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Instructor's Manual and Test Bank to accompany Meteorology Today, 10th Edition
Jonathan D. W. Kahl
University of Wisconsin-Milwaukee
Chapter 7
Precipitation
Summary
This chapter examines the processes that produce precipitation and looks at the different types of
precipitation that can fall from clouds.
The chapter begins with a more detailed look at the formation and growth of cloud droplets. “The
Freezing of Tiny Cloud Droplets”, the first of many focus sections, describes the formation and growth of
ice embryos. Cloud droplets or ice crystals are themselves too small and light to be able to reach the
ground as precipitation. Raindrops can form rapidly in warm clouds, however, when water droplets
collide and coalesce. Formation of rain by this process works best in thick clouds which have strong
updrafts. In cold clouds it is possible for ice crystals to grow when surrounded by supercooled water
droplets. Attempts to enhance precipitation by cloud seeding are reviewed. A second focus section
discusses the process by which precipitation is artificially enhanced by cloud seeding.
Precipitation can reach the ground in a variety of forms depending on the type of cloud producing
it and also on the atmospheric conditions between the cloud base and the ground. The myths of the tearshaped raindrop and the idea that below-freezing temperatures are required for snowfall are exposed in
separate focus sections. The mechanisms of freezing rain and sleet formation are described. The
“squeakiness” of snow and the hazardous phenomenon of aircraft icing are described in interesting focus
sections. Finally, students learn how rain and snowfall amounts can be measured using simple
instruments or estimated remotely using Doppler radar or satellite-based instruments.
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Teaching Suggestions
1.
Measure and compare the fall speeds of different size drops (fine mist, drops from an eye
dropper, small and large water balloons).
2.
Have the class design a precipitation-enhancement experiment. What method for enhancing
precipitation will be used? What materials will be needed? How will the experiment’s success be
assessed?
3.
rain.
Discuss what atmospheric conditions would cause a meteorologist to forecast sleet or freezing
4.
Encourage students to discuss the design of an automatic snowfall measurement device.
5.
Have students list the various ways in which standard and tipping bucket rain gauges might either
overestimate or underestimate the actual amount of rainfall.
6.
When cloud drops grow into raindrops, their volume increases by a factor of 10,000. In
cumulonimbus clouds, this growth takes place in about 30 minutes or less. Challenge students to suggest
other natural phenomena with similar growth rates.
7.
Encourage students to photograph snowflakes and post them on the class Facebook page.
8.
Before spring or winter break, have students speculate on what clouds might look like from
above. Encourage any students who might be traveling by airplane to photograph clouds from the plane
and share them with the class when they return.
9.
Show a surface weather chart and identify areas with different precipitation types.
10.
Stage a debate as to which is the best way to measure precipitation: Doppler radar or tipping
bucket rain gauge.
Student Projects
1.
Rainfall measurement, the construction of simple and inexpensive rain gauges, and student
projects have been discussed by J.T. Snow and S.B. Harley (ref: "Basic Meteorological Observations for
Schools: Rainfall," Bull. Amer. Meteorol. Soc., 69, 498-507, 1988).
2.
Have students explain why rain falls to the ground but clouds do not.
3.
Have students measure snow depth with a yardstick in three representative areas. Average the
results. Given the variation in the three measurements, what level of confidence can be placed in the
average?
4.
Have students review the document “An Overview of National Weather Service Quantitative
Precipitation Estimates” and summarize the strengths and weaknesses of rain gauge and tipping bucket
rainfall measurements. The document is available at:
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http://www.nws.noaa.gov/im/pub/tdl97-4.pdf
5.
Have students give several reasons why rain gauges are not effective for snowfall
measurements.
Answers to Question for Review
1.
An ordinary cloud droplet is extremely small, having an average diameter of 20 µm or 0.002 cm,
which is about 100 times smaller than a typical raindrop.
2.
They evaporate before reaching the ground.
3.
Large droplets overtake and collide with smaller drops in their path, merging the cloud droplets
by collision and causing them to stick together (coalescence) and grow larger.
4.
The cumulonimbus cloud because it's thicker, giving more opportunity for cloud droplets to
collide and coalesce.
5.
