4.1 The Atmosphere

Layers of the atmosphere

The atmosphere has four main layers based on temperature changes. The troposphere (0–12 km) is where weather and most life occurs; temperature decreases with altitude. The stratosphere (12–50 km) contains the ozone layer, which absorbs harmful UV radiation and causes temperature to increase with altitude. Above that are the mesosphere and thermosphere.

Composition of the atmosphere

Earth's atmosphere is about 78% nitrogen (N₂) and 21% oxygen (O₂), with trace gases like argon, carbon dioxide, and water vapor making up the rest. Though present in small amounts, greenhouse gases (CO₂, CH₄, H₂O) strongly influence climate by absorbing infrared radiation.

Atmospheric circulation

Uneven solar heating drives global convection currents that form the three-cell circulation model (Hadley, Ferrel, and Polar cells) in each hemisphere. This circulation, combined with Earth's rotation (Coriolis effect), creates global wind belts and pressure zones that distribute heat and moisture, shaping regional climates and biome distribution.

Key terms

Troposphere

Lowest atmospheric layer (0–12 km); contains most weather and life; temperature decreases with altitude.

Stratosphere

Layer above troposphere (12–50 km) containing the ozone layer, which absorbs UV radiation; temperature increases with altitude.

Atmospheric composition

~78% nitrogen, ~21% oxygen, with trace gases like argon, CO₂, and water vapor.

Coriolis effect

The deflection of moving air and water caused by Earth's rotation, shaping global wind patterns.

Three-cell circulation model

Hadley, Ferrel, and Polar cells in each hemisphere that redistribute heat and moisture around the globe.

Try an AP-style question

In which layer of the atmosphere does almost all weather occur?

1. Mesosphere
2. Troposphere
3. Thermosphere
4. Stratosphere