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Precipitation
Chapter 7
Precipitation Processes
• Precipitation is any form of water that falls
from a cloud and reaches the ground.
• How do cloud drops grow?
– When air is saturated with respect to a flat
surface it is unsaturated with respect to a curved
droplet of water….the smaller droplet
evaporates more rapidly …Curvature Effect
– At equilibrium, the vapor pressure over a curved
droplet of water is greater than that over a flat
surface
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To keep tiny cloud droplets in equilibrium with surrounding air,
the air must be supersaturated …RH> 100%
The curved line represents the relative humidity needed to
keep a droplet in equilibrium with its environment.
For a given droplet size, the droplet will evaporate and shrink
when the relative humidity is less than that given by the curve.
The droplet will grow by condensation when the relative
humidity is greater than the value on the curve.
• Collision & Coalescence
– Plays a significant role in clouds with
tops warmer than -15C (5 F)
– Droplets of different sizes collide and
coalesce into larger droplets
Collision and coalescence.
(a) In a warm cloud composed only of small cloud droplets of uniform size, the
droplets are less likely to collide as they all fall very slowly at about the same speed.
Those droplets that do collide, frequently do not coalesce because of the strong
surface tension that holds together each tiny droplet.
(b) In a cloud composed of different size droplets, larger droplets fall faster than
smaller droplets. Although some tiny droplets are swept aside, some collect on the
larger droplet’s forward edge, while others (captured in the wake of the larger droplet)
coalesce on the droplet’s backside.
A cloud droplet rising then falling through a warm cumulus cloud can grow by collision
and coalescence, and emerge from the cloud as a large raindrop.
“Warm Cloud” means all levels are above freezing
Most important factor in production of raindrops is cloud’s liquid water content.
Other factors:
• range of droplet size
• cloud thickness
• updrafts
• electrical charges
 Ice-Crystal (Bergeron) Process
 Wegener-Bergeron-Findeisen process
○ Occurs in cold clouds - mixture of ice & water
○ Ice crystals grow at expense of surrounding water droplets
○ Saturation vapor pressure greater over water as compared to ice.
In a saturated environment, the water droplet and the ice crystal are in equilibrium, as
the number of molecules leaving the surface of each droplet and ice crystal equals the
number returning. The greater number of vapor molecules above the liquid indicates,
however, that the saturation vapor pressure over water is greater than it is over ice.
The difference in saturation vapor pressure between supercooled water and ice at
different temperatures.
The ice-crystal
(Bergeron) process.
(1) The greater number of
water vapor molecules
around the liquid droplet
causes water molecules
to diffuse from the liquid
droplet toward the ice
crystal.
(2) The ice crystal absorbs
the water vapor and grows
larger, while
(3) the water droplet grows
smaller.
Precipitation Processes
• Cloud Seeding began in 1940’s
– Inject cloud with small particles that act as
condensation nuclei, starting the precipitation
process.
– NEED CLOUDS: seeding does not generate clouds
– Cold clouds with a low seed ration best
– Dry ice, silver iodide
Natural Seeding
Ice crystals falling from a dense cirriform cloud into a lower nimbostratus cloud. This
photo was taken at an altitude near 6 km (19,700 ft) above western Pennsylvania. At
the surface, moderate rain was falling over the region.
Natural seeding by cirrus clouds may form bands of precipitation
downwind of a mountain chain.
Precipitation in Clouds
• Starts quickly
• Most precipitation formed through accretion
• Many times rain starts out as ice
How ice crystals grow and produce precipitation in clouds with a low liquid-water content
and a high liquid-water content.
Precipitation Types
• Rain: 6mm max falling drop of liquid water
– Drizzle less than 0.5 mm
– Virga – evaporates
before reaching ground
Which of the three drops drawn here represents the real shape of a falling raindrop?
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Snow
 Most precipitation starts as snow
 Freezing level, snow & cloud appearance, fall
streaks, drifting snow, blizzard
 A blanket of snow is a good insulator
Snow scatters sunlight more effectively than rain. Consequently, when you look toward
the sun, the region of falling precipitation looks darker above the melting level than
below it.