Homogeneous freezing: Molecules within the water droplet join together in a rigid pattern to form
an ice structure which then serves as an ice nucleus. Contact freezing: Freezing nuclei immersed within a
liquid drop. Accretion: Ice crystals colliding with supercooled water droplets.
6.
Larger droplets. The chances of an ice embryo growing large enough to freeze water before the
embryo is broken up by thermal agitation increase with bigger volumes of water. Consequently, only
larger cloud droplets can freeze by homogeneous freezing at temperatures warmer than -40°C.
7.
Because there are many more cloud condensation nuclei than ice nuclei, cold clouds contain
many more liquid droplets than ice particles, even at temperatures as low as -10°C.
8.
The main premise is that the saturation vapor pressure just above a water surface is greater than
the saturation vapor pressure above an ice surface. This difference in vapor pressure causes water vapor
molecules to move (diffuse) from the droplet toward the ice crystal. The removal of vapor molecules
reduces the vapor pressure above the droplet. Since the droplet is now out of equilibrium with its
surroundings, it evaporates to replenish the diminished supply of water vapor above it. This process
provides a continuous source of moisture for the ice crystal, which absorbs the water vapor and grows
rapidly. Hence, during the ice-crystal (Bergeron) process, ice crystals grow larger at the expense of the
surrounding water droplets.
9.
Cumulonimbus clouds normally contain large convection currents causing heavy showers.
Continuous rain usually falls from a layered cloud that covers a large area and has smaller vertical air
currents.
10.
Snowflakes that fall through moist air that is slightly above freezing slowly melt as they descend.
A thin film of water forms on the edge of the flakes, which acts like glue when other snowflakes come in
contact with it. In this way, several flakes join to produce giant snowflakes often measuring several
centimeters or more in diameter. These large, soggy snowflakes are associated with moist air and
temperatures near freezing. However, when snowflakes fall through extremely cold air with a low
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moisture content, small, powdery flakes of “dry” snow accumulate on the ground.
11.
Droplets.
12.
The first ingredient in any seeding project is the presence of clouds, as seeding does not generate
clouds.
13.
Overseeding can produce too many ice crystals. When this phenomenon occurs, the cloud
becomes glaciated (all liquid droplets become ice) and the ice particles, being very small, do not fall as
precipitation. Since few liquid droplets exist, the ice crystals cannot grow by the ice-crystal (Bergeron)
process; rather, they evaporate, leaving a clear area in a thin, stratified cloud. Because dry ice can produce
the most ice crystals in a supercooled cloud, it is the substance most suitable for deliberate overseeding.
14.
When cirriform clouds lie directly above a lower cloud deck, ice crystals may descend from the
higher cloud and seed the cloud below. As the ice crystals mix into the lower cloud, supercooled droplets
are converted to ice crystals, and the precipitation process is enhanced. Sometimes the ice crystals in the
lower cloud may settle out, leaving a clear area or “hole” in the cloud.
15.
The below-cloud air must have a low relative humidity.
16.
Advantages: A blanket of snow is a good insulator. A light, fluffy covering of snow protects
sensitive plants and their root systems from damaging low temperatures by retarding the loss of ground
heat. Snow can prevent the ground from freezing downward to great depths. The accumulation of snow in
mountains provides for winter recreation, and the melting snow in spring and summer is of great
economic value in that it supplies streams and reservoirs with much-needed water. Disadvantages: Rapid
melting of the snowpack may flood low-lying areas. Too much snow on the side of a steep hill or
mountain may become an avalanche as the spring thaw approaches. The added weight of snow on the roof
of a building may cause it to collapse, leading to costly repairs and even loss of life. Each winter, heavy
snows clog streets and disrupt transportation.
17.
For sleet to form, surface temperatures must be below freezing.
typically well above freezing during hailstorms.
Surface temperatures are
18.
Sleet is a frozen raindrop. Freezing rain is a supercooled raindrop that freezes upon impact with
the surface.
19.
Hail is produced in a cumulonimbus cloud when graupel, large frozen raindrops, or just about any
particles (even insects) act as embryos that grow by accumulating supercooled liquid droplets. Violent,
upsurging air currents within the cloud carry small embryos high above the freezing level. As the
embryos pass through regions of varying liquid water content, a coating of ice forms around them and
they grow larger and larger. When the ice particles are appreciable size, they become too large and heavy
to be supported by the rising air, and they then begin to fall as hail. As they slowly descend, the hailstones
may get caught in a violent updraft only to be carried upward once again to repeat the cycle.
