1 Atmospheric Structure and Energy

Atmospheric composition

Earth's atmosphere is the layer of gases held around the planet by gravity. Its composition and vertical structure shape how energy and water move through the air, providing physical foundations for weather.

Dry air near Earth's surface is mostly nitrogen, at about $78\%$, and oxygen, at about $21\%$. Argon makes up nearly $1\%$; carbon dioxide, ozone, methane, and other gases occur in much smaller amounts. Water vapor is also present, but its concentration varies greatly with place and time, from nearly zero in very dry air to several percent in warm, humid air. This variable water vapor is essential to clouds and precipitation.

Atmospheric layers

Atmospheric pressure and density generally decrease with height because gravity pulls most air molecules toward the surface. Scientists commonly divide the atmosphere into layers according to how temperature changes with altitude. Layer boundaries vary in height, so stated altitudes are approximate.

• Troposphere: Extends from the surface to roughly $8\text{–}15\,\mathrm{km}$, depending on location and season. It contains most atmospheric mass and nearly all weather and atmospheric water vapor. Temperature usually decreases with height.

• Stratosphere: Extends above the troposphere to about $50\,\mathrm{km}$. The ozone layer absorbs much of the Sun's harmful ultraviolet radiation; temperature generally increases with height through much of this layer.

• Mesosphere: Extends to about $85\,\mathrm{km}$. Temperature generally decreases with height, reaching very low values near the upper boundary.

• Thermosphere: Lies above the mesosphere. It absorbs some incoming high-energy solar radiation, and temperatures rise sharply even though the air is extremely thin.

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

The tropopause separates the troposphere from the stratosphere. The troposphere contains most atmospheric water vapor and is heated mainly from below, so rising and sinking air there helps produce clouds, winds, and storms. The ozone layer is in the stratosphere, not the troposphere.

Solar heating and energy balance

The Sun supplies most of the energy that drives weather. Solar energy arrives mainly as shortwave radiation. Clouds, the atmosphere, and bright surfaces reflect some of it back to space; the rest is absorbed by the atmosphere or Earth's surface. The warmed surface then emits energy mainly as longwave infrared radiation. Greenhouse gases and clouds absorb and emit some of this infrared energy, affecting how quickly heat escapes to space.

Heating is uneven. Earth's curved surface receives more direct sunlight in some places than others, and the Sun's angle and day length change with latitude, season, and time of day. Land and water also warm and cool at different rates. These contrasts create temperature and pressure differences that help set air in motion. For example, land may warm more quickly than nearby water on a sunny day, encouraging air to rise over land and drawing cooler air inland as a sea breeze.

How heat moves

Heat moves through the atmosphere and between the surface and air in three main ways:

• Radiation transfers energy by electromagnetic waves and can travel through space. Sunlight warms Earth without needing air to carry it.

• Conduction transfers heat through direct contact. The ground can warm the thin layer of air touching it, but air is a poor conductor.

• Convection transfers heat through the movement of a fluid, such as air. Air warmed near the surface becomes less dense and can rise; cooler, denser air moves in to replace it. Rising air expands in lower pressure and cools, while sinking air is compressed and warms. Convection helps form clouds and drives many local and larger-scale atmospheric motions.

The water cycle

The water cycle is the continuous movement of water among Earth's surface, the atmosphere, and subsurface storage; it has no single starting point. Solar energy helps evaporate water from oceans, lakes, and soil. Plants add water vapor through transpiration; together, evaporation and transpiration are called evapotranspiration. Ice and snow can also change directly into vapor by sublimation. Winds transport water vapor through the atmosphere.

When moist air rises, it expands in lower pressure and cools. If it cools enough to reach saturation, water vapor can condense onto tiny particles in the air, forming cloud droplets. Below freezing, ice crystals may also form. Cloud particles grow through collisions and other processes. When they become heavy enough to fall, water returns to the surface as precipitation, such as rain, snow, sleet, or hail. Some water runs off into streams and oceans, some infiltrates the ground, and some is stored as snow, ice, lakes, or groundwater.

Latent heat and cloud formation

Water's phase changes also move energy. Evaporation absorbs energy from the surface and stores it in water vapor, so evaporation can cool the surface. Condensation releases that latent heat into the surrounding air. This added heat can help rising air remain buoyant and strengthen atmospheric convection, linking surface heating, atmospheric motion, clouds, and precipitation.

On a warm, humid afternoon, the Sun can heat the ground, which warms nearby air. If that air rises, it expands and cools. Water vapor may then condense into a cloud; condensation releases latent heat, which can further support rising motion. If cloud particles grow sufficiently, precipitation may fall.