Chapter 5

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Chapter 5

Cloud Development and Precipitation

Summary

This chapter examines atmospheric stability and factors that affect the development of clouds.

The treatment continues with a discussion of precipitation processes and the different types of precipitation that can be produced by clouds.

The concept of stable and unstable equilibria is introduced using an instructive analogy. Stability in the atmosphere depends on the change of temperature in a moving parcel relative to its surroundings. In a stable atmosphere, a parcel which is given an upward push will become colder and denser than its surroundings and will resist further upward motion. Clouds which form in a stable atmosphere tend to develop horizontally and have a layered structure.

In an unstable atmosphere, a rising air parcel will become warmer and less dense than its surroundings and will continue to move upward on its own, often forming cumuliform clouds. The rising air motions that are needed to form clouds can be produced in a variety of ways including convection, topographic uplifting, convergence, and lifting at frontal boundaries. A variety of factors can affect atmospheric stability such as warming or cooling at the ground and the influx of warm or cold air at upper levels.

In the second part of the chapter, the student learns that the water droplets or ice crystals found in clouds are themselves too small and not heavy enough to be able to reach

the

ground as precipitation. Larger raindrops can form rapidly in warm clouds when water droplets collide and coalesce. Formation of rain by this process works best in thick clouds with strong updrafts. The formation and growth of ice crystals, which is important for precipitation formation in cold clouds, is discussed An understanding of the processes that produce precipitation has led to attempts to enhance precipitation by cloud seeding. The effectiveness of cloud seeding is examined in a focus section.

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

Rain and snowfall amounts can be measured using simple instruments or estimated remotely using weather radar.

Teaching Suggestions, Demonstrations and Visual Aids

1. Free convection can be demonstrated by filling a dark colored balloon with hydrogen or helium gas. Tape a small piece of paper to the balloon to add weight. Trim the paper until the balloon is just heavy enough that it will sink very slowly when released. Heat the balloon using an infrared heat lamp or a high-wattage flood light. As the balloon warms, it will expand slightly and begin to rise. The balloon will float out of range of the heat lamp, cool and sink back toward the floor. The demonstration can be repeated several times.

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Or, alternatively, plunge a helium-filled balloon in liquid nitrogen. Remove the balloon and quickly place it on a table top. The balloon will expand as it warms. Eventually, the density of the gas in the balloon will be less than that of the surrounding air and the balloon will float upward off the table.

Convection currents can also be demonstrated by mixing a small amount of aluminum powder in a test tube of water. Shake the test tube to get a good homogeneous distribution of the particles.

Currents will be established when the bottom of the test tube is warmed (ref: E.J. Carlone,

"Convection Demonstration,"

The Phys. Teacher,

p. 464, October, 1983).

2. An interesting class discussion can sometimes be generated by writing two statements, "rising air cools" and "warm air rises," on the board and asking the students whether the statements are contradictory.

3. This chapter might be an appropriate place to introduce Archimedes' Law. Students should be able to understand that the vertical pressure gradient force and the downward pull of gravity are generally very nearly balanced in the atmosphere. A slight change in an air parcel's density

(with respect to its surroundings) will upset the balance and cause the parcel to rise or sink For a surprising demonstration of density differences, place a can of diet and sugar -sweetened soft drink in an aquarium filled with water. The can of diet drink will generally float, while the sweetened beverage will sink. A large amount of corn syrup is added to soft drinks and the resulting mixture has a density greater than water (refs: Toepker, T.P., "Floaters and Sinkers,"

The Phys. Teacher, p.

164, March, 1986 and Heckathorn, D., "A Density Demonstration",

The

Phys. Teacher, p.

39, Jan., 1987).

4. Temperature inversions and atmospheric stability are discussed in "Temperature

Inversions Have Cold Bottoms" in

What Light Through Yonder Window Breaks

by C.F. Bohren.

5. Satellite cloud photographs often illustrate stable and unstable atmosp heric conditions.

A spotty cumulus cloud pattern is often visible when cold air moves out over warm ocean water, for example.

6. Fill a glass tube (18 to 24 inches tall and 2 to 3 inches in diameter) with glycerin (or corn syrup). Glycerin is sufficiently viscous that steel ball bearings dropped into glycerin will accelerate and quickly attain their terminal velocity. The terminal velocity can readily be measured and the dependence of fall velocity on bearing diameter can be investigated (the terminal velocity should be proportional to the square of the ball bearing radius). Typical fall speeds for 3/32" and 3/8" diameter bearings are 1 to 2 cm/sec and 20 cm/sec, respectively.

7.

Ask students to explain why, when they’ve depressed the valve on an inflated bicycle tire, their finger gets cold.

Student Projects

1. Strong radiation inversions frequently develop overnight in many locations. Have students plot an early morning sounding and identify the top of the inversion layer. In larger cities, an associated hazy layer may be visible over the city in the early morning. Plot the

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afternoon sounding for the same day and determine whether the inversion layer has disappeared.

Are any other inversion layers visible? Have the students investigate what meteorologic al conditions favor strong inversions and pollution episodes for their location. (Tabulated sounding data will generally use pressure as the vertical coordinate; in this case it is probably sufficient to assume a 1 mb decrease per 10 meters)

2. Add the relative humidity (corresponding to the high and low temperatures) to the above graphs. Which moisture variable, dew point or RH, gives a better indication of how humid the air feels?

