Pressure at any level in the
atmosphere depends on (is determined by) the weight of the air
We used a pile of bricks (each brick represents a layer
to help visualize and understand why pressure decreases with
increasing altitude. A pile of bricks though can lead to the
air pressure exerts force in just a downward direction. A better
representation might be a "people pyramid."
If the bottom person in the stack above were standing on a
scale would measure the total weight of all the people in the
pile. That's analogous to sea level pressure being determined by
the weight of the all the air above.
The bottom person in the
picture above must be strong and be able to push upward with enough
force to support the people above. That is equivalent to the
bottom layer of the
atmosphere having enough pressure, pressure that points up, down, and
sideways, to support the weight of the air above. This is
illustrated further in the following figure
Air pressure is a force that pushes
downward, upward, and
If you fill a balloon with air and then push downward on it, you can
feel the air in the balloon pushing back (pushing upward). You'd
see the air in the balloon pushing sideways as well.
pressure in the four tires on your automobile pushes down on the road
(that's something you would feel if the car ran over your foot) and
with enough force to keep the 1000 or 2000 pound vehicle off the
Air is compressible, so a pile of mattresses might be a more
realistic representation of
layers of air in the atmosphere. We can use mattresses to
understand how air density changes with increasing altitude.
The mattress at the
bottom of the
pile feels the weight of all the mattresses above and is compressed the
most. The mattresses higher up aren't squished as much because
is less weight remaining above. The same is true with layers of
air in the atmosphere.
Here's the same idea applied to the atmosphere:
There's a lot of information in
this figure. It is worth
spending a minute or two looking at it and thinking about it.
1. You can first notice and remember that pressure
with increasing altitude. 1000 mb at the bottom decreases to 700
mb at the top of the picture.
Each layer of air contain the same amount (mass) of air. You
tell because the pressure decrease as you move upward through each
layer is the same (100 mb). Each layer contains 10% of the air in
the atmosphere and has the same weight.
2. The densest air is found in the bottom
layer. That is because each layer has the same amount of air
(same mass). The bottom layer is compressed the most so it has
smallest volume. Mass/( small volume)
gives a high density. The top layer has the same amount of air
but about twice the volume. It therefore has a lower density.
3. You again notice something that we covered earlier: the most
rate of pressure decrease with increasing altitude is in the densest
air in the bottom air layer. It takes almost twice the distance
for pressure to decrease from 800 mb to 700 mb in the top most layer
where the air density is lower.
So far we
have looked at how pressure and air density change with increasing
altitude. In the last part of this lecture we will have a quick
look at how air temperature
The atmosphere can be split
depending on whether
temperature is increasing or decreasing with increasing altitude.
The two lowest layers are shown in the figure above. There are
additional layers (the mesosphere and the thermosphere) above 50 km but
we won't worry about them.
1. We live in
the troposphere. The troposphere is found, on average, between 0
and about 10 km altitude, and is where temperature usually decreases
increasing altitude. [the troposphere is usually a little higher
in the tropics and lower at polar latitudes]
The troposphere contains most of the water vapor
in the atmosphere (the water vapor comes from evaporation of ocean
water) and is
where most of the clouds and weather occurs. The
troposphere can be stable or unstable (tropo means to turn over and
refers to the fact that air can move up and down in the
2a. The thunderstorm shown in
the figure indicates unstable conditions, meaning that strong up and
down air motions are occurring. When the thunderstorm reaches the
top of the troposphere, it runs into the bottom edge of the
stratosphere which is a very stable layer. The
air can't continue to rise into the stratosphere so the cloud
flattens out and forms an anvil (anvil is the name given to the flat
top of the thunderstorm). The
flat anvil top is something
that you can go outside and see and often marks the top of the
2b. The summit of Mt. Everest is a little over 29,000
ft. tall and is
close to the top of the troposphere.
2c. Cruising altitude in a passenger jet is usually between
30,000 and 40,000, near or just above the top of the troposphere, and
at the bottom of the stratosphere.
3. Temperature remains constant between 10 and 20 km
increases with increasing altitude between 20 and 50 km. These
two sections form the stratosphere. The stratosphere is a
very stable air layer. Increasing temperature with increasing
altitude is called an
inversion. This is what makes the stratosphere so stable.
4. A kilometer is one
thousand meters. Since 1 meter is about 3 feet, 10 km is about
30,000 feet. There are 5280 feet in a mile so this is about 6
is usually close enough in this class).
5. Sunlight is a mixture of ultraviolet (7%),
visible (44%), and
infrared light (49%). We can see the visible light.
5a. On average about 50% of the sunlight
arriving at the top of
the atmosphere passes through the atmosphere and is absorbed at the
ground (20% is absorbed by gases in the air, 30% is reflected back into
space). This warms the ground. The air in contact with the
ground is warmer than air just above. As you get further and
further from the warm ground,
temperature decreases with increasing altitude in the
5b. How do you explain increasing temperature with
altitude in the stratosphere?
The ozone layer is found in the stratosphere
(peak concentrations are found near 25 km altitude). Absorption
ultraviolet light by ozone warms the air in the stratosphere and
explains why the air can warm. The air in the stratosphere is
much less dense (thinner) than in the troposphere. So even though
there is not very much UV light in sunlight, it doesn't
take as much energy to warm this thin air as it would to warm denser
air closer to the ground.
6. That's a manned
and Paul Kipfer are
inside. In 1926 they made the first trip into the
stratosphere. It really was quite a daring trip at the
time at the
and they very
nearly didn't survive it.