20.
Because cumulonimbus clouds (thunderstorms) are more common in summer.
21.
Standard rain gauge, tipping bucket rain gauge, weighing-type rain gauge.
22.
(a) Doppler radar is like conventional radar in that it can detect areas of precipitation and measure
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rainfall intensity.
(b) the Doppler radar uses the principle called Doppler shift to measure the speed at which falling
rain is moving horizontally toward or away from the radar antenna.
Answers to Question for Thought
1.
Large ice crystals fall with their flat surfaces parallel to the ground. This produces a large flat
surface area-to-weight ratio and, hence, a large amount of air resistance which reduces the ice crystal's
rate of fall.
2.
The turbulent vertical motions and towering extent of a warm cumulus cloud will accelerate the
collision-coalescence process of producing rain. In a cold stratus cloud vertical motions are small, the
liquid water content is low, and the collision-coalescence process is not as effective in initiating
precipitation, especially when the air temperature is quite low.
3.
This is an example of the curvature effect. The relative humidity in a cloud is measured with
respect to a flat water surface. When the air is saturated in a cloud (relative humidity equals 100 percent),
it is unsaturated with respect to a curved droplet of pure water. The droplet of pure water is not in
equilibrium so it evaporates.
4.
In this example, the most important process would be the ice crystal process because the
collision-coalescence process requires that cloud droplets be of varying size so that drops will fall at
different speeds.
5.
One reason appears to be that in clouds that form over land there are larger concentrations of
nuclei than in clouds that form over water. Hence in clouds forming over land there are more, but
smaller, cloud droplets. Because the clouds that form over water usually contain fewer nuclei, they
contain larger droplets and a wider distribution of droplet sizes. This enhances the collision-coalescence
process and makes these clouds more efficient at producing rain.
6.
The blizzard occurs about 4 km (13,000 ft) above the surface in the middle of a violent
thunderstorm.
7.
This snowfall pattern could be the result of waves forming in the upper troposphere as air flows
from west to east over the mountains. Cirriform clouds form in the rising part of the wave (wave crest)
and seed ice crystals into a lower supercooled cloud layer, enhancing precipitation. The wave crests are
probably located above Denver, and above the region 150 km east of Denver.
8.
A large drop has greater surface area than a small drop, thus the frictional drag of the air it’s
falling through may tear it apart into smaller drops.
9.
Lead has a deleterious effect on the human body, especially the nervous system and kidneys.
10.
"Holes" are occasionally produced in altocumulus clouds when cirriform clouds are above them.
Ice crystals fall from the cirriform clouds and mix with the supercooled cloud droplets of the altocumulus,
converting many water droplets into ice crystals. The ice crystals grow larger by the ice crystal process
and eventually fall from the altocumulus, leaving the appearance of a hole in the cloud. Another
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possibility is that an aircraft has penetrated the altocumulus and either engine exhaust or vibrations (or
both) have changed the supercooled cloud droplets into ice.
11.
If it is raining heavily and there is a strong temperature inversion, freezing rain can fall when
surface temperatures are -12oC. The presence of a strong inversion ensures that cloud temperatures will
be much higher than surface temperatures, thus the precipitation is rain rather than snow. Heavy rain
(large drops) means large fall velocities, thus the raindrops become supercooled (but don’t freeze) as they
fall toward the ground.
12.
When snow becomes mixed with sleet, it often indicates that warm air aloft has moved into the
region, causing the snowflakes to, at least partially, melt and then refreeze in the colder surface air.
Frequently this is the first indication that warm air is moving into the region. Often, continued influx of
warm air, first aloft, then at the surface, will raise the air temperature to the point that the snowflakes melt
and become raindrops.
13.
Possible reasons include: the snow partially melted; blowing and drifting; the water content of the
snow changed over time; measurement error; the snow partially sublimated.
Answers to Critical Thinking Questions
Figure 7.9.
No. The difference in saturation vapor pressures over supercooled water and ice reaches
a maximum at around -12oC. Close to 0oC this difference is much smaller.
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Multiple Choice Exam Questions
1.
