Atmospheric Properties

The atmosphere is a layer of gaseous elements that surrounds the Earth and differentiates the Earth's environment from outer space. The atmosphere retains gases produced by chemical reactions and protects the Earth's surface from both solar and cosmic radiation, enabling life on Earth. The troposphere contains the greatest density of gases in the atmosphere, while the stratosphere contains the ozone layer, which protects the earth from cosmic radiation.

Composition of the Atmosphere

The atmosphere is a gaseous layer surrounding the Earth that is generated by chemical activity on the Earth’s surface and retained by the planet’s gravitational field. The atmosphere has no exact upper limit but gradually dissipates with increasing distance from the surface, blending into outer space. The outermost layer of the earth’s atmosphere is known as the exosphere, which stretches from approximately 500 to more than 1,000 kilometers (310 to 620 miles) from the earth’s surface and gradually blends into outer space.

The Earth’s atmosphere is largely made up of nitrogen (N₂), which accounts for between 78 and 79 percent of the total gases found beneath the exosphere. Oxygen (O₂) is the next most abundant type of gas, typically representing between 20 and 21 percent of the remaining gases. The remaining 1 percent of the atmosphere is composed mostly of argon (Ar), which accounts for approximately 0.93 percent, and carbon dioxide (CO₂), which is present at levels of up to 0.031 percent.

The remaining atmosphere consists of minute quantities of trace gases, including hydrogen (H₂), helium (He), neon (Ne), ozone (O₃), and methane (CH₄). Though the trace gases are present only in minute quantities, many are essential for life on Earth. Ozone, for instance, makes up only 0.000002 to 0.000007 percent of the atmosphere. However, it is the most important element, as it blocks solar radiation; this radiation would otherwise be deadly to life on Earth.

Oxygen and hydrogen combine to form water vapor (H₂O), which varies in concentration depending on circulation patterns. In extremely humid areas, water vapor can compose more than 4 percent of the atmosphere. Water vapor is responsible for all precipitation on Earth, making it essential for life outside the marine environment.

Structure of the Atmosphere

More than 80 percent of atmospheric gases are concentrated within the lower levels of the atmosphere, approximately 16 km (10 mi) above sea level. Most of this bottom 16 km is taken up by the troposphere, which extends to approximately 10 km (6 mi) above the surface and is the layer of the atmosphere most affected by the chemical processes generated by life on Earth.

The troposphere is in a constant state of turbulence, as chemical reactions contribute new gases and absorb other gases from the air. Changes in temperature and pressure combined with the gravitational pressures on Earth’s surface cause weather patterns to emerge and thereby fuel cycles of exchange that move elements between the atmosphere and the earth. The troposphere is heated by a transfer of heat from the Earth's surface; temperatures decrease with increasing altitude. This pattern ends at the tropopause, the uppermost layer of the troposphere.

The stratosphere is the next lowest level of the atmosphere, extending from 10 km to approximately 50 km (6 to 31 mi) above Earth’s surface. Compared with the troposphere, the stratosphere is relatively free of turbulence and stratified into distinct layers of different gaseous compositions. At the lowest levels of the stratosphere, sufficient oxygen exists to support bacterial life. Commercial airliners also tend to fly in the lowest level of the stratosphere to avoid the turbulent weather patterns that dominate the troposphere.

The stratosphere contains a thin layer of ozone between 25 and 30 km (16 and 19 mi) above the Earth’s surface, which blocks solar radiation. Solar radiation, therefore, concentrates in the upper mid- and upper levels of the stratosphere. For this reason, temperature in the stratosphere increases with altitude until it reaches the topmost layer, called the stratopause.

Above the stratopause is the mesosphere, which extends from 50 to 80 km (31 to 50 mi) above the Earth. Within the mesosphere, the effect of solar radiation reflected by the ozone layer is reduced, and temperatures again begin to decrease with altitude. In addition, CO₂ trapped in this region further reduces the temperature, making the mesosphere the coldest region in the atmosphere. Temperatures can drop below -90 degrees Celsius (-194 degrees Fahrenheit) in the upper reaches of the mesosphere.

