LAPSE RATES EXERCISE
Vocabulary
normal lapse rate, dry adiabatic lapse rate, wet adiabatic lapse rate, latent heat of vaporization; latent heat of condensation, windward, leeward
Behavioral Objectives
The student will be able to calculate when the air will be saturated given sufficient data.
The student will be able to calculate the equivalent sea level temperature, the temperature, or the altitude when given the other two.
The student will be able to determine the level of condensation and the temperature at point C on the mountain using the correct lapse rates given the base temperature, elevation, and moisture information.
Introduction
Temperature differences in the troposphere are a basic cause of many of the weather changes experienced at the surface of the earth. Several generalizations can be made concerning local temperature variations.
Temperatures decrease with increasing elevations. If a balloon were sent up, as it rose, the temperature would decrease fairly uniformly. The average or normal lapse rate says that the average change will be 3.5°F per 1000 feet elevation change. This change occurs partly due to the greenhouse effect and partly due to the lower density of the air. The normal lapse rate is especially useful to compare temperatures. For example, in order to accurately compare the temperature in New York City which is about at sea level (zero feet) with the temperature of the air of Denver, Colorado which is at about 5000' one would need to convert one of them so that they would both be at the same equivalent elevation so that the characteristics of the air masses would be similar. (See examples 1).
Air may rise for four reasons. Convectional lifting is caused by air at the surface being heated, thus, expanding, and rising. Frontal (cyclonic) lifting is caused where one air mass is forced to rise over another one. Convergence forces air to meet and rise because it can not move into the lithosphere. Orographic lifting occurs where air is forced to go over a landform barrier such as a mountain. All four of these cause the rising air to undergo pressure changes which also affect the temperature. When air is forced to rise, its temperature will decrease; and if air subsides, its temperature will increase.
When air is forced to move vertically, the average rate at which the temperature changes is 5.5°F per 1000'. This is called the dry adiabatic lapse rate. Recall, however that cold air can not hold as much water vapor as warm air and that cooling the air will cause it to approach and reach saturation. Also, remember that condensation is a warming process, releasing the latent heat of vaporization stored by evaporation. When the air is cooled below its dew point temperature, condensation occurs and adds heat to the air. For every 1000', 2.5°F of heat will be added. So, once condensation begins (or once the air cools below the dew point temperature) as the air rises, the temperature will drop 5.5°F per 1000' and 2.5°F of heat will be added by condensation so the result will be that the temperature will (5.5°F + 2.5°F = 3.0°F) drop only 3.0°F per 1000'. This change in temperature of 3.0°F per 1000' while condensation is occurring is called the wet adiabatic lapse rate.
So as air rises until condensation begins, the air will cool at the dry adiabatic lapse rate of 5.5°F per 1000'; and after condensation begins, the air will cool at the wet adiabatic lapse rate of 3.0°F per 1000'. As the air comes down, its temperature will rise, so it will increase its ability to hold water vapor and so its relative humidity will be getting lower which means condensation will NOT occur. Therefore, the air will warm at the dry adiabatic lapse rate of 5.5°F per 1000'. (See example 2).
See the Lapse Rate Summary
Diagram
Weather greatly influences our everyday lives. Temperature and moisture are only two of the factors involved in the complex interaction which occurs between the atmosphere and the earth's surface. The more information that is obtained, the better will be the understanding and predictions about what the weather will be and how it will affect human activities.
Review Questions
As elevation increases, the temperature will _______________.
As elevation decreases, the temperature will _______________.
The normal lapse rate is _______________°F per_______________' and is used for _______________.
The dry adiabatic lapse rate is _______________°F per _______________' and is used when air is _______________ until _______________ begins.
The wet adiabatic lapse rate is _______________°F per _______________' and is used when air is _______________ while _______________ is occurring.
Answers: decrease; increase; 3.5, 1000, comparisons; 5.5, 1000, moving, condensation; 3.0, 1000, moving, condensation.
Examples 1
a. If the temperature were 76°F at sea level, what would the temperature of the air be at an airplane flying 4000 feet overhead?
This does not involve any vertical air movement, so the rate used would be the normal lapse rate. The temperature decreases with altitude.
4000 feet
 0 feet
4000 feet difference in elevation
(simple math) four times the rate for 1000' = 4000'
3.5 x 4 = 14.0°F change in temperature
76°F  14°F = 62°F at 4000'
OR (algebra)
3.5°F = X
1000 4000'
1000x = 3.5°F (4000')
1000x = 14000
x = 14°F
The temperature changes 14°F and drops.