The dangling white streamers of ice crystals beneath these cirrus clouds are known as
fallstreaks. The bending of the streaks is due to the changing wind speed with height.
Figure 7.17
Common ice crystal forms (habits).
• Sleet: air below freezing, then travels
through a layer of air above freezing,
begins to melt and then falls through a
layer of air below freezing just above the
ground surface.
• Freezing Rain: ground surface is freezing
as rain hits the surface it freezes.
An accumulation of rime ice forms on tree branches as supercooled fog droplets
freeze on contact in the below-freezing air.
A heavy coating of freezing rain (glaze) covers Syracuse, New York, during January,
1998, causing tree limbs to break and power lines to sag.
Average annual number of days with freezing rain and freezing drizzle over the United
States. (NOAA)
Vertical temperature profiles (solid red line) associated with different forms of
precipitation.
• Snow Grains: solid equivalent of drizzle,
no bounce or shatter
• Snow Pellets: larger than grains, bounce,
break, crunch underfoot
• Graupel: ice particle accumulation with
rime
• Hail: graupel acts as embryo in intense
thunderstorm, grow through aggregation
as pushed back up by updraft.
The formation of snow pellets. In the cold air of a convective cloud, with a high liquidwater content, ice particles collide with supercooled cloud droplets, freezing them into
clumps of icy matter called graupel. Upon reaching the relatively cold surface, the
graupel is classified as snow pellets.
A snowflake becoming a rimed snowflake, then finally graupel (a snow pellet).
The accumulation of small hail after a thunderstorm.
The hail formed as supercooled cloud droplets collected on ice
particles called graupel inside a cumulonimbus cloud.
This whopping hailstone fell on Vivian, South Dakota, on July 23, 2010. It had a record
diameter of 8 inches, weighed a record 1.94 pounds and had a circumference of 18.6
inches.
Stepped Art
Measuring Precipitation
• Instruments
– Rain gauge: standard, tipping bucket,
weighing
• Snow: average depth at 3 locations, 10:1 water
equivalent
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Radar ( Radio Detection and Ranging)
 Transmitter generates energy toward target,
returned energy measured and displayed
 1 cm wavelength – cloud droplets
 3-10cm wavelengths - larger - Precipitation
○ Brightness of echo = amount/intensity of rain
Stepped Art
Doppler Radar
• 1990’s replaced conventional
• Has capacity to measure speed of rain
moving toward and away from antenna
• “ see” rotation and tornado generating
storms
(a) Doppler radar display showing precipitation intensity over North Carolina for August
27, 2011, as Hurricane Irene moves onshore.
(b) Doppler radar display showing 1-hour rainfall estimates over North Carolina for
August 27, 2011. Notice that in some places Doppler radar estimated that more than
1.50 inches of rain had fallen in one hour. (NOAA/NWS
Doppler radar display showing 1-hour rainfall amounts over Oklahoma for April 24,
1999.
Measuring Precipitation
• Measuring from space
– Specific satellites designed to assess clouds,
atmospheric moisture, and rain
○ TRMM Tropical Rainfall Measuring Mission:
– Precipitation Radar
– Microwave Imager
– Visible and Infrared Scanner
○ CloudSat – 2006 , 700km orbit
– Cloud Profiling Radar – vertical profiling
A satellite and radar image of Hurricane Humberto obtained by the TRMM satellite on
September 13, 2007. Precipitation rates (lowest in blue, highest in dark red) were
obtained by the satellite’s Precipitation Radar and Microwave Imager. The rainfall
estimates are overlain on the infrared image of the storm.
The National Weather Services across the United States are upgrading to better radar
technology that can provide meteorologists better radar data that can improve warnings
and lead time.
The upgrade, called Dual Polarization (aka dual-pol), will be installed in all 160
National Weather Service offices. The standard Doppler radar transmits and receives
information horizontally. With the dual-pol upgrade, meteorologists will be able to
transmit and receive information in both horizontally and vertically in the atmosphere.
These upgrades will improve the accuracy and identification of precipitation types,
precipitation estimates, and even be able to view tornadoes better. These upgrades will
continue across the United States and all offices should have the dual polarization
technology installed by May 2013.