Atmosphere of Earth

The atmosphere of Earth consists of a layer of mixed gas (commonly referred to as air) that is retained by gravity, surrounding the Earth's surface. It contains variable quantities of suspended aerosols and particulates that create weather features such as clouds and hazes. The atmosphere serves as a protective buffer between the Earth's surface and outer space. It shields the surface from most meteoroids and ultraviolet solar radiation, reduces diurnal temperature variation – the temperature extremes between day and night, and keeps it warm through heat retention via the greenhouse effect.

Earth's atmosphere from space, showing a blue layer at the stratosphere, above the clouds of the troposphere. The Moon is visible as a crescent in the background.

By mole fraction, dry air contains 78.08% nitrogen, 20.95% oxygen, 0.93% argon, 0.04% carbon dioxide, and small amounts of other trace gases. Air also contains a variable amount of water vapor, on average around 1% at sea level.

The atmosphere has a mass of about 5.15×10¹⁸ kg, three quarters of which is within about 11 km (6.8 mi) of the surface. The atmosphere becomes thinner with increasing altitude, with no definite boundary between the atmosphere and outer space. The Kármán line at 100 km (62 mi) is often used as a conventional definition of the edge of space. Several layers can be distinguished in the atmosphere based on characteristics such as temperature and composition, namely the troposphere, stratosphere, mesosphere, thermosphere, and exosphere.

Composition

Main article: Atmospheric chemistry

Earth's atmosphere is mostly made up of three gases: nitrogen, oxygen, and argon. There is also a small amount of water vapor, which changes depending on where you are and the weather. Other gases, called trace gases, include carbon dioxide, methane, and ozone.

The air also contains tiny particles like dust, pollen, and sea spray. Sometimes, pollution from human activities can add other substances to the air. Up to about 100 kilometers above Earth, these gases mix together evenly. Higher up, the gases start to separate, with lighter gases floating higher.

Major constituents of air^: 258
Dry air
Gas		Volume fraction^(A)		Mass fraction
Name	Formula	in ppm^(B)	in %	in ppm	in %
Nitrogen	N₂	780,840	78.084	755,200	75.52
Oxygen	O₂	209,460	20.946	231,400	23.14
Argon	Ar	9,340	0.9340	12,900	1.29
Carbon dioxide	CO₂	412	0.0412	626	0.0626
Neon	Ne	18.2	0.00182	12.7	0.00127
Helium	He	5.24	0.000524	0.724	0.0000724
Methane	CH₄	1.79	0.000179	0.99	0.000099
Krypton	Kr	1.14	0.000114	3.3	0.00033
If air is not dry:
Water vapor^(D)	H₂O	0–30,000^(D)	0–3%^(E)
The total ppm above adds up to more than 1 million (currently 83.43 above it) due to experimental error. Notes ^(A) In the atmosphere the pressure is low enough for the ideal gas laws to be correct within 1%. Therefore, the mole fraction is very close to the volume fraction.^: 4 ^(B) ppm: parts per million by molecular count ^(C) The concentration of CO₂ has been increasing in recent decades, as has that of CH₄. ^(D) Water vapor is about 0.25% by mass over full atmosphere ^(E) Water vapor varies significantly locally

Stratification

Air pressure and density decrease with altitude in Earth's atmosphere. Temperature changes in more complex ways, sometimes staying the same or even increasing with height. These temperature patterns help scientists divide the atmosphere into five main layers, each with its own typical height above Earth:

Afterglow of the troposphere (orange), the stratosphere (blue) and the mesosphere (dark) at which atmospheric entry begins, leaving smoke trails, such as in this case of a spacecraft reentry

Exosphere: 700–10,000 km (435–6,214 mi)
Thermosphere: 80–700 km (50–435 mi)
Mesosphere: 50–80 km (31–50 mi)
Stratosphere: 12–50 km (7–31 mi)
Troposphere: 0–12 km (0–7 mi)

The exosphere is the outermost layer, very thin and often considered part of space. The thermosphere is very hot but so thin that it doesn’t feel hot. The mesosphere is the coldest part of the atmosphere. The stratosphere contains an ozone layer that protects Earth from harmful sun radiation. The troposphere is where all our weather happens, including rain, snow, and wind.

Physical properties

Main article: Atmospheric pressure

The Earth's atmosphere is a layer of gases that surrounds our planet and is held in place by gravity. At sea level, the pressure of this air is about 101,325 pascals, which scientists call one standard atmosphere. This pressure helps protect us from the harsh conditions of space.

As you go higher in the sky, the air gets thinner and the pressure drops. Most of the atmosphere is close to the ground—about half of it is below 5.6 kilometers, and nearly all of it is within 100 kilometers. Even above this point, we still see effects like colorful lights in the sky called auroras.

Optical properties

Circulation

Main article: Atmospheric circulation

Atmospheric circulation is the way air moves around Earth, helping to spread heat from the sun. The basic pattern stays the same because of Earth's rotation and the way the sun shines more strongly on the equator than on the poles. The tilt of Earth also causes seasons, changing where the warmest air is.

Air flows in three big loops, called cells, based on latitude. Near the equator, warm air rises and moves toward the poles in the Hadley cell. In the middle latitudes, air moves toward the poles in the Ferrel cell. In the far north and south, the Polar cell sends air rising again toward the poles. The edges between these cells create jet streams, fast-moving air currents high in the sky that help move weather systems.

Evolution of Earth's atmosphere

See also: History of Earth and Paleoclimatology

The Earth's atmosphere has changed a lot over time. In the very beginning, during the Hadean eon, the atmosphere was made of gases like hydrogen, water vapor, methane, and ammonia from the solar system. Later, as Earth cooled and volcanoes became active, the atmosphere changed to include nitrogen, carbon dioxide, and other gases.

As life developed, plants began changing the atmosphere through photosynthesis, producing oxygen. This led to a major change called the Great Oxygenation Event about 2.4 billion years ago. Over time, the amount of oxygen in the atmosphere has gone up and down, influenced by living things and natural processes like volcanoes. Today, Earth's atmosphere protects us and helps control our climate.

Main article: Geological history of oxygen

Air pollution

Main article: Air pollution

Air pollution happens when harmful substances like chemicals, tiny particles, or biological materials get into the air we breathe. Human activities such as population growth, industrialization, and the use of cars have increased these pollutants, leading to problems like smog, acid rain, and health issues.

Since the Industrial Revolution, human actions have also raised the levels of greenhouse gases such as carbon dioxide and methane in the atmosphere. This increase, along with the loss of forests and wetlands, has caused the Earth’s temperature to rise. This warming can lead to serious environmental changes, including higher sea levels, more extreme weather, and damage to wildlife and ecosystems.