AAbove 80 km (50 mi), the mesosphere gradually blends into the ionosphere, the general term used for the atmospheric region between 80 and 800 km (497 mi) above sea level. This portion of the atmosphere is most susceptible to solar and cosmic radiation and is, therefore, populated by ionized gas. Ionization occurs when streams of free electrons in cosmic radiation impact with atoms and cause them to exist in an ionized state where free electrons and ions dominate the environment.

The thermosphere is where spacecraft enter low Earth orbits, and electromagnetic fields create the visual effects known as auroras. The atmospheric atoms and molecules in the thermosphere increase in temperature as they react with solar and cosmic radiation. The temperature of atoms in the thermosphere can reach more than 2,000 degrees Celsius (3,632 degrees Fahrenheit), but the concentration of atoms is so low that the region would still feel cold to any life-form. Past 300 km (186 mi) from the surface, the concentration of atoms is so low that it becomes difficult to measure any further increase in temperature, and the environment begins to resemble the vacuum of outer space.

Beyond the thermosphere is the exosphere, where atoms and molecules leave Earth’s atmosphere and exit into space. While it is still possible to observe minute quantities of gas in the exosphere, the region is essentially a vacuum; the extremely low density of atoms means little temperature variation within the region.

The circulation of atmospheric gas depends on several interrelated factors, including temperature, pressure, and the Earth's rotation. The most critical factor in determining the movement of atmospheric gas is the uneven distribution of heat over the Earth. This creates convection currents, which move air vertically through the atmosphere.

Because of the Earth’s shape and orientation relative to the sun, more solar radiation falls on the equator than the poles. Warm air rises in the atmosphere and loses density as it cools, creating low and high-pressure areas. Coupled with variations in Earth’s topography and the contribution of ocean currents driven by thermal energy from beneath the crust, these warming and cooling cycles give rise to circulation cells that create wind and weather patterns and distribute solar energy across the planet.

The path of wind currents over the Earth’s surface is complicated by the Coriolis effect, an observational phenomenon caused by Earth’s rotation. Because the Earth is rotating, an object moving in a straight line above the Earth's surface will appear to veer to the right or left before landing. The observed variance is not actually a result of the object’s movement but is caused by the Earth’s continued rotation while the object is moving, thereby altering the landing point concerning the observed direction of travel.

Earth rotates in a counterclockwise direction; therefore, in the Northern Hemisphere, the Coriolis effect appears to shift the path of a moving object to the right, while movement seems to shift to the left in the Southern Hemisphere. At the equator, there is no net movement in either direction, and an object will appear to follow a straight path to its destination. Because of this rotational effect, winds originating in the north and flowing south will arrive on Earth as if moving in a southwest direction. In contrast, winds originating in the south and heading north will appear to be traveling in a southeastern direction.

Three primary circulation systems act on the Earth. They are known as circulation cells or wind belts. The first of these systems is the tropical or Hadley cell, which begins at the equator. Here, warm air rises in response to solar heating until it meets the tropopause, spreading north and south toward the poles.

Two tropical cells exist, one to the north and one to the south of the equator, from 0 to 30 degrees in either direction. Because of the Coriolis effect, wind from the tropical cells flows either southeast or northeast from the cells, creating the trade winds, which were named because of their importance to merchant mariners. At the equator, there is virtually no net movement of wind, creating a dead zone called the doldrums.

At about 30 degrees east or west, the cooled air moving poleward descends to the Earth, creating two more areas—the horse latitudes—with little directional air current. Most of the cooled air returns to the equator and is recycled within the tropical cell; however, some air pushes farther toward the pole and becomes part of what meteorologists call the midlatitude or Ferrel cell.

