MET 1010 - Study Guide for Chapter 7, Spring, 1999

The following are definitions and issues from the chapter that you need to understand.

1. The term "parcel of air" refers to a given sample volume of air that may have some special feature making it different from the air in which it resides. Perhaps the parcel is forced up over a hill. Perhaps its temperature and density are different. For example, air over a pond may have more moisture and higher temperature than the air over the nearby land. In order to study this parcel, we image it to be enclosed in a large "bag" of very thin material that can expand or contract along with the parcel without affecting the parcel itself.

2. The term ambient air refers to the air surrounding the parcel. Ambient air is not the parcel but rather is the air through which the parcel moves.

3. Atmospheric instability or stability is a condition that exists in the ambient air at a given place and a given height range. If the air at a point A is stable, this means that a parcel of air at point A is inhibited from undergoing convection. If the air at point A is unstable, this means that a parcel once formed is able to convect upward.

4. The temperature lapse rate for the ambient air refers to the air temperature as a function of height. Ambient air does not rise or fall. It simply states the temperature at various heights. If we measure the temperature every 100 m from ground level up to 10 km over Gainesville today at 4:00PM, we can plot these values and say that we have the ambient air temperature lapse rate. Words equivalent to ambient air temperature lapse rate are static air lapse rate and environmental lapse rate. The lapse rate can vary widely. Normally, air temperature decreases with height, but at a given time, temperature may decrease or increase with height. If the temperature increases at successive values upward, we have a temperature inversion.

5. As a parcel of air rises, that parcel expands and cools. To good approximation, as the parcel rises, no heat energy from outside the parcel has time to enter or leave through the bag. This describes an adiabatic process. For this special case, the parcel cools at a known rate. If instead of rising, the parcels falls, the reverse is true, and the parcel increases in temperature at a given rate. Though the process is more complex than our rules indicate, the following rates of change of temperature (cooling or heating) apply for a parcel as it rises or falls:

a) Dry adiabatic lapse rate (DALR) - 1° C for every 100 meters. This rate applies only if the air is not condensing. Dry does not mean that no water vapor is in the parcel, only that the parcel is not condensing.

b) Wet adiabatic lapse rate (WALR) - 0.6° C for every 100 meters. This rate applies whenever condensation occurs within the parcel. The reason for the difference is that as a parcel condenses, the latent heat of condensation is deposited within the parcel. This latent heat comes from within the parcel, not through the "bag" walls, and this is why the process is still called adiabatic.

 5. The main factor that tells us whether an air parcel is more dense or less dense than the ambient air is its relative temperature. If the parcel's temperature > the ambient air temperature, the parcel is less dense than the ambient air and is buoyant. If the parcel's temperature < the ambient temperature, the parcel is denser than the ambient air and sinks.

6. When an air parcel is buoyant, it has a tendency to rise of its own just like a buoyant object in a pool of water will rise toward the surface. But just knowing a parcel is buoyant does not mean we know how far the parcel will rise. As the parcel rises, it cools at the dry or wet adiabatic rate. If this means that by rising, the parcel cools more than the ambient air, it will reach to a height where it is denser than the ambient air and will stop rising. As an example, consider the case where the temperature of the ambient air is 30° C at the surface, 29.5° C at a height of 100 m and 29° C at a height of 200 m. A parcel of dry air at the surface happens to have a temperature of 31° C. Because at the surface, the parcel has a greater temperature than the ambient air, it is buoyant and will start to rise. When the parcel reaches 100 m, its temperature has gone down by 1° C to 30° C. SInce the ambient air at this height is 29.5° C, the parcel is still buoyant! The parcel continues to rise, but not for long. At a height of 200 m, the rising parcel will cool by another 1° C to 29° C. At this same height the ambient air also has a temperature of 29° C. The result is that at 200 m the parcel is no longer buoyant and wil stop having a tendency to rise. This particular environmental lapse rate allows parcels to undergo convection for a small distance upward but prevents large scale convection. This lapse rate describes air that is said to be stable.