Which below best describes the solute effect?
a. keeps water droplets from freezing at temperatures below 32F
b. removal of pollutants from the atmosphere by cloud droplets
c. water droplets dissolve hygroscopic nuclei and condensation can occur at relative
humidities less than 100 percent
d. evaporation of cloud droplets and grow of ice crystals in a cold cloud
ANSWER: C
2.
Which statement below best describes the curvature effect?
a. Large cloud droplets fall faster than small droplets.
b. Small droplets evaporate more quickly than large droplets.
c. Small droplets collide and coalesce more easily than larger droplets.
d. It explains the six-sided shape of ice crystals.
ANSWER: B
3.
to the
Condensation onto hygroscopic nuclei is possible at relative humidities less than 100 percent due
a.
b.
c.
d.
curvature effect.
electrical charge on these nuclei.
solute effect.
crystalline structure of these nuclei.
ANSWER: C
4.
Which of the following is NOT an important factor in the production of rain by the collisioncoalescence process?
a. the updrafts in the cloud
b. relative size of the droplets
c. the number of ice crystals in the cloud
d. cloud thickness
e. the electric charge of the droplets
ANSWER: C
5.
Which cloud type below will only produce precipitation by the collision-coalescence process?
a. a thick, cold nimbostratus cloud
b. a thick, warm cumulus cloud
c. a thick, cold cumulus cloud
d. a thick, supercooled cumulonimbus cloud with abundant nuclei
e. a supercooled cumulus congestus cloud
ANSWER: B
6.
Large raindrops fall ____ than smaller raindrops, and have a ____ terminal velocity than small
raindrops.
a. faster, lesser
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b. faster, greater
c. slower, lesser
d. slower, greater
ANSWER: B
7.
Which cloud would most likely produce drizzle?
a. stratus
b. cumulus
c. cumulus congestus
d. cirrostratus
e. cumulonimbus
ANSWER: A
8.
If you observe large raindrops hitting the ground, you could probably say that the cloud overhead
was ____ and had ____ updrafts.
a. thick, weak
b. thick, strong
c. thin, weak
d. thin, strong
ANSWER: B
9.
If rain falls on one side of a street and not on the other side, the rain most likely fell from a
a. nimbostratus cloud.
b. stratus cloud.
c. cumulonimbus cloud.
d. altostratus cloud.
e. altocumulus cloud.
ANSWER: C
10.
During the ice crystal process of rain formation,
a. only ice crystals need be present in a cloud.
b. ice crystals grow larger at the expense of the surrounding liquid cloud droplets.
c. the temperature in the cloud must be -40C (-40F) or below.
d. the cloud must be a cumuliform cloud.
ANSWER: B
11.
The temperature at which you would expect a cloud to become completely glaciated is
a. 0C (32F).
b. -5C (23F).
c. -18C (0F).
d. -40C (-40F).
ANSWER: D
12.
Homogeneous nucleation occurs when
a. water vapor condenses onto hygroscopic nuclei.
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b. water vapor condenses onto hydrophobic nuclei.
c. water vapor condenses without nuclei.
d. all the condensation nuclei are exactly the same.
ANSWER: C
13.
Which of the following statements is NOT correct?
a. Generally, the smaller the pure water droplet, the lower the temperature at which it will
freeze.
b. Ice nuclei are more plentiful in the atmosphere than condensation nuclei.
c. Much of the rain falling in middle northern latitudes begins as snow.
d. Ice crystals may grow in a cold cloud even though supercooled droplets do not.
ANSWER: B
14.
Ice nuclei may be
a. ice crystals.
b. certain clay minerals.
c. bacteria in decaying plant leaf material.
d. all of the above
ANSWER: D
15.
Supercooled cloud droplets are
a. ice crystals surrounded by air warmer than 0C (32F).
b. liquid droplets that are cooler than the air around them.
c. liquid droplets observed at temperatures below 0C (32F).
d. water droplets that have had all their latent heat removed.
ANSWER: C
16.
At the same sub-freezing temperature, the saturation vapor pressure just above a liquid water
surface is ____ the saturation vapor pressure above an ice surface.
a. greater than
b. the same as
c. less than
ANSWER: A
17.
Contact nucleation is
a. the freezing of supercooled droplets by contact with a nucleus.
b. the sticking together of ice crystals to make a snowflake.
c. the joining of many nuclei to form an ice nucleus.
d. the freezing of supercooled droplets when they come into contact with a supercooled
surface.
