The Atmosphere

The atmosphere is the blanket of gases wrapped around Earth, held in place by gravity. It gives you air to breathe, blocks harmful radiation, burns up meteors, and keeps the planet warm enough to live on. Without it, daytime would hit over 100 °C and nights would drop below -150 °C. Basically, game over.

It's also surprisingly thin. If Earth were an apple, the breathable part of the atmosphere would be thinner than the skin.

Dry air is a mix of gases:

• Nitrogen: about 78%
• Oxygen: about 21%
• Argon: almost 1%
• Carbon dioxide: about 0.04%. Tiny amount, huge impact on climate.

Air also holds water vapor (from 0 to 4%, depending on where you are) and tiny solid bits called aerosols: dust, smoke, salt, and pollen. Wildfire smoke that fills Idaho skies in late summer is made of aerosols.

Air has weight. The whole column of air above you presses down with about 14.7 pounds on every square inch. That's air pressure (also called atmospheric pressure). You don't get crushed because the air inside your body pushes back.

Air pressure is measured with a barometer. As you go higher, there's less air above you, so pressure drops. That's why your ears pop driving up to a ski resort and why a sealed bag of chips puffs up in the mountains.

Idaho Falls sits at about 4,700 feet. The air here is already about 15% thinner than at sea level. Athletes from the coast notice.

Scientists divide the atmosphere into layers based on how temperature changes with height:

1. Troposphere (0 to about 12 km): where we live and where all weather happens. It holds about 75% of the air's mass. Temperature drops as you go up, about 6.5 °C per kilometer. That's why mountaintops have snow.
2. Stratosphere (12 to 50 km): home of the ozone layer, which absorbs harmful ultraviolet (UV) radiation from the Sun. Because ozone absorbs that energy, temperature rises with height here. Jets often fly in the lower stratosphere to avoid weather.
3. Mesosphere (50 to 85 km): the coldest layer, down to about -90 °C. Meteors burn up here as shooting stars.
4. Thermosphere (85 to about 600 km): the air is super thin, but the few particles are blazing hot. The northern lights happen here, and so does the International Space Station's orbit.
5. Exosphere: the outer edge, where the atmosphere fades into space.

The boundary between the troposphere and stratosphere is called the tropopause.

Ozone is a form of oxygen with three atoms (O₃) instead of two. In the stratosphere it acts like sunscreen for the whole planet. In the 1980s, scientists discovered a growing "ozone hole" over Antarctica, caused by chemicals called CFCs used in spray cans and refrigerators. In 1987, almost every country signed the Montreal Protocol to ban them. The ozone layer is slowly healing, one of science's big success stories.

The Sun heats Earth, and that heat moves in three ways:

• Radiation: energy traveling as waves, like sunlight warming your face. No contact needed.
• Conduction: heat moving by direct touch, like a hot sidewalk warming the air right above it.
• Convection: warm air rises because it's less dense, and cool air sinks to replace it, forming currents. This builds clouds and drives wind.

About 30% of sunlight bounces right back to space off clouds, ice, and snow. How reflective a surface is, is called its albedo. Fresh snow has a high albedo; dark pavement has a low albedo.

Earth's surface absorbs sunlight and gives off heat (infrared energy). Greenhouse gases like water vapor, carbon dioxide, and methane absorb some of that heat and send it back down, keeping Earth warm. This natural greenhouse effect keeps the planet's average around 15 °C. Without it, Earth would average about -18 °C, a frozen snowball. More greenhouse gases trap more heat, which you'll explore in Chapter 22.

Wind is air moving from areas of high pressure to areas of low pressure. Uneven heating causes the pressure differences. Warm air rises and creates low pressure; cool air sinks and creates high pressure.

• Local winds: like sea breezes and mountain and valley breezes. In Idaho's valleys, warm air flows up the mountains during the day and cool air drains down at night.
• Global winds: huge belts like the trade winds near the equator and the prevailing westerlies over the U.S., bent by the Coriolis effect. The westerlies are why most of Idaho's weather arrives from the west.
• Jet streams: fast rivers of wind high in the troposphere, which steer storms across the country.