User:Milton Beychok/Sandbox

The Earth's atmosphere is an envelope of gas that surrounds the Earth and extends from the Earth's surface out thousands of kilometres, becoming increasingly thinner (less dense) with distance but always held in place by Earth's gravitational pull. The atmosphere contains the air we breathe and it holds clouds of moisture (water vapor) that become the water we drink. It protects us from harmful solar radiation and warms the Earth's surface by heat retention. In effect, the atmosphere is an envelope that protects all life on Earth.

The atmosphere is a mixture of gases we call "air". On a dry volume basis, it consists of about 78% nitrogen and 21% oxygen. The remainder of about 1% contains argon, carbon dioxide and very small amounts of other gases. The atmosphere is rarely, if ever, completely dry. Water vapor is almost always present up to about 4% of the total volume. In desert regions, when dry winds are blowing, water vapor in the air will be nearly zero. This climbs in other regions to about 3% on extremely hot and humid days. The upper limit, approaching 4%, is for tropical areas.

The atmosphere has a total mass of about 5 × 1015 metric tons and about 80% of that mass is within about 12 kilometres (7.5 miles) from the Earth's surface.

There is no definite boundary between the atmosphere and outer space. It slowly becomes thinner and fades into space. The distance from the Earth's surface that is frequently regarded as the boundary between the atmosphere and outer space is about 100 kilometres (62 miles) and that boundry is called the Kármán line.

Structure of the atmosphere
As shown in the adjacent diagram, Earth's atmosphere has five primary layers, referred to as spheres. From the lowest to the highest layer, they are the Troposphere, Stratosphere, Mesosphere, Thermosphere and the Exosphere. The four boundaries between the primary layers, referred to as pauses, are the Tropopause, Stratopause, Mesopause and the Thermopause. In more detail:


 * Troposphere: The troposphere is the lowest layer of the atmosphere and extends from the Earth's surface to about 7 kilometres (4 miles) high at the north and south poles and 17 kilometres (11 miles) high at equator. This is where we live. As the density of the air in this layer decrease with height, the air temperature in the troposphere also decreases with height. The temperature drops from about 17 °C at the surface to about -50 °C at the top of the troposphere. All of our weather occurs in the troposphere. The troposphere contains about 80% of the total mass of the atmosphere. In fact, 50% percent of the total mass of the atmosphere is located in the lowest 5 to 6 kilometres of the troposphere.


 * Tropopause: The boundary between the troposphere and the stratosphere. Together, the troposphere and the tropopause are known as the lower atmosphere.


 * Stratosphere: From the Latin word "stratus" meaning spreading out. The stratosphere extends from the troposphere's 7 - 17 km range to about 51 km. Temperature increases with height. The stratosphere contains the ozone layer, the part of the Earth's atmosphere which contains relatively high concentrations of ozone. "Relatively high" means a few parts per million—much higher than the concentrations in the lower atmosphere but still small compared to the main components of the atmosphere. It is mainly located in the lower portion of the stratosphere from approximately 15 - 35 km above Earth's surface, though the thickness varies seasonally and geographically.


 * Stratopause: The boundary between the stratosphere and the mesosphere. The atmospheric pressure here is 1/1000th of the atmospheric pressure at sea level.


 * Mesosphere: From the Greek word "μέσος" meaning middle. The mesosphere extends from about 50 km to the range of 80 - 85 km. Temperature decreases with height, reaching -100 C in the upper mesosphere. This is also where most meteors burn up when entering the atmosphere.


 * Mesopause: The temperature minimum at the boundary between the thermosphere and the mesosphere. It is the coldest place on Earth, with a temperature of -100 C.


 * Thermosphere: From 80 - 85 km to over 640 km, temperature increasing with height. The temperature of this layer can rise to 1500 C. The International Space Station orbits in this layer, between 320 and 380 km.


 * Thermopause: The boundary above the thermosphere, it varies in height from 500 - 1000 km.


 * Exosphere: Extends from an altitude range of 500 to 1000 kilometres (310 to 620 miles) up to an altitude of 10,000 kilometres (6,200 miles). The exosphere contains free-moving particles that may migrate into and out of the magnetosphere or the solar wind.

The Ionosphere and the Ozone Layer are also parts of the Earth's atmosphere:


 * Ionosphere: The part of the atmosphere that is ionized by solar radiation stretches from 50 to 1000 km and typically overlaps both the exosphere and the thermosphere. It plays an important part in atmospheric electricity and forms the inner edge of the magnetosphere. Because of its charged particles, it has practical importance because it influences, for example, radio propagation on the Earth. It is responsible for auroras.


 * Ozone Layer: Though part of the Stratosphere, the ozone layer is considered as a layer of the Earth's atmosphere in itself because its physical and chemical composition is far different from the Stratosphere. Ozone (O3) in the Earth's stratosphere is created by ultraviolet light striking oxygen molecules containing two oxygen atoms (O2), splitting them into individual oxygen atoms (atomic oxygen); the atomic oxygen then combines with unbroken O2 to create O3. O3 is unstable (although, in the stratosphere, long-lived) and when ultraviolet light hits ozone it splits into a molecule of O2 and an atom of atomic oxygen, a continuing process called the ozone-oxygen cycle. This occurs in the ozone layer, the region from about 10 to 50 km above Earth's surface. About 90% of the ozone in our atmosphere is contained in the stratosphere. Ozone concentrations are greatest between about 20 and 40 km, where they range from about 2 to 8 parts per million.

The average temperature of the atmosphere at the surface of Earth is 14 C or 15 C, depending on the reference.

Composition of the atmospheric air
Filtered air includes trace amounts of many of the chemical elements. Substantial amounts of argon, nitrogen, and oxygen are present as elementary gases. Note the major greenhouse gases: water vapor, carbon dioxide, methane, nitrous oxide, and ozone. Many additional elements from natural sources may be present in tiny amounts in an unfiltered air sample, including contributions from dust, pollen and spores, sea spray, vulcanism, and meteoroids. Various industrial pollutants are also now present in the air, such as chlorine (elementary or in compounds), fluorine (in compounds), elementary mercury, and sulfur (in compounds such as sulfur dioxide [SO2]).