Although we are not aware of it, we live at the bottom of an ocean of air. The atmosphere of our planet is pressing down on us all the time. On earth at sea level the pressure is 14.7 pounds per square inch. Living things on earth are adapted to this, and breathe well at this pressure. When we climb up mountains there is less air pressure at higher altitudes, so we pant and puff, trying to get enough oxygen into our lungs.

     Air pressure is the weight of the atmosphere directly above us, pushing down on our heads and bodies. The air pushes sideways, too, at the same pressure. The more molecules of air there are, the more weight presses on us. On earth we define 14.7 pounds per square inch as one atmosphere. Our bodies are adapted to this level of pressure. If we were put into an airless environment, our bodies would explode. This happens to deep sea fish when we pull them up from the ocean bottoms, and subject them to a drastic change of pressure..

     It is possible to compress air to higher pressures. When you put air into your car tires it is at perhaps 30 pounds per square inch. The molecules of air inside the tire push on the wall, keeping the tire from going flat. We put higher air pressures in spray cans so that the contents will fly out when we press the button.

This is how air pressure works.

The pressure is affected by two things:

• how much atmosphere there is, and
• the mass of the planet, which determines the gravity that pulls the air down.

This pile of books illustrates the pressure effect: the more books there are, the more pressure there is on the ones at the bottom. The ones at the top feel very little pressure.

     Not all planets have the same air pressure. Venus has about the same mass as the earth, but it has many more gas molecules in its atmosphere. At ground level, the atmospheric pressure of Venus is 90 atmospheres:

90 atmospheres times 14.7 pounds per square inch = 1323 pounds per square inch.

This is air pressure of more than half a ton. Our bodies would be crushed under so much weight.

     On Mars the atmospheric pressure is less than on earth. Mars is a smaller planet, and so has less mass and less gravitational pull. Mars also has a very thin atmosphere. The air pressure on the surface of Mars is less than 1/100 of the pressure on earth. So, let's do the math:

14.7 pounds per square inch times 1/100 =

14.7 divided by 100 = .147 pounds per square inch

There is no way that we could breathe this atmosphere and live.

Sea Level 14.7 pounds per square inch

10,000 Feet 10.2 pounds per square inch

20,000 Feet 5.7 pounds per square inch

30,000 Feet 4.5 pounds per square inch

40,000 Feet 2.8 pounds per square inch

The higher you go, the less air is pressing down and sideways on you!

Mountaineers call the area above 20,000 feet "The Death Zone" because humans cannot get enough oxygen to live for long at this elevation. Although it is possible to survive for some days, the brain suffers damage from oxygen starvation.