1 The Atmosphere and Its Energy

Atmospheric Composition

Earth’s atmosphere is a thin envelope of gases held close to the planet by gravity. It supplies air and water vapor, moderates Earth’s temperature, and transports energy around the globe. Air is densest near the surface and becomes thinner with altitude.

Composition of air

Dry air near Earth’s surface is mostly nitrogen, at about $78\%$, and oxygen, at about $21\%$. Argon makes up just under $1\%$, while carbon dioxide and other trace gases occur in much smaller amounts. Water vapor varies considerably: it may be nearly absent in cold, dry air and may reach several percent in warm, humid air. Although scarce, some trace gases—especially water vapor and carbon dioxide—absorb outgoing infrared energy and influence Earth’s energy balance.

Atmospheric Layers

Scientists commonly divide the atmosphere into layers according to how temperature changes with height. The boundaries are approximate and vary with latitude and season.

• Troposphere: Extends from the surface to about $8\text{–}15\ \text{km}$. It contains most of the atmosphere’s mass and nearly all weather. Temperature generally decreases with height.

• Stratosphere: Extends above the troposphere to about $50\ \text{km}$. Ozone absorbs ultraviolet radiation, so temperature generally increases with height.

• Mesosphere: Extends from about $50$ to $80\ \text{km}$. Temperature generally decreases with height, and many meteors burn up here.

• Thermosphere: Begins roughly $80\ \text{km}$ above Earth and extends upward for several hundred kilometers. Sparse gases absorb energetic solar radiation, and temperatures increase with height. Auroras occur in this region.

• Exosphere: The extremely thin outer region, which gradually merges into space.

The Ionosphere is a region of electrically charged particles formed largely by solar radiation. It overlaps parts of the mesosphere and thermosphere; it is not a separate temperature layer.

Solar Energy and Earth’s Energy Balance

The Sun provides the main energy that drives the atmosphere. Incoming sunlight is mostly shortwave radiation, including visible light and ultraviolet. Clouds, aerosols, and bright surfaces reflect some sunlight back to space; gases and particles absorb some; and much of it passes through the air to warm land and ocean.

Earth’s warmed surface emits energy mainly as longwave infrared radiation. Greenhouse gases and clouds absorb and emit some of this infrared energy. This warms the lower atmosphere and surface while also allowing energy to escape to space. Over long periods, Earth’s temperature depends on the balance between absorbed solar energy and energy returned to space.

Why heating is uneven

Sunlight is more concentrated near the equator because Earth is curved, while it arrives at a lower angle near the poles. Earth’s axial tilt changes the angle and duration of sunlight through the seasons. Land and ocean also warm and cool at different rates. These contrasts create temperature differences, which help set the atmosphere in motion.

How Atmospheric Circulation Develops

Atmospheric circulation is the large-scale movement of air that redistributes heat and moisture. One example is convection: air warmed at the surface expands, becomes less dense, and rises. Cooler, denser air sinks and flows in to replace it. This transfers energy vertically, both in small rising air currents and on the larger scale of global circulation.

Horizontal temperature differences create pressure differences. Air tends to move from higher pressure toward lower pressure; this pressure-gradient force helps produce wind. Earth’s rotation deflects large-scale moving air through the Coriolis effect: air turns to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. Consequently, global circulation is organized into broad wind belts and circulation cells rather than one simple flow from equator to pole.

Global Circulation Cells and Heat Transport

In the idealized three-cell model, each hemisphere has three broad circulation zones:

• Hadley cell (tropics): Strong heating near the equator causes air to rise. Aloft, the air moves poleward, cools, and sinks in the subtropics. Near the surface, air flows back toward the equator; the Coriolis effect turns this return flow into the trade winds.

• Ferrel cell (midlatitudes): Between the tropics and polar regions, prevailing westerly winds are part of a more complex circulation shaped by neighboring cells and moving weather systems.

• Polar cell: Cold air tends to sink over the poles and flow toward lower latitudes near the surface. Earth’s rotation helps produce polar easterly winds.

These cells are a simplified average, not rigid loops. Their boundaries and strengths shift with the seasons, and actual winds vary with continents, oceans, and weather.

Moving energy through water

Solar heating evaporates water. Winds transport water vapor, and condensation releases latent heat into the air. Together with ocean circulation, these processes move energy from warmer regions toward cooler ones and help shape climate.