ANSWER: A
18.
The growth of a precipitation particle by the collision of an ice crystal (or snowflake) with a
supercooled liquid droplet is called
a. accretion.
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b. spontaneous nucleation.
c. condensation.
d. deposition.
ANSWER: A
19.
Cloud seeding using silver iodide only works in
a. cold clouds composed entirely of ice crystals.
b. warm clouds composed entirely of water droplets.
c. cold clouds composed of ice crystals and supercooled droplets.
d. cumuliform clouds.
ANSWER: C
20.
What are the two main substances used in cloud seeding?
a. lead iodide, dry ice
b. silver iodide, lead iodide
c. ice crystals, flood powder
d. dry ice, sea salt
e. silver iodide, dry ice
ANSWER: E
21.
After a rainstorm, visibility typically
a. deteriorates.
b. is unaffected.
c. improves.
ANSWER: C
22.
Rain which falls from a cloud but evaporates before reaching the ground is referred to as
a. sleet.
b. virga.
c. graupel.
d. dry rain.
ANSWER: B
23.
The most common ice crystal shape is
a. graupel.
b. dendrite.
c. rime.
d. virga.
ANSWER: B
24.
Fall streaks usually ____ before reaching the ground.
a. evaporate
b. condense
c. sublimate
d. deposit
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ANSWER: C
25.
Which below best describes why a fluffy covering of snow is able to protect sensitive plants and
their root systems from damaging low temperatures?
a. Snow is a good insulator.
b. Melting snow releases latent heat.
c. Snow is a good emitter of infrared energy.
d. Snow is a good reflector of sunlight.
ANSWER: A
26.
Large, heavy snowflakes are associated with
a. dry air and temperatures well below freezing.
b. moist air and temperatures well below freezing.
c. dry air and temperatures near freezing.
d. moist air and temperatures near freezing.
ANSWER: D
27.
Fall streaks most often form with
a. nimbostratus clouds.
b. cumulonimbus clouds.
c. stratus clouds.
d. altostratus clouds.
e. cirrus clouds.
ANSWER: E
28.
A true blizzard is characterized by
a. low temperatures.
b. strong winds.
c. reduced visibility.
d. blowing snow.
e. all of these
ANSWER: E
29.
The largest snowflakes would probably be observed in ____ air whose temperature is ____
freezing.
a. moist, near
b. dry, near
c. moist, well below
d. dry, well below
ANSWER: A
30.
In order for falling snowflakes to survive in air with temperatures much above freezing, the air
must be ____ and the wet bulb temperature must be ____.
a. unsaturated, at or below freezing
b. unsaturated, above freezing
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c. saturated, at or below freezing
d. saturated, above freezing
ANSWER: A
31.
In the winter you read in the newspaper that a large section of the Midwest is without power due
to downed power lines. Which form of precipitation would most likely produce this situation?
a. snow
b. hail
c. freezing rain
d. sleet
e. rain
ANSWER: C
32.
Which is NOT a correct association?
a. snow grains - hail
b. ground blizzard - drifting and blowing snow
c. snow squall - intense snow shower
d. sleet - ice pellet
e. freezing rain - glaze
ANSWER: A
33.
Which of the following might be mistaken for hail?
a. virga
b. graupel
c. dendrite
d. supercooled droplet
ANSWER: B
34.
Which type of precipitation would most likely form when the surface air temperature is slightly
below freezing and the air temperature increases as you move upward away from the ground?
a. freezing rain
b. hail
c. rain
d. snow
e. drizzle
ANSWER: A
35.
The primary method used in preventing the growth of large, destructive hailstones is to inject a
thunderstorm with large quantities of
a. silver iodide.
b. ice crystals.
c. dry ice.
d. hydrophobic nuclei.
e. hailstone embryos.
ANSWER: A
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36.
Precipitation with the greatest size (diameter) is
a. the snow pellet.
b. the snow grain.
c. a hailstone.
d. sleet.
e. a rain drop.
ANSWER: C
37.
An amount of precipitation measured to be less than one hundredth of an inch (0.25 mm) is called
a. a trace.
b. drizzle.
c. light rain.
d. mist.
ANSWER: A
38.
