Fluid Pressure

1. The atmosphere is a thin layer of gasses that surrounds Earth, and held in place by gravity.

The atmosphere is very thin compared to the rest of Earth.

2. The atmosphere is a mixture of gasses, including 78% nitrogen, 21% oxygen, 0.93% argon, 0.04% carbon dioxide, water vapor, and others.

The air we breathe isn’t all oxygen. It isn’t even

mostly oxygen.

3. Air pressure refers to the weight of the atmosphere pushing down on everything at the surface. Air pressure is not the same everywhere in the atmosphere.

Does air weigh anything? Let’s find out.

4. We currently have 250 miles of gasses pushing on us in the form of the atmosphere, about 15 pounds per square inch.

Hold out your hand. The 75 miles of air above you is pushing down on your hand with the weight of a dishwasher.

Why aren’t you crushed by the weight of the atmosphere?

5. The atmosphere is a fluid. Here are some rules all fluids follow:

a. A fluid is any substance that flows. That includes all liquids and gasses.

Liquids flow Gasses flow

You are surrounded by air the same way you would be surrounded by water if you were at the bottom of a swimming pool.

b. As you go deeper in a fluid, pressure increases. As you go higher in a fluid, pressure decreases.

This spiny anglerfish is immune to the pressure of the deep. But if you bring it to the surface, it dies from the pressure difference.

Where is the pressure higher? Where is it lower?

Death Valley is the lowest spot on Earth, at 282 feet below sea level

c. Low air pressure is associated with storms, tornadoes and hurricanes. High pressure is associated with sunny, dry conditions.

High pressure, 1020 millibars

Low pressure, 960 millibars

6. We measure air pressure with a barometer.

Aneroid barometer Mercury barometer

7. Most weather is caused by the air trying to equalize pressure. Warm air (less dense) has low pressure and cold air (more dense) has high pressure.