Name ____________________________________________
Lab 4
Vertical Structure of the Atmosphere
Materials
Calculator
Ruler
Introduction
The first in-class activity in meteorology introduces the concept of atmospheric pressure. We will construct and interpret a
number of graphs to measure how pressure, density, and temperature change with height above the Earth’s surface. We will
focus on how these relationships are expressed in the troposphere, which is where weather processes occur.
Introduction
The atmosphere is a compressible fluid, made up of gases whose molecules are pulled to the Earth’s surface by gravity. As a
result, the molecules that make up the atmosphere are most compressed close to Earth’s surface, and atmospheric density
decreases most rapidly with height there. (Figure 4-1)
Figure 4-1
Although the boundary between Earth’s surface and the atmosphere is obvious, there is no clear “top” to the atmosphere. It
thins out with increasing height, but never actually ends. However, since very few gas molecules within Earth’s gravitational
field exist beyond 100 kilometers (km), we can consider this height to be an arbitrary “top” to the atmosphere.
We may use a simple rule to describe the rate at which density decreases with height:
For every 5.6 km you ascend, there is half the atmospheric mass above you as when you started.
1.
Using the above rule, indicate the percentage of the atmosphere above and below each height in Table 4-1.
TABLE 4-1
Height (km)
% of Atmosphere Above
% of Atmosphere Below
33.6
28.0
22.4
16.8
11.2
5.6
Sea level
I hereby declare upon my word of honor that I have neither given nor received any unauthorized aid on
this work.
______________________________________________________
2.
Use the data in Table 4-1 to construct a graph using the grid below. The vertical axis is divided into 17 equally spaced
intervals. Label this axis “Height Above Sea Level (km)” and label the intervals. Label the horizontal axis “Percentage
of the Atmosphere Above” and label its intervals (0-100%). You will use the graph to answer questions 3-7.
3.
Jet airplanes travel at about 11.2 km above sea level (≈37,000 ft). Approximately what fraction of the atmosphere is
above these jets?
4.
a. ________%
Approximately what fraction is below the summit of Mt. McKinley in Alaska (6.19 km, 20,320 ft)?
5.
a. ________%
Approximately what fraction is above Pike’s Peak in Colorado (4.34 km, 14,110 ft)?
a.
________%
Since barometric pressure reflects the weight of the atmosphere above a point, there is also a close relationship between
height and atmospheric pressure. We can assume that 100% of the atmospheric mass lies above sea level and exerts a pressure
of approximately 1000 millibars (mb). Since atmospheric mass at 5.6 km is 50% of its sea-level value, the pressure at this height
is half of that exerted at sea level.
6. Using this relationship, add a pressure scale beneath the percent scale in your graph and label it “Pressure (mb).”
7.
Using this new scale, estimate the pressure at the three levels used in questions 3-5.
a.
Height of jet airplanes
________mb
b.
The top of Mt. McKinley
________mb
c.
The top of Pike’s Peak
________mb
Changes in Temperature with Height
We define four layers of the atmosphere (the troposphere, stratosphere, mesosphere, and thermosphere) according to their
average lapse rate—the rate at which temperature changes with height. You will demonstrate this change in Lab 4A by
graphing the lapse rate at different altitudes. Before continuing with the remainder of this activity, complete Lab 4A.
Although the lapse rate at any given time or place will differ from this average, the graph you created in Lab 4A provides a
starting point for understanding the temperature profile of the atmosphere. The tropopause, stratopause, and mesopause
mark the top (end) of each layer.
8.
Ozone is a good absorber of ultraviolet radiation from the sun. How does its highest concentration at 20-30 km
influence the stratospheric lapse rate?
Tropospheric Lapse Rate
Let us examine in more detail the environmental lapse rate in the troposphere—the layer in which most weather phenomena
occur. When averaged over all seasons, air temperature is 15°C at the Earth’s surface and decreases by 6.5°C per kilometer in
the lowest 11 km. Consequently, this decrease is often referred to as the average lapse rate—the average for all locations and
seasons. Of course, the measured atmospheric lapse rate for a specific time and place will likely differ from the average.
9.
Table 4-2 on the next page lists atmospheric temperatures measured at various heights above Key West, Florida and
Fairbanks, Alaska on December 8, 1996, 12:00 GMT. Plot the data in the graph below the table. As a reference, plot
the average tropospheric lapse rate (6.5°C/km) on the same graph.
10. Which station has the higher surface temperature?
11. Which station has the higher temperature at 10 km?
12. Which station has the higher temperature at 16 km?
13. How does the temperature profile in the lowest 1 km differ between the two stations?
14. The tropopause marks the top of the troposphere and is defined as the height of coolest tropospheric temperature.
Record the tropopause height at Key West and Fairbanks and their respective temperatures in Table 4-3 below.
TABLE 4-3 TROPOPAUSE HEIGHT AND TEMPERATURE
Station
Height (km)
Temperature (°C)
Key West
Fairbanks
TABLE 4-2
Key West, Florida
Height (m)
Surface
Temperature
(°C)
24
Fairbanks, Alaska
Height (m)
Surface
Temperature
(°C)
-26
800
3300
5300
7500
12000
16000
17000
20500
28000
18
7
-6
-21
-57
-74
-74
-62
-56
800
3300
5300
7500
12000
16000
17000
20500
28000
-11
-7
-22
-30
-60
-51
-48
-47
-42
Review Questions
15. In your own words, describe how air pressure, density, and temperature vary with height in the troposphere.
16. The thickness of the troposphere varies from place to place and from day to day. What influences this thickness?
17. Do you think air pressure changes faster as you move horizontally or vertically? Explain your reasoning. (As a
reference, an average hurricane has a radius of approximately 600 km and a central pressure of 950 mb.)