Atmospheric Pressure: How and why it changes

A graph is a picture. A picture is worth a thousand words.

One of the best ways to learn about atmospheric pressure on Earth is to create your own graph. A graph [Math Standard: using graphs to illustrate the relationship between two concepts.] will illustrate the relationship between air pressure and altitude.

Graph X and Y Axis Terminology

Several terms that you will be using for your graph are:

1. X-axis: mmHg - millimeters of mercury is the unit of measure used by NASA to talk about air pressure.

2. Y-axis: Altitude is the height in feet above the surface of the Earth at sea level.

Steps to create your Altitude/Air Pressure Graph

To create your graph, follow these steps. As you proceed you will discover how air pressure changes with altitude.

1. Create a Table.

a) Create a table with three columns headed by:
Column 1) Fact #.
Column 2) Atmospheric Pressure (mmHg).
Column 3) Altitude.
b) The title of the Table is "Altitude and Atmospheric Pressure."
c) Use the FACTS below to fill in the table.
d) Example:

Fact # The Altitude (Y-axis) The Atmospheric Pressure (mmHg) (X-axis
1. 0 760
2. 5000 632
3.


e) Complete this table using the facts below.

Facts:
#
1) At sea level, or 0' altitude, the atmospheric pressure = 760 mmHg at STP.
2) At an altitude of 5,000', the atmospheric pressure = 632 mmHg.
3) At an altitude of 6,288', the atmospheric pressure = 605 mmHg
4) At an altitude of 10,000', the atmospheric pressure = 523 mmHg.
5) At an altitude of 12,000', the atmospheric pressure = 485 mmHg.
6) At an altitude of 15,000', the atmospheric pressure = 429 mmHg.
7) At an altitude of 18,000', the atmospheric pressure = 50% of sea level at STP. (Calculate the atmospheric pressure.)
8) At an altitude of 29,028', the atmospheric pressure = 215 mm Hg
9) At an altitude of 35,200', the atmospheric pressure is 25% of sea level at STP. (Calculate the atmospheric pressure.)
10) At an altitude of 40,000', the atmospheric pressure is 140 mmHg.
11) At an altitude of 50,000, the atmospheric pressure is 87 mm Hg.
12) At an altitude of 62,000', the atmospheric pressure = 51.71 mm Hg.
13) At an altitude of 65,000', the atmospheric pressure = 45 mm Hg.
14) At 110,000' altitude, the atmospheric pressure = 7.6 mm Hg.

2. Create a Graph

a. On 8 ½" x 11" graph paper, draw an x and y-axis and label both as described. Entitle the graph, "Atmospheric Pressure Vs. Altitude."

1) Draw the x-axis, or the horizontal axis, along the 11" side of the graph paper. Begin the x-axis three squares from both the left edge and "bottom" of the paper. Label the x-axis Atmospheric Pressure in mmHg. Number the x-axis from 0 to 800 in increments of 50 (0, 50, 100, 150, etc. to 800). The x-axis represents atmospheric pressure from 0 mmHg to 800 mmHg.

2) Create the y-axis by drawing a vertical line from the left edge of the x-axis to the top of the graph paper. Label the y-axis, or the vertical axis, Altitude in thousands of feet (000) and number it from 0 to 130 in increments of 10 (0, 10, 20, 30, etc. to 130). The three "0's in parentheses in your y-axis label mean that every increment of 10 on your y-axis represents 10,000 feet. The y- axis represents the altitude from 0 feet, or sea level, to 130,000 feet, where there is virtually no air molecules left to create a measurable pressure.

(Author's note: It may be desirable to insert at this point a sample of the graph with axes and labels.)

b. Use a pencil and create a graph from the information in your Table. Use a Pencil!

c. After you have plotted the points on your graph, connect them. The line through the points represents the relationship between atmospheric pressure and altitude.

d. Remember, all these air pressure measurements might be different if we were not working at STP. It gets much colder that 59 Fahrenheit as we go higher in space. The air pressure inside a hurricane can cause the atmospheric pressure to drop as much as 50mmHg or more. Your curve, however, gives you a good picture of how air pressure decreases as altitude increases. This relationship is an excellent example of what scientists call an "inverse relationship," meaning when one variable increases, the other decreases.

3. In class tomorrow you will be able to check your graphing skills. You can compare your graph to an annotated version of the same graph. This version, called Graph B will include various physical features of the Earth's surface and notes regarding mankind's relationship to atmospheric pressure.