A thin shell of gas
Earth’s atmosphere is a layer of gas held in place by gravity. It feels enormous from the ground, but compared with the size of the planet it is very thin. Most of the air by mass lies within about the lowest 15 to 20 kilometers, a tiny fraction of Earth’s radius of roughly 6,400 kilometers. There is no sharp upper edge; the air simply thins out gradually into space.
Despite its thinness, this shell does several jobs at once. It supplies the oxygen we breathe, holds in enough warmth to keep water liquid, absorbs harmful ultraviolet light, burns up most incoming small meteors, and carries water around the globe.
What the air is made of
Dry air near the surface is roughly 78 percent nitrogen and 21 percent oxygen by volume, with about 1 percent argon. The remaining fraction includes carbon dioxide, which is only around 0.04 percent but has an outsized effect on temperature, along with traces of neon, helium, methane and other gases. Water vapor varies widely, from almost none in cold deserts to a few percent in humid tropical air.
The mixture is kept fairly uniform by mixing in the lower atmosphere, so the proportions of nitrogen and oxygen are similar at the top of a mountain and on a beach. What changes with height is how much air there is, not what it is made of.
Air pressure
Air has weight. Each square centimeter of surface at sea level has the weight of a column of air above it pressing down, an average of about 1 kilogram’s worth. Standard sea-level pressure is about 101,300 pascals, or 1,013 hectopascals, which weather reports often quote in millibars or hectopascals.
Pressure falls as you climb because there is less air above you. The fall is not linear: it is steepest near the ground. Roughly half of the atmosphere’s mass lies below an altitude of about 5.5 kilometers, which is why climbers at great heights often need supplemental oxygen. The percentage of oxygen is the same, but each breath contains fewer molecules.
Pressure differences from place to place also drive wind, as air moves from higher to lower pressure, a theme that sits at the heart of weather systems.
The atmospheric layers
Scientists divide the atmosphere into layers mainly by how temperature changes with height. The atmospheric layers, from the ground up, are:
- Troposphere. The lowest layer, where we live and where nearly all weather occurs. It extends to roughly 8 kilometers over the poles and perhaps 15 to 18 kilometers over the tropics. Temperature generally falls with altitude here, which is why mountaintops are cold.
- Stratosphere. Above the troposphere, extending to about 50 kilometers. Temperature rises with height because the ozone layer absorbs ultraviolet radiation from the Sun. Commercial jets often cruise near its lower boundary, where the air is calmer.
- Mesosphere. From about 50 to 85 kilometers. Temperatures drop again, reaching some of the coldest in the atmosphere. Many meteors burn up here.
- Thermosphere. From about 85 kilometers to several hundred kilometers. Very little air is present, and absorption of intense solar radiation makes the temperature of the sparse gas very high, although it would not feel hot to a person because there are so few particles to transfer heat. The International Space Station orbits in this region, and auroras occur here.
- Exosphere. The outermost region, where air gradually fades into space.
The boundaries between layers have their own names, such as the tropopause and stratopause. A commonly used convention places the edge of space at 100 kilometers, known as the Karman line, though the choice is partly a matter of definition rather than a physical wall.
The ozone layer and ultraviolet light
Most of the atmosphere’s ozone is located in the stratosphere. Ozone molecules absorb much of the Sun’s high-energy ultraviolet radiation, which can damage living tissue. International agreements to phase out chemicals that deplete ozone have been widely credited with helping the layer recover, though it is a gradual process.
Ozone behaves differently at ground level, where it is a pollutant that can irritate the lungs. The same molecule is helpful high up and harmful near us, which is a good example of how location changes the story. Other ground-level air pollutants, such as particles and nitrogen oxides, are a separate and important health topic.
Heat, greenhouse gases and temperature
The atmosphere lets much of the incoming visible sunlight reach the surface. The warmed surface then radiates infrared energy back upward, and certain gases, including water vapor, carbon dioxide and methane, absorb part of it and re-emit it in all directions. This slows the loss of heat to space and is the natural greenhouse effect. Without it, Earth’s average surface temperature would be well below freezing. Changes in the amounts of those gases influence the balance, which is the subject of work on climate change drivers.
A look at Venus atmosphere, with its extremely dense carbon dioxide, shows how strongly the composition and thickness of an atmosphere can shape a planet’s surface conditions.
Why the atmosphere matters for life
- Breathable oxygen. Oxygen in the air is largely a product of living things, especially photosynthesizing organisms, accumulated over a very long time.
- Protection. Along with the ozone layer, the atmosphere cushions the surface from meteoroids and absorbs some harmful radiation. Another protector, the planet’s magnetic field, works at a larger scale and deflects charged particles from the Sun.
- Climate stability. The atmosphere redistributes heat from the tropics toward the poles through winds and storms.
- Water supply. Evaporation, condensation and precipitation move fresh water across land, a process described by the water cycle.
The atmosphere is a fluid
It helps to think of air as a fluid, like a very light ocean. It flows, rises when heated, sinks when cooled and carries energy and moisture. Many everyday phenomena, from sea breezes to jet streams, are simply consequences of uneven heating of this fluid and of Earth’s rotation. That is why understanding the layers and pressure is the first step toward understanding almost everything else about weather and climate.