Summary:
- Air has mass, volume, and, of course, density.
- Density is a measure of how closely molecules are packed together.
- Since air is a gas, the molecules can pack tightly or spread out.
- At higher altitudes, there is less air pushing down from above, and gravity is weaker farther from Earth's center.
- At sea level, the atmosphere presses down with a force of about 1 kilogram per square centimeter (14.76 pounds per square inch) Air pressure is defined as the weight of the air pressing against a given area.