7. Now let us define the consequences of three different environmental lapse rates. Define
ELR as an abreviation for environmental lapse rate (can vary widely)

DALR as an abreviation for dry adiabatic lapse rate (assumed to be 1° C/100 m)

WALR as an abreviation for wet adiabatic lapse rate (assumed to be 0.6° C/100 m)
Then:

a) If the ELR < WALR, the atmosphere is said to be absolutely stable.
b) If the WALR < ELR < DALR , the atmosphere is said to be conditionally unstable.
c) If the ELR > DALR, the atmosphere is said to be absolutely unstable.


8. Whenever the atmosphere is stable in a given region, air tends to move horizontally, not vertically. If the atmosphere is absolutely stable over a given height range, any clouds that form in that range are likely to be one of the stratus types, depending on height.

9. The ELR can change very much over a 24-hour period. Review the reasons air can be made stable presented on page 170 - 171. Note that all of the reasons come from processes in which air aloft is warmed relative to surface air or surface air is cooled relative to air aloft.

10. The most subtle process is the one which leads to a subsidence inversion. Assume a layer of air that is 1 km thick with the bottom of the layer at a height of 5 km, at which height the atmospheric pressure is close to 1/2 the surface pressure. Also assume that the temperature of this layer is -10° C at the bottom and -17° C at the top of the layer. This lapse rate (see 7 above) is a conditionally unstable layer. Now suppose some unknown process causes the base of this layer of air to be forced downward by 2 km, where the pressure is greater. The bottom of the layer would then increase adiabatically by 20° C to +10° C. But what about the top of the layer? Since the layer must be compressed as it subsides, the top will move downward farther than the bottom by a couple hundred meters. Hence, the top will warm by 22° C or 23° C to a value of about +6° C. As the layer moves downward it has changed from conditionally unstable to absolutely stable. Subsidence always make a layer more stable. It can even produce a temperature inversion provided the change in altitude is sufficient and provided the layer is stable at the starting altitude.

11. Air becomes more stable under the following conditions:

a) Radiational cooling of the ground in turn cools surface air and will tend to make a stable surface layer of air.
b) Whenever air moves over a cool surface, the air becomes more stable.
c) If a layer of air is forced to subside, the layer becomes more stable, as discussed earlier.
d) For a layer of air in an altitude range A up to height B, the layer will get more stable if either cold air advection occurs near A or if warm air advection occurs near height B.
  12. Air becomes less stable under the reverse conditions: a) Heating of the ground in turn heats surface air and makes it less stable, even unstable.
b) Whenever air moves over a warm surface, the air becomes less stable, even unstable.
c) For a layer of air in an altitude range A up to height B, the layer will get less stable if either warm air advection occurs near A or if cold air advection occurs near B.
d) When a layer of air is forced to rise, the layer becomes less stable because of the vertical stretching implied earlier.
e) When dry air that occupies an altitude range from height A to height B is mixed with itself, the ELR tends to change toward the DALR. Read about this on page 174.
f) Radiational cooling from cloud tops will often destabilize the layer associated with the clouds. Thus, for example, altocumulus can transform to altocumulus castellanus.
  13. Most clouds form from one of four mechanisms: a) convection of air parcels as thermals, b) forced lifting of a layer of air by a "physical barrier" such as an upslope topography or a frontal system (which we have yet to discuss), and c) the convergence of surface air d) mixing as in the production of mixing fog, as discussed in Chapter 6.

14. A thermal is a rising (convecting) parcel. However, all the air in a region can not rise at the same time. Instead certain parcels are "favored" because of a local turbulence or because of differential heating. Therefore, relatively small thermals rise as the rest of the environmental air gradually subsides. These rising thermals condense at that height at which the temperature in the parcel cools to the dew point temperature.

