CHAPTER 7. Precipitation Processes
Chapter Overview:
The chapter augments concepts of precipitation development. Various types of precipitation are
discussed along with atmospheric conditions that produce them. The chapter concludes with a
discussion of precipitation measuring devices.
Chapter at a Glance:
Not all clouds precipitate due to the small size and slow fall rates of average cloud drops. Rapid
cloud drop growth rates are required for precipitation to form.
• Growth of Cloud Droplets - Gravity and frictional drag with atmospheric gases balance
to achieve terminal velocity for any falling object. Because of the small size of cloud
drops, terminal velocities do not exceed even weak updrafts. When drops achieve
approximately one million times the volume of the average cloud drop, the drops will have
sufficient terminal velocities to overcome updrafts.
A. Growth by Condensation - Condensation about condensation nuclei initially
forms most cloud drops. Condensation is only a valid form of growth until the drop
achieves radii of about 20 µm due to the overall low amounts of water vapor
available. Condensation alone is insufficient to generate precipitation.
B. Growth in Warm Clouds - Clouds with temperatures above freezing dominate
the tropics and the mid-latitudes during warm months. In these warm clouds,
collision-coalescence is the process that generates precipitation. Due to high
terminal velocities very large collector drops begin the process.
1. Collision - Collector drops collide with smaller drops and collisions
occur. Due to the amount of compressed air below the collector drop, there
is an inverse relationship between collector drop size and collision
efficiency. Because of this, collisions typically occur between a collector
drop and fairly large cloud drops instead of very small ones, which get
pushed aside.
2. Coalescence - When collisions occur, drops either bounce apart or
coalesce into one larger drop. Coalescence efficiency is very high
indicating that most collisions result in coalescence. <Web>
C. Growth in Cool and Cold Clouds - Cool month mid-latitude and high latitude
clouds are typically classified as cool clouds as average temperatures are well
below freezing. However, these clouds can be composed of liquid water,
supercooled water, and/or ice with the later two dominating when temperatures are
between -4oC (25oF) and -40oC (F). The coexistence of ice and supercooled water
is critical to the creation of cool cloud precipitation, the so-called Bergeron process.
Saturation vapor pressure of ice is less than that of supercooled water and water
vapor such that during coexistence, water will sublimate directly onto ice. Ice
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crystals then grow rapidly at the expense of supercooled drops. Collisions between
falling ice crystals and water drops cause further growth through riming and
aggregation.
1. Riming and Aggregation - Riming, or liquid water freezing onto ice
crystals, ensures rapid ice crystal growth. In some cases, aggregation, or
the joining of multiple ice crystals through the bonding of surface water,
rapidly builds ice crystals to the point that they overcome updrafts.
Collision combined with riming and aggregation allows the formation of
precipitation sized drops within ½ hour of initial formation.
• Forms of Precipitation - Warm clouds produce only rain but cool clouds may produce a
wide variety of precipitation types dependent upon ambient conditions.
A. Snow - Snow results from the Bergeron process, riming, and aggregation.
<Web> Snowflakes may have a wide assortment of shapes and sizes depending on
moisture content and temperature of the air. Snowfall distribution in North
America is highly related to north-south aligned mountain ranges and the presence
of the Great Lakes. Lake effect snows develop in lee of the Lakes as the warm lake
surfaces decrease low-level stability and add moisture to the cooler air advecting
over. Topographic features aid in the development of precipitation downwind. At
extremely low temperatures it is still possible for snow to form, however, it is
unlikely due to the low moisture content of the air.
B. Rain - Warm clouds produce rain exclusively throughout the tropics. Cool
clouds may produce rain when surface temperatures are above freezing. <ME7.1>
1. Rainshowers - Showers are episodic precipitation events usually
associated with convective activity and cumulus clouds. Due to various
terminal velocities, drops tend to be large and widely spaced to begin, and
then smaller drops become more prolific.
2. Raindrop Shape - Raindrops begin with a spherical shape, then as
frictional drag increases, their shape changes to that of a mushroom.
Eventually, as drag increases, the drops flatten. As drag overcomes the
surface tension of water, the drops split. Splitting ensures not only a
maximum drop size of about 5 mm, but also a continuation of the
collision-coalescence process.
C. Graupel and Hail - Ice crystals that undergo extensive riming lose their
original six-sided shape, smooth out, and fall to the ground as graupel. Graupel
may either fall to the ground or provide a nucleus for hail formation. Concentric
layers of ice indicate the formation conditions of hail. Hail forms originally as
graupel that is carried aloft in an updraft. At very high altitudes, water accreting to
the graupel freezes, forming a layer. The process is repeated until the hailstone fall
rate exceeds updraft strength and the hail falls to the surface. Hailstones are very
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heavy as the process ensures a composition high in water and low in air. They are,
therefore, capable of tremendous amounts of damage, especially in the Great Plains
which experiences a high frequency of hail events. <Web> <ME7.2; 7.3>
D. Sleet - Sleet begins first as ice crystals, which melt into rain through a mid-level
inversion. The rain then passes through colder air near the surface which freezes it
into a frozen raindrop.
