7 The Atmosphere, Weather, and Climate

Atmospheric composition and layers

Earth’s atmosphere is a mixture of gases held near the planet by gravity. Dry air consists mostly of nitrogen and oxygen, with argon and much smaller amounts of carbon dioxide and other trace gases. Water vapor varies considerably: it may be nearly absent in very dry air or make up several percent of warm, humid air. Even at low concentrations, water vapor, carbon dioxide, methane, and other greenhouse gases influence how the atmosphere absorbs and emits heat.

Air pressure and density decrease with altitude. The atmosphere’s layers are identified by how temperature changes as altitude increases:

• Troposphere: The lowest layer. Most atmospheric mass and water vapor are here, and nearly all weather develops in this layer. Temperature generally decreases with altitude.

• Stratosphere: Above the troposphere. Its ozone layer absorbs much of the Sun’s ultraviolet radiation, and temperature generally increases with altitude.

• Mesosphere: Above the stratosphere, where temperature generally decreases with altitude.

• Thermosphere: A very thin upper layer where gases absorb energetic solar radiation and temperature rises with altitude.

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

Layer boundaries shift, so their altitudes are approximate. The atmosphere’s composition and structure provide the setting for energy transfer and weather.

Energy balance and heat transfer

The Sun supplies most of the energy that drives Earth’s weather and climate. Because Earth is curved, sunlight is concentrated over a smaller area near the equator and spread across a larger area near the poles. A surface also absorbs more energy when the Sun is high in the sky than when sunlight arrives at a low angle.

Clouds, airborne particles, ice, and bright surfaces reflect some incoming sunlight back to space. The rest is absorbed by the atmosphere, land, and oceans. Earth then emits energy as infrared radiation. Greenhouse gases absorb some of this outgoing radiation and emit infrared energy in all directions. This natural greenhouse effect keeps Earth warmer than it would be without those gases.

Over long periods, global temperature is relatively stable when energy entering the Earth system balances energy leaving it. A sustained imbalance changes the heat stored by the planet and, in turn, its climate. Because heating is uneven, the atmosphere and oceans transport energy from warmer regions toward cooler ones. The surface warms nearby air, rising air carries energy upward, and evaporation moves energy into water vapor. When water vapor condenses to form clouds and precipitation, it releases heat.

Takeaway: Earth’s energy balance connects incoming sunlight, reflected energy, outgoing infrared radiation, and the movement of heat through the atmosphere and oceans.

Weather and the moving atmosphere

Weather describes atmospheric conditions over a short period at a particular place. It includes temperature, air pressure, humidity, wind, cloud cover, and precipitation. These conditions change as air moves and interacts with Earth’s surface.

Uneven heating creates differences in air temperature and pressure. Warm air tends to rise, while cooler, denser air tends to sink. Air moving from areas of high pressure toward areas of lower pressure produces wind. Earth’s rotation deflects large-scale winds; this is called the Coriolis effect. As a result, global wind patterns do not flow simply north or south.

Water also cycles between the surface and the atmosphere. Evaporation adds water vapor to the air. As air cools, water vapor can condense into cloud droplets or ice crystals. If these grow heavy enough, they fall as precipitation, such as rain or snow. Rising air often cools and forms clouds, while sinking air often warms and becomes drier.

Takeaway: Differences in heating and pressure set air in motion, while changes in water’s state help form clouds and precipitation.

Climate patterns and their causes

Climate is the long-term pattern of weather in a region, commonly described using averages and variations over about 30 years. It includes typical precipitation, seasons, and winds, as well as the frequency of extreme conditions—not only average temperature. A single cold or rainy day does not by itself define a region’s climate.

Several factors interact to shape climate:

• Latitude and seasons: Low latitudes receive more direct sunlight on average. Earth’s tilted axis changes the angle and duration of sunlight over the year, producing seasons, especially at middle and high latitudes.

• Global circulation: Broad atmospheric circulation helps move heat and moisture. Warm air rises near the equator and generally sinks in the subtropics, contributing to wet tropical regions and many dry subtropical regions. Seasonal shifts in tropical winds can bring wet and dry monsoon seasons.

• Oceans and currents: Water heats and cools more slowly than land, moderating temperatures near coasts. Ocean currents move heat and affect the moisture available to the atmosphere.

• Elevation and mountains: Higher elevations are generally cooler. Mountains can force moist air upward, bringing precipitation to the windward side. Air descending on the leeward side is often drier, creating a rain shadow.

• Large-scale ocean–atmosphere patterns: El Niño and La Niña alter winds and ocean temperatures in the tropical Pacific, shifting weather and precipitation patterns in many regions.

Two places at similar latitudes can have different climates if one is beside the ocean and the other is inland or behind a mountain range. Climate results from the combined effects of sunlight, circulation, oceans, elevation, and local geography.

Takeaway: Climate is a long-term pattern shaped by interacting global processes and local conditions; it cannot be inferred from a single day’s weather.