15. The condensation height, if any, and the height of the cloud top can be estimated if we know the ELR and if we know the temperature and dew point of the parcel at the ground. In the most simple model, here is how to calculate the height of cloud bottoms and tops:

a) As an unsaturated parcel rises, it must cool at the DALR of 1° C/100 m.
b) As the parcel rises, its dew point will decrease at a lapse rate of about 0.2° C/100 m.
c) When the parcel temperature and the dew point meet, the cloud base will form.
d) Above the base, the parcel condenses as it rises. Therefore the parcel now cools at the WALR. The dew point also must drop at the same rate as the WALR.
e) When the parcel's temperature equals the environmental temperature, the parcel is no longer buoyant, and the cloud tops out. The air in the parcel then moves laterally, and the droplets usually evaporate into the environmental air.
f) ALL THIS ASSUMES THAT BETWEEN GROUND LEVEL AND THE CLOUD TOP, THE PARCEL'S TEMPERATURE > THE ENVIRONMENTAL AIR TEMPERATURE.
  16. Remember that the ELR is a quantity that is measured at a given time and place. It varies! The height to which the cloud reaches depends on the ELR.

17. We have neglected several significant processes that make calculations more difficult. One important item is the entrainment of environmental air into the sides of the cloud. The mixing that occurs during entrainment and also the mixing and release of latent heat that occurs as the cloud evaporates gradually makes the convective region less and less unstable. finally, as the sun gets low stability returns and no more convection is possible.

18. A calculation will be done in class as part of the discussion on how to estimate cloud heights. One of the most useful points to understand is that you can estimate the height of the bottom of a convective cloud by knowing the temperature and dew point of the parcel at the starting point. Since the parcel temperature drops 1° C for every 100 m, and since the dew point drops by 0.2° C for every 100 m, the difference between these values drops by 0.8° C for every 100 m. When the two are equal, the condensation level is reached. For example, suppose the temperature and dew point of a parcel are 30° C and 20° C, respectively. The difference is therefore 10° C. This difference will vanish if the parcel rises by (10° C divided by 0.8° C/100m) = 1250 meters.

19. The same logic is used for orographic lifting. We will do one of these in class.

20. A result of orographic lifting is that the windward side of a mountain range gets more cloudiness and precipitation than the leeward side. This produces what is called a rain shadow. If the cloudiness vanishes as the lifted air goes over the top of the range and starts down the lee side, then the descending air must heat at the DALR, whereas on the way up the windward side the air must have cooled at the WALR wherever condensation occurred. The net effect is that the air flowing down the lee side is generally hot and dry. Temperature in the valley below on the lee side is many degrees higher than the starting temperatures on the windward side.

21. For students who are interested, the proper way to compute cloud heights is by means of adiabatic charts, as discussed on pages 182 and 183. This material will not be tested.

22. You are expected to read the chapter with an eye to the information and examples presented. In particular, understand the general conditions that lead to cloud types and conditions that lead to changes from one type to another.

 Chapter 7 - Stability and Cloud Development 1. Which one or more of these are considered adiabatic processes?
a) A parcel of air is heated by tthermal contact with the warm ground.
b) A parcel of air sinks, compresses and warms.
c) A parcel of air is forced up a mountain ridge and cools.

(1) a (2) b (3) c (4) a and b (5) b and c

2. We assume in this course for the purpose of calculation that the WALR is 6° C/km. This is only approximate. Near the earth's surface, does a rising parcel of saturated air cool more rapidly when its temperature is (1) low? (2) high? Can you give a reason for your choice?

3. What term applies when the ELR is less than the WALR?

(1) conditionally unstable (2) absolutely stable (3) absolutely unstable (4) neutral

4. In conditionally unstable air, the ELR is __ than the DALR and __ than the WALR.

(1) greater, less (2) greater, greater (3) less, greater (4) less, less

5. Which one or more of the following can make a layer of air less stable?
a) mix the air in the layer, b) lift the entire layer of air,
c) cool the upper part of the layer by radiational cooling.

