snowflakes.
Ice Crystal Growth
Ice crystal growth
is the dominant
cause of
precipitation in
nimobstratus and
cumulonimbus
clouds, both of
which have lower
liquid content than
warm-layered
clouds such as
stratus.
Cloud Seeding
Artificial seeding, such as Silver Iodide, and natural seeding, such as
cirriform ice crystals, are available to increase the number of
condensation nuclei and encourage precipitation.
Forms of precipitation
• Rain
-drizzle (diameters smaller than 0.5mm)
-virga (rain evaporating prior to reaching the surface)
• Snow
-flurry (light snow from cumulus clouds)
-squall (intense snow shower)
-blizzard (heavy snow and winds > 30knots)
• Sleet (snow that partially melts and freezes again into ice)
• Freezing Rain (supercooled liquid water that reaches the
ground
• Hail
Evaporating Rain
Rain falling
into low
humidity air
below will
cause the
drops to
decrease in
size, possibly
evaporating
into streaks of
dry air as in
this virga.
Snowflakes & Snowfall
Snowflakes are crystalline structures that can have plate,
column, dendrite, or needle forms.
Air temperature and humidity determine crystal form, and
dendrite is the most common habit.
Sleet & Freezing Rain
Environmental
temperatures may
reveal a warm zone
between two
freezing layers.
Snow falling into
the warm zone will
melt and either
a) fall as rain and
refreeze on contact
with the ground, or
b) refreeze and fall
as sleet.
Sleet & Freezing Rain
Four vertical temperature profiles are shown to illustrate the phase change
that a snowflake may experience in its path toward earth's surface.
Rime & Freezing Rain
Rime, a granular ice,
accumulates when super cooled
fog droplets touch a frozen
surface.
Freezing rain creates incredible
strain on branches and other
structures, resulting in costly
damages.
Snow Grains & Snow Pellets
Flat and long snow grains fall as frozen drizzle, too small to bounce
or shatter.
Snow pellets are larger, and have a rounded layer of rimed ice that
creates air bubbles and a bounce.
Hailstones & Damage
Updrafts in a towering cumulus cloud recirculate graupel
through an accretion and freezing process that produces large
heavy hailstones.
Hailstreaks
Such storms cause regular property damage, but only 2 U.S.
deaths in the 20th Century.
Hailstreaks
Coffeyville Hailstone
Regular and polarized light images of the 14 cm diameter
hailstone that fell in Kansas in 1970.
Hailstorms cause severe damage to crops and other structures.
Farmers name for it?
Aurora Hailstone
Previous hailstone record holder in the U.S.
17.8 cm in diameter
June, 2003
Vivian Hailstone
Largest hailstone on record in the U.S.
20.32 cm in diameter, 2 lbs in weight
July, 2010