If a city were to receive 1/2 inch of rain in the morning and then 5 inches of snow that afternoon,
about how much precipitation would the weather service report for that day?
a. 5 1/2 inches
b. 1/2 inch
c. 1 inch
d. 10 inches
ANSWER: C
39.
After a snowstorm, the newspaper reports that Buffalo, New York, received 1.50 inches of
precipitation. If we assume an average water equivalent ratio for this snowstorm, then Buffalo received
about ____ inches of snow.
a. 3
b. 1.5
c. 10
d. 9
e. 15
ANSWER: E
40.
On average, the water equivalent of 10 inches of snow is about ____ inches of water.
a. 0.5
b. 1
c. 2
d. 2.5
e. 5
ANSWER: B
41.
Radar gathers information about precipitation in clouds by measuring the
a. energy emitted by the precipitation particles.
b. absorption characteristics of falling precipitation.
c. amount of energy reflected back to a transmitter.
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d. amount of sunlight scattered off the precipitation.
e. amount of solar energy passing through the cloud.
ANSWER: C
42.
In a typical advancing winter storm, which of the following sequences of precipitation types is
most likely to occur?
a. rain, freezing rain, snow, sleet
b. rain, sleet, freezing rain, snow
c. freezing rain, rain, sleet, snow
d. rain, freezing rain, sleet, snow
ANSWER: D
43.
Freshly fallen snow ____ sound waves.
a. amplifies
b. absorbs
c. none of the above
ANSWER: B
44.
The main difference between a cloud drop and a raindrop is its
a. color.
b. size.
c. shape.
d. none of the above
ANSWER: B
45.
A typical raindrop is about ____ than a typical cloud droplet.
a. one thousand times smaller
b. one thousand times larger
c. one hundred times smaller
d. one hundred times larger
ANSWER: D
46.
Saturation vapor pressure ____ as temperature increases.
a. increases
b. decreases
c. can't answer: it depends on whether the cloud has ice crystals or cloud droplets.
ANSWER: A
47.
Large cloud droplets fall faster than small cloud droplets because
a. the ratio of the drop's surface area to its weight is larger.
b. the ratio of the drop's surface area to its weight is smaller.
c. gravity acts more strongly on larger droplets.
d. the speed of the falling drop increases until the air resistance equals the pull of gravity.
ANSWER: B
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48.
The lightest form of rain is
a. shower.
b. cloudburst.
c. virga.
d. drizzle.
ANSWER: D
49.
The difference in saturation vapor pressure between supercooled water and ice reaches a
maximum at about -12C. This means that the ice crystal process will be most effective in producing
precipitation when cloud temperatures are
a. much colder than -12C.
b. around -12C.
c. much warmer than -12C.
ANSWER: B
50.
Satellites measure rainfall from space using which instrument?
a. radar
b. tipping-bucket rain gauge
c. ceiliometer
d. all of the above
e. none of the above
ANSWER: A
Essay Exam Questions
1.
Is silver iodide used as a cloud seeding agent in warm or cold clouds? Why?
2.
Explain why it is possible for an ice crystal to grow in a cold cloud even though the supercooled
water droplets surrounding the ice crystal do not.
3.
Would you expect the largest forms of precipitation particles to occur during the warmest or the
coldest time of year? Explain.
4.
What is the main difference between a raindrop and a cloud droplet?
5.
The first raindrops to reach the ground at the beginning of a rain shower are often very large.
Explain why this is so.
6.
Thunderstorm cloud bases are generally higher above the ground in Arizona than in Florida.
Why?
7.
Briefly describe the differences between snow, freezing rain, sleet and hail.
8.
About how large can raindrops get? Why can't they get any larger?
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9.
What shape does a large raindrop have? What forces determine this shape?
10.
Would you expect the heaviest snowfall to occur on an unusually cold night or a night when the
temperature was just a little below freezing?
11.
When precipitation is falling in the form of sleet, what conclusions can you make regarding the
vertical profile of temperature between the ground and the cloud?
12.
Explain why it is much more difficult to measure snowfall amount than rainfall amount.
13.
Design an automated device for measuring snowfall. How might it work?
14.
Given that precipitation can be accurately measured at surface weather stations, explain why it is
desirable to measure rainfall from space.
15.
How is precipitation measured using Doppler radar?