3. 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. Am. Meteorol. Soc.,

69, 498-507, 1988).

4. Use the Atmospheric Basics/Layers of the Atmosphere section of the

ThomsonNow web site to explore the vertical profile of temperature at a location near you. What is the environmental lapse rate of the first 500 meters of the atmosphere?

What is the stability of this layer?

5. Using the Adiabatic activity on the ThomsonNow web site, make a graph of the altitude of cloud formation (plot this on the vertical scale) vs. dew point depression

(plotted on the horizontal scale). Dew point depression is the difference between the temperature and the dew point temperature. Explain the shape of the graph.

Answers to Questions for Review

1. A process in which an air parcel expands and cools, or compresses and warms, with no interchange of heat with its surroundings.

2. By comparing the air temperatures at two different altitudes.

3. During a moist adiabatic process the heat added during condensation offsets some of the cooling due to expansion, so the air no longer cools at the dry adiabatic rate but at a lesser rate called the moist adiabatic rate.

4. The atmosphere tends to become more stable as the air aloft warms or the surface air cools.

If the air aloft is being replaced by warmer air (warm advection), and the surface air is not changing appreciably, the environmental lapse rate decreases and the air becomes stable.

Similarly, if the surface air cools and the aloft air temperature remains relatively constant, the air becomes more stable. The atmosphere becomes more unstable as the environmental lapse rate steepens; that is, as the air temperature drops rapidly with increasing height.

5. Conditional instability means that if the atmosphere is unsaturated it is stable, but if it is saturated it is unstable. Saturation.

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6.

7.

Because during an inversion the environmental lapse rate is not only small, it's negative.

Stable: stratus and nimbostratus. Unstable: vertical development clouds.

8. Because surface temperatures are generally warmer in the afternoon, providing better conditions for convection.

9. Sinking air between the clouds compensates for the rising air beneath the clouds.

10. Because the tropospause represents a transition to the extreme stability of the stratosphere, and a rising cloud tends to flatten out as it reaches this altitude.

11. Surface heating and free convection: Air in contact with the warm surface becomes warmer than its surroundings and rises. Example: cumulus cloud. Topography: Horizontally moving air forced to rise over a mountain range. Example: orographic clouds, which form on the upwind side of the range. Widespread ascent: Rising air due to convergence in low pressure systems.

12. Because the clouds and precipitation generally form on the upwind side of the range, and by the time the air gets to the leeward side, there's not much moisture left.

13. Leeward.

14. 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.

15. They evaporate before reaching the ground.

16. 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.

17. 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.

18. By injecting small particles that will act as nuclei into the cloud, so that the cloud particles will grow large enough to fall to the surface as precipitation.

19. 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.

20. Rain consists of drops with diameters greater than 5 mm; drizzle consists of drops with

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diameters less than 5 mm.

21. The showers falling from cumuliform clouds are usually brief and sporadic, as the cloud moves overhead and then drifts on by. If the shower is excessively heavy, it is termed a cloudburst.

Beneath a cumulonimbus cloud, which normally contains large convection currents, it is entirely possible that one side of a street may be dry (updraft side), while a heavy shower is occurring across the street (downdraft side). Continuous rain, on the other hand, usually falls from a layered cloud that covers a large area and has smaller vertical air currents. These are the conditions normally associated with nimbostratus clouds.

22. No matter how cold the air becomes, it always contains some water vapor that could produce snow. In fact, tiny ice crystals have been observed falling at temperatures as low as 47°C (

53°F).

23. Because the wind at higher levels moves the cloud and ice particles horizontally more quickly than do the slower winds at lower levels, fallstreaks often appear as dangling white streamers.

24. Sleet is a frozen raindrop. Freezing rain is a supercooled raindrop that freezes upon impact with the surface.

25. For sleet to form, surface temperatures must be below freezing. Surface temperatures are typically well above freezing during hailstorms.

26. A standard rain gauge consists of a funnel-shaped collector attached to a long measuring tube. The cross-sectional area of the collector is ten times that of the tube. Hence, rain falling into the collector is amplified tenfold in the tube, permitting measurements of great precision—to as low as one-hundredth (0.01) of an inch.

Answers to Questions for Thought and Exploration

1. The temperature changes measured by the thermometer hanging freely will reflect the temperature of the air at different altitudes. The temperature changes measured by the thermometer enclosed in the balloon will reflect the dry adiabatic lapse rate, because the air inside the balloon is behaving like an air parcel, expanding adiabatically as the climber ascends.

2. North Pole. At the tropics, where there is abundant water vapor, latent heat of condensation will partially offset dry adiabatic cooling. At the North Pole, where there is very little water vapor, there will also be very little latent heat of condensation.

3. The surface must warm or the air aloft must cool, or both.

4. When precipitation-bearing winds are persistently from the east.

5. All three observers could have measured the correct amounts. Possible reasons for the different measurements include: the snow partially melted; blowing and drifting; the water content of the snow changed over time; measurement error; the snow partially sublimated.

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6. There are more likely to be updrafts in the tropical cumulus clouds.

7. 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.

8. 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 collisioncoalescence process and makes these clouds more efficient at producing rain.

9. The blizzard occurs about 4 km (13,000 ft) above the surface in the middle of a violent thunderstorm.

10. 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.

11. 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.

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Instructor’s Manual

Chapter 5: Cloud Development and Precipitation

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