Definition of Atmosphere

Thin sphere around the Earth consisting of

the mixture of gases we call air, held in place by gravity. The

most abundant gas is nitrogen (78 percent), followed by oxygen

(21 percent), argon (0.9 percent), carbon dioxide (0.036

percent), and minor amounts of helium, krypton, neon, and

xenon. Atmospheric (or air) pressure is the force per unit area

(similar to weight) that the air above a certain point exerts on

any object below it. Atmospheric pressure causes most of the

volume of the atmosphere to be compressed to 3.4 miles (5.5

km) above the Earth’s surface, even though the entire atmosphere

is hundreds of kilometers thick.

The atmosphere is always moving, because more of the

Sun’s heat is received per unit area at the equator than at the

poles. The heated air expands and rises to where it spreads

out, then it cools and sinks, and gradually returns to the

equator. This pattern of global air circulation forms Hadley

cells that mix air between the equator and mid-latitudes. Similar

circulation cells mix air in middle to high latitudes, and

between the poles and high latitudes. The effects of the

Earth’s rotation modify this simple picture of the atmosphere’s

circulation. The Coriolis effect causes any freely

moving body in the Northern Hemisphere to veer to the

right, and toward the left in the Southern Hemisphere. The

combination of these effects forms the familiar trade winds,

easterlies and westerlies, and doldrums.

The atmosphere is divided into several layers, based

mainly on the vertical temperature gradients that vary significantly

with height. Atmospheric pressure and air density

both decrease more uniformly with height, and therefore

they do not serve as a useful way to differentiate the atmospheric

layers.

The lower 36,000 feet (11 km) of the atmosphere is

known as the troposphere, where the temperature generally

decreases gradually, at about 7.0°F per mile (6.4°C per km),

with increasing height above the surface. This is because the

Sun heats the surface, which in turn warms the lower part of

the troposphere. Most of the atmospheric and weather phenomena

we are familiar with occur in the troposphere.

Above the troposphere is a boundary region known as

the tropopause, marking the transition into the stratosphere

that continues to a height of about 31 miles (50 km). The

base of the stratosphere contains a region known as an

isothermal, where the temperature remains the same with

increasing height. The tropopause is generally at higher elevations

in the summer than the winter, and it is also the region

where the jet streams are located. Jet streams are narrow,

streamlike channels of air that flow at high velocities, often

exceeding 115 miles per hour (100 knots). Above about 12.5

miles (20 km), the isothermal region gives way to the upper

stratosphere where temperatures increase with height, back

to near surface temperatures at 31 miles (50 km). The heating

of the stratosphere is due to ozone at this level absorbing

ultraviolet radiation from the Sun.

The mesosphere lies above the stratosphere, extending

between 31 and 53 miles (50–85 km). An isothermal region

known as the stratopause separates the stratosphere and

mesosphere. The air temperature in the mesosphere decreases

dramatically above the stratopause, reaching a low of –130°F

(–90°C) at the top of the mesosphere. The mesopause separates

the mesosphere from the thermosphere, which is a hot

layer where temperatures rise to more than 150°F (80°C).

The relatively few oxygen atoms at this level absorb solar

energy, heat quickly, and may change dramatically in

response to changing solar activity. The thermosphere continues

to thin upward, extending to about 311 miles (500 km)

above the surface. Above this level, atoms dissociate and are

able to shoot outward and escape the gravitational pull of

Earth. This far region of the atmosphere is sometimes

referred to as the exosphere.

In addition to the temperature-based division of the

atmosphere, it is possible to divide the atmosphere into different

regions based on their chemical and other properties.

Using such a scheme, the lower 46.5–62 miles (75–100 km)

of the atmosphere may be referred to as the homosphere,

where the atmosphere is well mixed and has a fairly uniform

ratio of gases from base to top. In the overlying heterosphere,

the denser gases (oxygen, nitrogen) have settled to the base,

whereas lighter gases (hydrogen, helium) have risen to greater

heights, resulting in chemical differences with height.

The upper parts of the homosphere and the heterosphere

contain a large number of electrically charged particles

known as ions. This region is known also as the ionosphere,

which strongly influences radio transmission and the formation

of the aurora borealis and aurora australis.