E. Freezing Rain - Freezing rain forms through the exact same process as sleet
except that the height of the inversion is lower in altitude. Because the layer of
sub-freezing temperatures near the surface is shallow, sleet cannot form. Instead,
freezing of the water drop occurs upon contact with the surface. Freezing rain
produces very dangerous surface conditions as the surface becomes coated in ice.
<Web>
• Measuring Precipitation - Precipitation is measured at many locations using various
methods to estimate the amounts of different types.
A. Raingages - Standard raingages, with a 20.3 cm (8") collecting surface and 1/10
area collector, are used to measure liquid precipitation. Depth of water level in the
gage conveys a tenfold increase in total precipitation. Automated devices such as
tipping-bucket and weighing-bucket raingages provide both a record of
precipitation amount and the time of the event. Though widespread on continents,
raingages are scarce over the oceans, 70% of the Earth’s surface.
1. Raingage Measurement Errors - Foremost of the many errors
associated with raingages is the fact that they are point estimates. Wide
variation across small spatial scales leads to measurement inaccuracies.
Problems of under recording also occur as a result of evaporation of water
from the gage and deflection of precipitation from the collecting surface by
wind. Overestimation stems from wind related blowing, failing to
completely empty the gage after measurement, and placing the gage on
non-level surfaces.
2. Precipitation Measurement by Weather Radar - Due to recent
technological advancements, radar can now be used to estimate
precipitation amounts. As the information is in real-time, such estimates
are useful in short-term forecasting applications.
B. Snow Measurement - Raingages are inadequate at measuring frozen
precipitation. Therefore, measurements of accumulated snow are used and a water
equivalent of the snow, a 10-to-1 ratio assumed. Automated snow pillows now
replace more antiquated measuring techniques at many locations. <Web> The
snow pillows detect snow weight and convert this directly into water equivalent.
• Cloud Seeding - Two methods are used to either further stimulate the precipitation
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process or to trigger the precipitation process in non-precipitating clouds. Dry ice is used to
lower cloud drops to a freezing point in order to stimulate ice crystal production leading to
the Bergeron process. Silver iodide initiates the Bergeron process by directly acting as
freezing nuclei. Under ideal conditions, a rarity, precipitation enhancement may be up to
10% greater than without seeding. However, it is thought that seeding contributes little.
Legalities with downwind locations also present additional seeding concerns.
Chapter Boxes:
7-1 Physical Principles: Why Cloud Droplets Don’t Fall - Frictional drag with
atmospheric gases, relative to an objects mass, slows falling objects. The downward force
of gravity is directly opposed by frictional drag such that a falling object accelerates only
until its speed balances with drag. This equals the objects terminal velocity and is related
to the objects mass, as gravity is proportional to mass. Drag is also dependent on the mass
of the object and the fall rate. Therefore, small cloud drops are easily suspended in air as
they have relatively small terminal velocities. By contrast, larger, heavier drops may
exceed even strong updrafts.
7-2 Physical Principles: The Effect of Hail Size on Damage - Hail damage increases
rapidly with increasing hailstone size as the amount of kinetic energy in a falling hailstone
increases to the fourth power of its radius. <Web>
Related Web Sites:
Snow: www-nsidc.colorado.edu
Snow Pillow: www.csac.org/Professional/weather/snotel.html
Collision-Coalescence; Freezing Rain; Graupel:
http://thor.creighton.edu/Summaries/chap5/tsld016.htm
Hail: www.chaseday.com/hail.htm
Radar: http://weather.noaa.gov/radar/mosaic/DS.p19r0/ar.us.conus.shtml
http://www.intellicast.com
Rainfall Maps: http://lwf.ncdc.noaa.gov/oa/climate/severeweather/rainfall.html
Snow Cover: http://www.weather.unisys.com/surface/snow_cover.html
Media Enrichment:
ME7.1 - Movie of global precipitation climatology.
ME7.2 - Large hailstone image.
ME7.3 - Image of hailstones.
Key Terms:
drag
shower
cool cloud
collector drop
cloud seeding
coalescence
warm cloud
hail
aggregation
snow pillow
rain
raingage
terminal velocity
cold cloud
graupel
Bergeron process
riming
sleet
tipping-bucket gage
freezing rain collision
weighing-bucket gage
collision-coalescence process
Review Questions:
1. What determines the terminal velocity of falling droplets and raindrops?
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Cloud droplets remain in balance between gravity, pulling downward, and pressure
gradients, pulling upward. The upward pulling gradient supports updrafts while gravity
supports downdrafts. The pull of gravity is related to mass and resulting terminal velocity.
Cloud drops remain in balance between these opposing forces. Or, put another way, a
droplet’s terminal velocity is proportionally offset by a supporting updraft. When droplet
size increases such that terminal velocity exceeds the supporting updraft, precipitation
occurs.
2. Describe the characteristics that distinguish warm, cool, and cold clouds.
Warm clouds, composed primarily of water, form in above freezing temperatures. These
clouds dominate the tropics through the year and mid-latitudes during warm months. Cool
clouds occur in the mid-latitudes during cool months and in high latitudes year round.
