Pushy Pressure

Pressure is the force of one object pushing on another. In the case of the air around you, it is the force of all the air molecules hitting your body. When you are standing on the ground, the pressure is the weight of all the air above you (all the way to the edge of the atmosphere) pushing down on you. Mind you, it doesn't just push down. All of those molecules are pushing sideways, up, diagonally, and every way imaginable.

Think about the desk your computer is on. If the force of the air were just down, it would probably collapse. Because the air also pushes up, the desk is able to stay in one piece and not collapse under the weight of your books. Think about when you go swimming. There is pressure all around you from the water. That pressure is from the water molecules around you and the air above the surface. The deeper you go, the more pressure, and you have to pop your ears.

A Little About Gases

The key to understanding pressure in the atmosphere is to understand how gases work. You can read about gases and how pressure can increase and decrease when forces change (like temperature and volume). We're also adding on the idea of density. Density is the amount of a substance in a specific area. Water has a greater density than ice, which has a greater density than water vapor. When you decrease the volume of a container (and keep the same amount of matter) you will increase the pressure. If you increase the temperature of a container, you will increase the pressure.

Water is a special case where the solid is actually less dense than the liquid form. Ice floats at the top of your soda because it is less dense than the surrounding liquid. The solid version of most compounds is more dense than the liquid version. Liquid states are always more dense that the gas state (under normal conditions).

Real World Explanations

So you have a hot day. Chances are the pressure will rise when it gets hotter. The molecules are getting more excited and have nowhere to go. They wind up pushing on everything with a greater force. Let's say you're up in the sky. There is less pressure because there are fewer molecules above you pushing on you.

That idea explains why the pressure is lower in Colorado than it is on a beach in California. Colorado has a higher altitude. When do you get the greatest pressure? On a hot day? No. Really cold days actually have a higher atmospheric pressure. Why? As the temperature drops, the molecules of the air around you begin to condense and are less excited. These compressed molecules actually create a greater pressure than the excited and hot ones on a warm day. You will be able to prove this fact if you visit Siberia, Russia. Go figure.

Up Up And Away

Let's talk about the very top of the atmosphere. As you move higher above the ground, the temperature will drop and then begin to rise again. The pressure continually decreases. How? There is a lot more energy hitting the Earth from the Sun as you move closer to the outer edge of the atmosphere. This extra energy causes the molecules to get excited and the temperature goes up. As the temperature increases, there is no pressure pushing on the molecules (like on the surface of the Earth) so they can spread out as much as they want. Only gravity pulls on them. They spread out so much that there is actually less pressure than on a place the same temperature lower in the atmosphere.

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