The mid-latitude cell blends cold air from the polar region with warmer air flowing from the equatorial zone. This mixture of temperatures and pressures creates complex wind patterns. Because both the tropical and polar winds blow in an easterly direction, the mid-latitude winds are caught between the two and tend to move in a westerly direction, creating what meteorologists call the prevailing westerlies. While westerly winds are typical, changes in wind direction are common in this region, leading to a high degree of temperature and pressure variation. This variability results in the temperate zone, marked by higher variation in seasonal temperatures and weather patterns than in tropical or polar regions.

At 60 degrees north and south, the mid-latitude cells meet the polar cells, which behave similarly to the tropical cells. Here, warm air rises again to the troposphere and moves poleward, eventually falling to the Earth at the poles and recycling into the polar cell. Where the mid-latitude and the polar cells meet, the winds of the polar cells tend to dominate, flowing over the weaker currents from the mid-latitude cells. This causes intense shifts in pressure that translate into strong winds and unpredictable changes in current direction. For this reason, the polar fronts are known for their potential to generate windstorms and cyclones.

Electromagnetism and the Atmosphere

Earth generates a magnetic field because of the inherent magnetism, heat, and the movement of material in the planet’s core. The magnetic field stretches beyond the exosphere and reaches thousands of kilometers into space. The magnetic field plays a role in protecting the Earth from solar radiation.

The sun constantly releases millions of tons of radiation, called the solar wind, into space. This solar wind permeates the entire heliosphere, which is the portion of space affected by the sun. Energetic particles in the solar wind bombard all the planets in the solar system.

The interaction of the solar wind and the Earth’s magnetic field creates the magnetosphere, a field of charged magnetic particles surrounding the Earth. The magnetosphere is not spherical but contains a long tail stretching thousands of kilometers into space on the side facing away from the sun. Some of the charged particles in the solar wind penetrate the magnetosphere, moving eventually into the thermosphere, where they interact with atmospheric atoms to create the optical phenomena known as auroras.

Without the magnetosphere, the charged particles of the solar wind would destroy the ozone layer in the Earth’s stratosphere, leaving the Earth vulnerable to increased solar radiation levels. Life on Earth, therefore, depends on both the shielding effects of the atmosphere and the magnetic field.

Global climate change has severely affected the Earth’s atmosphere in several ways. Human activities have raised the amount of greenhouse gases, especially carbon dioxide, in the atmosphere, altering its composition. Rising temperatures have allowed the atmosphere to retain more water vapor, which, in turn, causes a further rise in temperature. This phenomenon has been especially dire in the atmosphere above the Arctic and Antarctic regions. Changes in the circulation patterns of Earth’s atmosphere and its chemistry have also been brought about by anthropogenic climate change. Due to these issues, the air quality within Earth’s atmosphere has declined, and extreme weather events have become increasingly common.

Principal Terms

atmosphere: gaseous “envelope” surrounding the earth that contains all gases produced by terrestrial sources

circulation cell: cyclic pattern of air movement within the atmosphere

convection: the vertical transport of atmospheric properties

Coriolis effect: the illusion of deflection observed when a body moves through the atmosphere with regard to an individual situated on the moving surface of the earth

magnetosphere: outer region of Earth’s ionosphere where the movement of particles is dominated by Earth’s magnetic field

ozone: a form of oxygen containing three joined oxygen atoms responsible for blocking much of the solar radiation that hits Earth’s atmosphere

stratosphere: uppermost region of the atmosphere able to support life; extends from 10 to 50 kilometers (6 to 31 miles) above Earth’s surface

thermosphere: outer region of the atmosphere between 80 and 800 kilometers (50 to 497 miles) from the surface where temperature increases with increasing altitude because of bombardment by solar radiation

topography: the relief features or surface configuration of a certain area

troposphere: the lowest level of Earth’s atmosphere extending to approximately 10 kilometers above sea level

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