They are comprised of water, supercooled water, and/or ice. Cold clouds are similar to
cool clouds except that they form in lower temperature situations and, therefore, are
composed of supercooled water and ice.
3. How do the growth processes of droplets in warm and cold clouds differ from each other?
Warm cloud droplets form through condensation and by collision-coalescence processes.
The latter process is responsible for the formation of precipitation within these clouds.
Cool and cold cloud droplets form through the Bergeron ice crystallization process
whereby ice crystals grow rapidly at the expense of supercooled water.
4. Why isn’t growth by condensation able to create precipitation size droplets on its own?
Condensation is only a viable form of growth until the drop achieves radii of about 20 µm
due to the overall low amounts of water vapor available. Condensation is also a very slow
process and therefore is insufficient to generate precipitation.
5. How do collision and coalescence work to increase the size of cloud droplets?
Collector drops collide with smaller drops and collisions occur. This typically occurs
between drops of similar size due to compressed air below the falling drop. When
collisions occur, drops either bounce apart or coalesce into one larger drop with a greater
terminal velocity.
6. Explain how variations in the saturation vapor pressure for ice crystals and supercooled water
droplets affect the development of precipitation.
The saturation vapor pressure of ice is lower than that for supercooled water and water
vapor. This is due to individual molecules of water being trapped in a frozen form in the
ice crystal. Because molecular motion is limited in this capacity, the ice crystal exerts less
pressure on the surrounding air than the freer moving liquid water or vapor molecules.
Thus, a saturation pressure gradient exists between the ice crystal and the supercooled
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water. In this situation, molecules of supercooled water are drawn to the ice crystal which
grows rapidly into a crystal capable of precipitating.
7. Why can’t the Bergeron process take place in warm clouds?
Because ice and supercooled water is integral to the process. A pressure gradient develops
between ice crystals and supercooled water which directs water drops on to the ice. This
causes the ice crystal to grow rapidly, thereby gaining sufficient terminal velocities to
develop precipitation.
8. What are riming and aggregation?
Riming refers to liquid water freezing onto ice crystals. Aggregation is the joining of
multiple ice crystals through the bonding of surface water.
9. Why is precipitation greater in Mississippi than in Michigan?
Mississippi is located nearer to a large water vapor source, the Gulf of Mexico. This,
combined with warmer overall temperatures ensures greater precipitation amounts per
individual storm than Michigan.
10. How do lakes enhance precipitation downwind?
They act as a source of water vapor integral to cloud and precipitation development.
11. Why do rain showers initially occur only as large drops?
Rain showers normally develop from convective activity associated with warm clouds.
Large droplet formation begins the collision-coalescence process as they have sufficient
terminal velocities to initiate the process.
12. Explain why the formation of sleet requires an inversion.
Sleet and freezing rain form under similar conditions. Both form as ice crystals in the
upper atmosphere. They fall through a warm inversion in the middle atmosphere and melt
into a rain drop. Both then pass through colder air near the surface. Sleet represents
droplets that re-freeze into an ice pellet before striking the surface. Freezing rain refers to
droplets that remain liquid in the air and freeze upon contact with the cold surface.
13. It is never too cold for snow to occur. Is that also true of sleet?
It is frequently too cold for the development of sleet, which requires a warm mid-level
inversion that melts falling ice crystals. These crystals then re-freeze near the surface. If
temperatures are exceedingly cold at the surface, it is unlikely that a mid-level inversion
exists.
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14. Why does hail consist of multiple layers of ice?
Hail is formed as graupel is carried aloft in an updraft. Water accreting to the graupel
freezes forming a layer. The process is repeated until the hailstone is too heavy to be
pushed aloft by the updraft. One may count the rings of a hailstone to determine how many
times the stone was pushed aloft before descending to the surface.
15. What are some of the inherent sources of error in raingages?
Most errors relate to the fact that raingage estimates are point source estimates. Wide
variations across small spatial scales leads to measurement inaccuracies. Problems of
under recording also occur as a result of evaporation of water from the gage and deflection
of precipitation from the collecting surface by wind. Overestimation stems from wind
related blowing, failing to completely empty the gage after measurement, and placing the
gage on non-level surfaces.
16. How do weighing-bucket and tipping-bucket gages measure rainfall?
Weighing buckets simply weighs accumulated water in the gauge and translates the weight
to a precipitation depth. A tipping bucket funnels precipitation into a pivoting bucket.
When a bucket fills, it tips downward and records rain equivalent to a certain depth
(usually 0.01 in). Both methods store data automatically.
17. Explain how snow pillows measure snow accumulation.
Snow pillows measure the weight of overlying snow and convert that weight to a water
equivalent.
18. What are the materials used in cloud seeding, and how do they stimulate (or inhibit)
precipitation.
Dry ice and silver iodide are the two most common materials used in cloud seeding. Dry
ice lowers cloud droplet temperature to the freezing point (-40oC), thus initiating ice
crystals and stimulating the Bergeron process. Silver iodide initiates the Bergeron process
directly by acting as freezing nuclei. Therefore, dry ice is used in warmer atmospheric
conditions. Seeding has been related to about a 10% increase in total precipitation under
ideal conditions.
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