76°F  14°F = 62°F at 4000'.
b. The temperature in Denver, Colorado, elevation 5000' is 53°F and the temperature in New York City at sea level is 68°F. Does the air in New York City actually contain more heat than the air in Denver?
5000'  0 = 5000' change in elevation
5000' / 1000' = 5 times the rate
No actual vertical movement of the air occurs; this is a comparison. The normal lapse rate is used.
5 x 3.5°F = 17.5°F
APPROACH A
Convert the temperature of New York to the temperature at 5000'.
68°F  17.5°F = 50.5°F is the temperature at 5000' above New York.
Denver is 53°F. New York's air does not contain more heat.
APPROACH B
Convert the temperature of Denver to what it would be at sea level.
53°F + 17.5°F = 70.5°F is the equivalent sea level temperature.
New York is 68°F. Denver contains more heat in its air than New York.
On all of the mountains used by Dr. Hill, the wind will be blowing across the mountain from A to X to B to C. See basic mountain and also see Lapse Rate Summary Diagram Point X represents the point where condensation begins. Note: The positioning of point X on the mountain does not represent its actual location as it may be at a higher elevation than point B, lower than point A, either at A or B, or anywhere in between points A and B.
Example 2
A = 3000'; B = 9000'; C = 3000' The temperature at point A is 79°F, and the dew point temperature is 57°F. What would be the equivalent sea level temperature of C?
From A to X the air cools 79°F 57°F = 22°F at the dry adiabatic lapse rate. 22/5.5 = 4 times or 4000' elevation change. X is at an elevation of (3000' + 4000' =) 7000'. From X to B the air cools at the wet adiabatic lapse rate. 9000'  7000' = 2000' elevation change. 2000'/1000 = 2 times the rate. 2 x 3.0 = 6°F change. 57°F  6°F = 51°F at B.
From B to C it warms at the dry adiabatic lapse rate. 9000'  3000' = 6000'/1000' = 6 times the rate. 6 x 5.5°F = 33°F change. 51°F + 33°F = 84°F at C. (Note that this is warmer than at A.)
C is at 3000', and the air can not physically move to sea level. Thus, this will be the equivalent sea level temperature. Since it is a comparison and involves no vertical air movement, it uses the normal lapse rate. 3000'  0'(sea level) = 3000' elevation change. 3000'/1000' = 3 times the rate. 3 X 3.5°F = 10.5°F change. 84.0°F + 10.5°F = 94.5°F is the equivalent sea level temperature of C.
Problems
1. Complete the following table.
2. A = 1000', B = 7000', C = 3000'. The temperature at A is 82°F and this air contains 5.7 grains of water vapor. Condensation will begin at _______________ degrees. This point is represented by X and is at an elevation of _______________ feet. The temperature at B is _______________. The temperature at C is _______________°F. The equivalent sea level temperature of the air at C would be _______________°F. 3. A = 2000', B = 19,000', C = 7000'. The equivalent sea level temperature of A is 87°F and the dew point temperature is 47°F. The temperature at A is _______________ degrees. The elevation of X, the height at which the cloud base will occur, is _______________'. The temperature at B is _______________°F. The temperature at C is _______________°F. The equivalent sea level temperature of the air at C would be _______________°F. If one assumes that when the air temperature reaches 32°F, snow falls, the height of the snowline on the windward side of the mountain would be _______________. 4. Use mountain chain A. A = 1000', B = 4000', C = 1000', D = 8000', E = 2000', F = 5000', G = 1000'
a. If the temperature at A is 68°F, the sealevel temperature would be _______________°F. b. If condensation occurs, the air temperature at C is _______________ (the same, colder, warmer) than at A because _______________. c. If condensation on Mt. X begins when the air is cooled to 50°F and the air at B is 40°F, the dew point on Mt. Y will be _______________°F. In this case, will condensation occur on Mt. Z? _______________ Why?_______________ d. If the dew point is not reached as the air moves over these mountains, the air temperature at G will be _______________(the same, colder, warmer) than the air at A. e. If the air becomes saturated at some time as it blows over the mountains, the temperature at G will be _______________(the same, colder, warmer) than the air at A. f. Which mountain peak will most often be covered by clouds?_______________ Answers: 1. 73.5°F, 105°F, 1500', 11°F, 63.7°F; 2. 60, 5000, 54, 76, 86.5; 3. 80, 8000, 14, 80, 104.5, 13,000; 4. a. 71.5°F; b. warmer, latent heat of condensation; c. 40°F, no, It does not reach the dew point temperature.; d. the same; e. warmer; f. Mt. Y. More Practice Fill in the table below.
