**Document ID**: 01_06_97_1

**Date Received**: 1997-01-06 **Date Revised**: 03-17-97 **Date
Accepted**: 03-17-97

**Curriculum Topic Benchmarks**: S1.3.2, S1.3.2, S1.2.4

**Grade Level**: \[6-8\] Middle School

**Subject Keywords**: scale, atmosphere

**Rating**: moderate

**How Thick is the Atmosphere?**

**By**: Martin J Ruzek, Universities Space Research Association, 8426
Polifka Road,

Whitelaw WI 54247 e-mail: ruzek@usra.edu

**From**: The PUMAS Collection http://pumas.jpl.nasa.gov

©1997, California Institute of Technology. ALL RIGHTS RESERVED. Based on
U.S. Gov\'t sponsored research

We\'ve all seen pictures of the Earth and its atmosphere as a series of
concentric circles, showing the troposphere, stratosphere, ionosphere,
etc., often looking like a bullseye target with the solid Earth in the
middle. But if we were to draw that picture to scale, what would it
really look like?

Let\'s start with the solid Earth, which has a radius at the equator of
about 6378 kilometers (or 3963 miles). With a compass (or a computer)
draw a circle scaled to fit on a single sheet of paper to represent the
Earth. If a distance of 1 cm on your paper equals 1000 km on Earth, then
the Earth is drawn as a circle with a radius of 6.378 cm

The atmosphere is interesting stuff. One of the things it happens to do
is get thinner the higher you go. At sea level, the atmosphere pushes
down with a weight of about 1 kg per square centimeter (or about 14.7
pounds per square inch). As you move up into the atmosphere, while you
are climbing a mountain or riding in an airplane for example, the
atmospheric pressure begins to decrease. When you reach an altitude of
about 5,500 meters (about 18,000 feet), the air pressure is only about
one half of what it was at sea level, which means that about half of the
Earth\'s atmosphere is already below you. Does that mean that you will
pass through the other half of the atmosphere and be in the vacuum of
space if you rise another 5,500 meters? Fortunately, no, since most
commercial passenger jets fly at an altitude of 11 - 13,000 meters.
There is still enough air at that altitude to keep jet engines running
and to flow over the wings to create enough lift to keep the plane
aloft. But the air pressure at 11,000 meters is only about one quarter
of that at sea level. In other words, for roughly each 5.5 km you
ascend, the atmospheric pressure halves. By the time you have reached an
altitude of 30 km (on your way into orbit aboard the space shuttle, for
instance), you have risen above 99% of the Earth\'s atmosphere, and by
100 km, 99.9999% of the atmosphere is below you.

So, lets see where this \"top\" of the atmosphere goes on your scale
drawing. (It\'s really not the top, since molecules of air continue to
extend into space, but virtually all of the atmosphere will lie below
this line) If your drawing uses the scale of 1 cm = 1000 km, you need to
add another circle with a radius just 1 mm larger than the circle
representing the Earth to show 100 km altitude.

I think you will be able to see from your drawing just how thin the
Earth\'s atmosphere is. The Earth is a big place, but the atmosphere
that supports life, creates our weather and keeps us protected from
harmful radiation is just a small portion of the planet we live on.

**Other activities**:

Make a graph plotting atmospheric pressure along the horizontal axis,
and altitude along the vertical axis. Does the atmosphere ever go away
completely?

Increase the scale of your drawing to 1 cm = 100 km or even 1 cm = 10 km
(you may need to use a piece of string to scale the Earth\'s radius if
you want to draw the curve of the surface to scale). Now you have room
to draw some other features within the atmosphere, such as:

Mt. Everest, the highest point on Earth, rises 8,848 meters above sea
level; the altitude where many commercial jets travel (11-13 km); the
top of the troposphere, below which most weather occurs (17 km); the
altitude of the ozone layer (10 - 40 km); the orbital altitude of the
space shuttle (225 - 400 km); the orbital altitude of geosynchronous
weather satellites (35,786 km); the altitude where most meteors burn up
as they enter the atmosphere (50 - 100 km)