2.50
3.00
4.00
5.00
6.00
7.00
8.00
9.00
In-class Assignment #8
Earth’s Atmosphere: Pressure Profiles & Pressure Patterns
The gases that make up air create pressure through their motion, size, and number. This pressure is exerted on all surfaces in contact with the air. The weight of the atmosphere, or air pressure, exerts an average force of approximately
1 kg/cm 2 (14.7 lbs/in 2 ) at sea level. Under the acceleration of gravity, air is compressed and therefore denser near the
Earth’s surface. The atmosphere rapidly thins with increasing altitude, a decrease that is measurable because air exerts its weight as pressure. Consequently, over half the total mass of the atmosphere is compressed below 5500 m (18000 ft), 75% compressed is compressed below 10700 m (35105 ft), and 90% is below 16000 m (52500 ft). All but 0.1% is accounted for at an altitude of 50000 m (163680 ft) or 50 km (31 mi).
Any instrument that measures air pressure is called a barometer. One type of barometer uses a column of mercury that is counterbalanced by the mass of surrounding air exerting an equivalent pressure on a vessel of mercury to which the column is attached. Evangelista Torricelli developed the mercury barometer. Another type of barometer, the aneroid barometer, is a small chamber that is partially emptied of air, sealed, and connected to a mechanism that is sensitive to changes in air pressure. As air pressure varies, the mechanism responds.
Altimetry is the measurement of altitude using air pressure. A pressure altimeter is an instrument that measures altitude based on a strict relationship between air pressure and altitude. Air pressure is measured within an altimeter by an aneroid barometer capsule with the instrument graduated in increments of altitude. A pilot must constantly adjust the altimeter, or “zero” the altimeter, during flight as atmospheric densities change with air temperature. Another type of altimeter sends and receives radio wavelengths between the plane and the ground to determine altitude – a radio altimeter.
Normal sea level pressure is expressed as 1013.2 mb (millibars) of mercury (a way of expressing force per square meter of surface area). At sea level standard atmospheric pressure is expressed in several ways:

14.7 lbs/in 2

29.9213” of Hg (mercury)

1013.250 millibars

101.325 kilopascals (kilopascal = 10 millibars)
Some convenient conversions are helpful:

1.0 in. (Hg) = 33.87 mb = 25.40 mm (Hg) = 0.49 lbs/in 2

1.0 mb = 0.0295 in. (Hg) = 0.75 mm (Hg) = 0.0145 lbs/in 2
The standard atmosphere for pressure in millibars and altitude in kilometers is given in the table below. This is used in the following activity.
Table 1.1
0.00
0.50
Altitude (km)
1013.25
954.61
898.76
Pressure (mb)
10.00
12.00
Altitude (km)
264.99
193.99
141.70
Pressure (mb)
1.00
1.50
2.00
845.59
795.01
14.00
16.00
18.00
103.52
75.65
746.91
701.21
616.60
540.48
472.17
411.05
356.51
308.00
20.00
25.00
30.00
35.00
40.00
50.00
60.00
70.00
55.29
25.49
11.97
5.75
2.87
0.79
0.23
0.06

1.
Using the supplied graph paper, plot the standard atmosphere of air pressure decrease with altitude presented in Table 1.1. After completing the plot, connect the data points with a line to complete the pressure profile of the atmosphere.
2.
The information in Table 1.1 allows a determination of the rate of pressure decrease with altitude, which is not at a constant rate. Remember that half of the weight of the total atmosphere occurs below 5500 m (18000 ft); at that altitude only about half of the total atoms and molecules of atmospheric gases remain to form the mass of the atmosphere. Determine the decrease in pressure between the following altitudes.
1 km interval difference in pressure: 10 km interval difference in pressure:
0 and 1 km ________mb; ________in
2 and 3 km ________mb; ________in
5 and 6 km ________mb; ________in
8 and 9 km ________mb; ________in
0 and 10 km ________mb; ________in
10 and 20 km ________mb; ________in
20 and 30 km ________mb; ________in
40 and 50 km ________mb; ________in
60 and 70 km ________mb; ________in 9 and 10 km ________mb; ________in
3.
Using the graph you prepared above approximate the answers to the following (assuming standard atmospheric conditions): a.
Mount Everest’s summit is 8850 m (29035 ft) above sea level. What is the barometric pressure there according to the standard atmosphere? ______________ b.
Mount McKinley, 6194 m (20320 ft); air pressure at the summit? ______________ c.
Mount Whitney, 4418 m (14494 ft); air pressure at the summit? ______________ d.
Yellowstone Lake, Yellowstone N.P., 2356 m (7731 ft); air pressure? ____________ e.
The Petronas Towers I and II, Kuala Lumpur, Malaysia, 452 m (1483 ft); air pressure? ___________ f.
In a commercial airliner taking you from San Francisco to New York at 12000 m (39400 ft), what percentage of atmospheric pressure is below your plane? ____________ i.
What percentage of atmospheric pressure resides above your flight altitude? ___________
4.
Why does atmospheric pressure decrease so rapidly with altitude?
5.
Using a black colored pencil, lightly draw lines connecting identical values of sea level pressure on the surface air pressure map below. These lines, called isobars, do not cross each other. Isobars are usually drawn for every four millibars, using 1000 millibars as the starting point. Therefore, these lines will have values of 1000, 1004,
1008, 1012, 1016, 1020, 1024, etc., or 996, 992, 988, 984, 980, etc.
Procedure
Begin drawing from the 1024 millibars station pressure over Salt Lake City, Utah (highlighted in gray). Draw a line to the next 1024 value located to the northeast. Without lifting your pencil draw a line to the next 1024 value located to the south and then to the one located southwest, finally returning to the Salt Lake City value. Remember, isobars are smooth lines with few, if any, kinks.
The result is an elongated circle, centered approximately over Eastern Utah. The line that was drawn represents the
1024 millibars line and you can expect the pressure to be 1024 millibars everywhere along that line. Repeat the procedure with the next isobar value. Remember, the value between isobars is 4 millibars. Since there are no 1028 millibars values on the map, then your next line will follow the 1020 millibars reports. Then continue with the remaining values until you have all the reports connected with an isobar.
Label each isobar with the appropriate value. Traditionally, only the last two digits are used for labels. For example, the label on the 1024 mb isobar would be 24. A 1008 mb isobar would be labeled 08. A 992 mb isobar will be labeled 92.
These labels can be placed anywhere along the isobar but are typically placed around edges of the map at the end of each line. For closed isobars (lines that connect) a gap is placed in the isobar with the value inserted in the gap.

Isobars can be used to identify "Highs" and "Lows".
* Label the center of the high pressure area with a large blue "H".
* Label the center of the high pressure area with a large red "L".
High pressure regions are usually associated with dry weather because as the air sinks it warms and the moisture evaporates. Low pressure regions usually bring precipitation because when the air rises it cools and the water vapor condenses.
* Shade, in green, the state(s) would you expect to see rain or snow.
* Shade, in yellow, the state(s) would you expect to see clear skies.
* Draw arrows around the "H" on your map to indicate the wind direction.
* Draw arrows around the "L" on your map to indicate the wind direction.