Atmospheric gases are being produced at approximately

the same rate that they are being destroyed or removed from

the atmospheric system, although some gases are gradually

increasing or decreasing in abundance as described below. Soil

bacteria and other biologic agents remove nitrogen from the

atmosphere, whereas decay of organic material releases nitrogen

back to the atmosphere. However, decaying organic material

removes oxygen from the atmosphere by combining with

other substances to produce oxides. Animals also remove oxygen

from the atmosphere by breathing, whereas oxygen is

added back to the atmosphere through photosynthesis.

Water vapor is an extremely important gas in the atmosphere,

but it varies greatly in concentration (0–4 percent)

from place to place, and from time to time. Water vapor is

invisible, and it becomes visible as clouds, fog, ice, and rain

when the water molecules coalesce into larger groups. Water

forms water vapor gas, liquid, and solid, and constitutes the

precipitation that falls to Earth and is the basis for the hydrologic

cycle. Water vapor is also a major factor in heat transfer

in the atmosphere. A kind of heat known as latent heat is

released when water vapor turns into solid ice or liquid water.

This heat is a major source of atmospheric energy that is a

major contributor to the formation of thunderstorms, hurricanes,

and other weather phenomena. Water vapor may also

play a longer-term role in atmospheric regulation, as it is a

greenhouse gas that absorbs a significant portion of the outgoing

radiation from the Earth, causing the atmosphere to warm.

Carbon dioxide, although small in concentration, is

another very important gas in the Earth’s atmosphere. Carbon

dioxide is produced during decay of organic material,

from volcanic outgassing, from cow, termite, and other animal

emissions, deforestation, and from the burning of fossil

fuels. It is taken up by plants during photosynthesis and is

also used by many marine organisms for their shells, made of

CaCO3 (calcium carbonate). When these organisms (for

instance, phytoplankton) die, their shells can sink to the bottom

of the ocean and be buried, removing carbon dioxide

from the atmospheric system. Like water vapor, carbon dioxide

is a greenhouse gas that traps some of the outgoing solar

radiation that is reflected from the Earth, causing the atmosphere

to warm up. Because carbon dioxide is released by the

burning of fossil fuels, its concentration is increasing in the

atmosphere as humans consume more fuel. The concentration

of CO2 in the atmosphere has increased by 15 percent

since 1958, enough to cause considerable global warming. It

is estimated that the concentration of CO2 will increase by

another 35 percent by the end of the 21st century, further

enhancing global warming.

Other gases also contribute to the greenhouse effect,

notably methane (CH4), nitrous oxide (NO2) and chlorofluorocarbons

(CFCs). Methane is increasing in concentration in

the atmosphere and is produced by the breakdown of organic

material by bacteria in rice paddies and other environments,

termites, and in the stomachs of cows. NO2, produced by

microbes in the soil, is also increasing in concentration by 1

percent every few years, even though it is destroyed by ultraviolet

radiation in the atmosphere. Chlorofluorocarbons have

received a large amount of attention since they are long-lived

greenhouse gases increasing in atmospheric concentration as

a result of human activity. Chlorofluorocarbons trap heat like

other greenhouse gases and also destroy ozone (O3), our protective

blanket that shields the Earth from harmful ultraviolet

radiation. Chlorofluorocarbons were used widely as refrigerants

and as propellants in spray cans. Their use has been

largely curtailed, but since they have such a long residence

time in the atmosphere, they are still destroying ozone and

contributing to global warming and will continue to do so

for many years.

Ozone (O3) is found primarily in the upper atmosphere

where free oxygen atoms combine with oxygen molecules

(O2) in the stratosphere. The loss of ozone has been dramatic

in recent years, even leading to the formation of “ozone

holes” with virtually no ozone present above the Arctic and

Antarctic in the fall. There is currently debate about how

much of the ozone loss is due to human-induced ozone loss

by chlorofluorocarbon production, and how much may be

related to natural fluctuations in ozone concentration.

Many other gases and particulate matter play important

roles in atmospheric phenomena. For instance, small amounts

of sulfur dioxide (SO2) produced by the burning of fossil

fuels mix with water to form sulfuric acid, the main harmful

component of acid rain. Acid rain is killing the biota of many

natural lake systems, particularly in the northeastern United

States, and it is causing a wide range of other environmental

problems across the world. Other pollutants are major causes

of respiratory problems, environmental degradation, and the

major increase in particulate matter in the atmosphere in the

past century has increased the hazards and health effects

from these atmospheric particles.

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