(1) b (2) c (3) a and b (4) b and c (5) a, b and c

6. In a rising parcel, what happens, if anything, to the dew point?

(1) increases (2) does not change (3) decreases

7. Which one of these environmental lapse rates is most unstable?

(1) 1° C/km (2) 3° C/km (3) 6° C/km (4) 9° C/km (5) 11° C/km

8. Which one or more of these can cause a layer of As clouds to change into Ac clouds?
a) The top of the cloud deck cools while the bottom part warms.
b) Convection cells begin in the altostratus layer.
c) The altostratus layer becomes more stable.

(1) a (2) b (3) c (4) a and b (5) a and c

9. A parcel of air moves downward from a point where the relative humidity is close to 100% and no clouds or fog is present. Which one or more of these are true?
a) The parcel will warm at the wet adiabatic lapse rate.
b) The parcel will warm at the dry adiabatic lapse rate.
c) Condensation is likely to occur.

(1) a (2) b (3) c (4) a and c (5) b and c

10. The ELR is known to be 8° C/km. This air has which one or more of these behaviors?
a) allows convection of dry air, b) allows convection of moist air,
c) is conditionally unstable.

(1) a (2) b (3) a and b (4) a and c (5) b and c

11. Convective clouds are observed to have their base at a height of 1000 m. The temperature is 12° C at this height. Which one of these is the temperature for air at the surface below?

(1) 22° C (2) 18° C (3) 14° C (4) 12° C (5) 10° C

12. Which one of the above answers is the dew point for air at the surface below?

13. What will be the height of the base of a cumulus cloud when the surface air temperature is 45° C and the dew point is 25° C?

(1) 1125 m (2) 2500 m (3) 4440 m (4) 5000 m (5) 6500 m

14. The surface air temperature is 66° F, and the base of a cumulus congestus cloud directly overhead is at a height of 2000 ft. What is the approximate temperature at an elevation of 5000 ft above you and inside the cloud? Assume a DALR of 5.5° F/1000 ft and a WALR of 3.0° F/1000 ft.

(1) 32° F (2) 41° F (3) 46° F (4) 49° F (5) 55° F

15. The temperature at the surface is 30° C. A cumulus cloud base is observed at a height of 1500 m, where the environmental air is known to be 13° C. Above 1500 m, the ELR is
0.35° C/100 m. The cloud will top out at a height of about ___ m.

(1) 1700 (2) 1900 (3) 2100 (4) 2300 (5) 2500

16. Air on the windward side of a mountain range has a temperature of 30° C and a dew point of 22° C. This air is forced to flow up and over the mountain range, which is 2000 m high. Orographic clouds form at a height of ___ m.

(1) 800 (2) 1000 (3) 1200 (4) 1500 (5) 1800

17. The air flowing over the mountain range in the previous question is of course affected by contact with the ground and by the sun's radiation. However, assume that the only mechanism in operation is adiabatic cooling and heating. Also assume that all of the condensed moisture falls out as rain on the windward side, so that the lee side is dry, and assume that the base of the lee side is at the same altitude as the base of the windward side. The temperature at the base of the lee side is ___ ° C.

(1) 26 (2) 28 (3) 30 (4) 32 (5) 34

18. In the above question, the dew point at the base of the lee side is about ___° C.

(1) 6 (2) 8 (3) 10 (4) 12 (5) 14
 
 

ANSWERS TO CHAPTER 7 QUESTIONS
7-1. (5) 7-6. (3) 7-11. (1) 7-16. (2)
7-2. (low) 7-7. (5) 7-12. (3) 7-17. (5)
7-3. (2) 7-8. (4) 7-13. (2) 7-18. (3)
7-4. (3) 7-9. (2) 7-14. (3)  
7-5. (5) 7-10. (5) 7-15. (4)