Precipitation
Cloud particles are so small that they
are easily kept afloat by even the
weakest updrafts.
This is why most clouds do not
produce rain.
For precipitation to occur, cloud particles must grow in size!
Cloud Constituents
• Condensation nuceli, r=0.1µm
.
• Typical Cloud Droplet, r=10µm
• Large Cloud Droplet, r=50µm
• Typical Rain Droplets, r=1000µm (1mm)
Rain drops large enough to fall must grow 100
times greater than typical Cloud droplets.
How do rain drops grow?
• Growth by condensation alone does not produce
droplets large enough to fall as rain.
• Collision and Coalescence
– Warm clouds (up to 30o N and S of the equator)
– Collector droplet over takes smaller droplets. They
collide and join together.
• Bergeron Process
– Saturation vapor pressure for ice is less than that for
supersaturated liquids, at the same temperature.
• Rimming and Aggregation
– Aides the Bergeron Process
Bergeron Process
1. Ice crystals and supercooled liquids co-exist in
clouds.
2. The vapor pressure of the air is equal to the
saturation vapor pressure. The air is saturated
with respect to the liquid droplets (balance
between evaporation and condensation)
3. For ice crystals the vapor pressure exceeds the
saturation vapor pressure and ice crystals begin
to grow.
4. As the ice crystals grow water vapor is depleted
in the air and the air becomes undersaturated for
water droplets. The liquid water evaporates.
5. Ice crystals continue to grow at the expensive of
liquid water.
How do ice crystals grow larger?
• Rimming and Aggregation
– Rimming - ice crystals act as nuclei and may collide
with supercooled liquids in clouds and the liquid may
freeze onto them.
– Aggregation - two or more ice crystals may collide and
stick together.
Forms of Precipitation
• 1) Snow - frozen, crystalline precipitation that
forms and remains in the ice stage as it descends
• Results from the growth of ice crystals through the Bergeron
process and rimming and aggregation.
• Occurs as six-sided crystals of a variety of forms
– Depends on temperature, cooling rate and moisture content.
• Sizes of crystals are 50 µm to 5 mm
• Snow pack - warmer moister
• Powder - colder, drier
Snow Crystals
Snow Crystals
Snow Crystals
Snow Crystals
Snow Crystals
Lake Effect
•
Snowfall is enhanced downwind of the great lakes
or any large body of water.
1.
Lakes or other bodies of water remain relatively warm into
earlier winter. (water has a higher heat capacity)
As cold air from the north flows over lakes, heat and water
vapor are transferred upward and the air becomes moist and
unstable.
As the air passes over the shore, the effects of topography
and vegetation slow the wind. The decrease in wind speed
results in (speed) convergence, which is a mechanism for
uplift.
2.
3.
Therefore passage of cold air over a lake provides three
mechanisms for cloud formation and precipitation.
1) Uplift
2) Unstable air
3) Moisture
Forms of Precipitation
• 2) rain - precipitation arriving at the surface in the
form of liquid droplets, usually between 5µm and
5mm in size.
• In the mid-latitudes( 30 - 60o) most precipitation is initiated
through the Bergeron process.
• 3) graupel - ice crystals that grow by rimming
sometimes have a porous with bubbles and a
spongey texture and milky appearance. The
original snow crystals are no longer identifiable
• 4) hail - layered ice crystals.
• 5) Sleet - precipitation in the form of ice pellets,
resulting when raindrops freeze before reaching
the surface.
• Requires
– Temperature inversion
– Usually a warm front
– Precipitation must fall through a thick section
of cold air.
• 6) Freezing rain - rain droplets that freeze as
they hit the surface forming a layer of ice on
the surface..
Measuring Precipitation
• Raingauge - instrument used to measure
amount of rainfall
– Collecting surface with 8 inch diameter.
– Precipitation funnels into a tube with 1/10 the
diameter of the collecting surface.
– The 10X amplification increases the precision
of the measurement.
Measuring Precipitation
• Tipping bucket - instrument used to measure
amount of rainfall
– Water is funneled (like a raingage) into one of two
pivoting buckets.
– One bucket is always upright and the other tipped
– Whent the bucket collects rain to a certain depth, it tips
over and the other bucket is brought upright.
– The tipped bucket triggers an electric current to a
computer and the time and amount of rain of recorded.
– Gives a measure of both amount and intensity of
rainfall.
Measuring Precipitation
• Weighing Bucket - similar to tipping bucket but
the weight of the water is converted to a depth of
rainfall.
Measuring Precipitation
•
Problems with raingages
1. Point measurements - they represent rainfall at a given
point.
2. Splash - water may splash off of the collecting surface.
3. Wind may deflect precipitation.
4. Water may bounce from the ground into the collecting
surface.
5. Measurements at sea.
Surface is not level
Ocean water splash
Measuring Precipitation
• Weather Radar - intensity of precipitation
is measured by emitting microwave
radiation with l = several centimeters.
–
Precipitation scatters the emitted radiation back to a
receiver which records the intensity of backscattered
radiation.
Measuring Precipitation
• Snow Measurement
–
Raingages are not reliable for snowfall measurement.
–
Depth of accumulated snow is measured directly and
converted to water equivalent (10:1)
–
Snow pillow - large mattress filled with anti-freeze
and connected to a pressure gauge.
Cloud Seeding
•
Injection of materials into non-precipitating
clouds
1.
Dry Ice (frozen CO2) - dry ice lowers the temperature enough
to initiate homogeneous nucleation of ice.
2.
Silver iodide - acts as nuclei to initiate the Bergeron process.
1.
Problems
1. Not cost effective
2. Drains moisture from the air which needs to be replenished,
affecting areas downwind
3. Poor results
Cloud Coverage
Term
Overcast
Broken
Scattered
Clear
Coverage (%)
>90
60 - 90
10-60
<10