Lecture 5 - Mass, weight, density, and pressure pt. 2

Pressure at any level in the atmosphere depends on (is determined by) the weight of the air overhead.

We used a pile of bricks (each brick represents a layer of air) to help visualize and understand why pressure decreases with increasing altitude. A pile of bricks though can lead to the belief that 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, the 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 sideways. 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.

The air 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 pushes upward with enough force to keep the 1000 or 2000 pound vehicle off the road.

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 their 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 decreases 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 can 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 the 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 rapid 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 changes with altitude.

The atmosphere can be split into layers 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 with 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 troposphere).

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 troposphere.

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 and then 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 miles (about 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, the air is colder and colder. This explains why air temperature decreases with increasing altitude in the troposphere.

5b. How do you explain increasing temperature with increasing altitude in the stratosphere?

The ozone layer is found in the stratosphere (peak concentrations are found near 25 km altitude). Absorption of 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 balloon; Auguste Piccard 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 time, and they very nearly didn't survive it.