Composition and Properties of the Atmosphere

We humans are designed to live in the conditions we do. As a result we tend not to notice the air we breathe and walk through on a daily basis.

Earth's atmosphere today is composed primarily of:

• Nitrogen (78.1%) - this molecule is what makes our skies look blue.
• Oxygen (20.9%) - Oxygen is, to many organisms (including us), a poison. Too much O₂ and animals sufficate and fires burn out of control. Too little and we sufficate. We need O₂ to be in that "Goldielocks" zone of "just right"!
• Water vapor (as much as 4%) - This amount varies depending upon where you are. Humid areas like rainforests have more, dry areas like deserts have less.

Our atmosphere also has mass. A one sq. cm column of air extending from sea level up to the top of the atmosphere weighs 1.04 kg (2.3 pounds). However, a one sq. ft column of the same height weights more than 1 ton. We don't notice this weight as we are adapted to it.

Air also moves vertically depending upon it's temperature and elevation.

Air at high elevations:
• Cooler
• Expands
• Water vapor tends to condense
Air at sea level:
• Warmer
• More compressed
• Can hold more water vapor

This rising and falling of air due to temperature is called adiabatic heating/cooling