Air Medal
Awarded by the United States Armed Forces
Type	Medal
Awarded for	Meritorious achievement while participating in aerial flight
Status	Currently awarded
Statistics
Established	Executive Order 9158, May 11, 1942 (as amended by E.O. 9242-A, September 11, 1942)
Precedence
Next (higher)	Meritorious Service Medals: Joint Service, Branch Service
Next (lower)	Air Force: Aerial Achievement Medal Army, Navy & Marine Corps: Commendation Medals
Ribbon (above), Reverse (below)

The Air Medal is a military decoration of the United States. The award was created in 1942, and is awarded for meritorious achievement while participating in aerial flight.^[1]

Criteria[link]

The Air Medal was established by Executive Order 9158, signed by Franklin D. Roosevelt, on 11 May 1942. The Air Medal was awarded retroactive to 8 September 1939. The medal is awarded to any person who, while serving in any capacity in or with the Armed Forces of the United States, shall have distinguished himself/herself by meritorious achievement while participating in aerial flight. ^[2][3][1]

During World War II the medal's award criteria varied widely depending on the theater of operations, the aircraft flown, and the missions accomplished. In Europe some bomber crewmembers received it for five sorties; elsewhere much higher criteria were used.^[4]

Awards may be made to recognize single acts of merit or heroism, or for meritorious service. Award of the Air Medal is primarily intended to recognize those personnel who are on current crew member or non-crew member flying status which requires them to participate in aerial flight on a regular and frequent basis in the performance of their primary duties. However, it may also be awarded to certain other individuals whose combat duties require regular and frequent flying in other than a passenger status, or individuals who perform a particularly noteworthy act while performing the function of a crew member but who are not on flying status. These individuals must make a discernible contribution to the operational land combat mission or to the mission of the aircraft in flight. ^[5]

Examples of personnel whose combat duties require them to fly include those in the attack elements of units involved in air-land assaults against an armed enemy and those directly involved in airborne command and control of combat operations. Awards will not be made to individuals who use air transportation solely for the purpose of moving from point to point in a combat zone.

Subsequent decorations of the Air Medal are denoted in the U.S. Army by award numerals displayed on the medal and ribbon. The Army originally used oak leaf clusters, however changed to numerals during the Vietnam War when the number of Air Medals awarded became too large to be annotated on a single ribbon. The Army and the Air Force also awards the Air Medal with a Valor device for acts of heroism.

The Secretary of the Air Force approved the award of the "V" Device for valor to Air Medals awarded for heroism effective 21 Oct 2004. It applies to all Air Force members (Active Duty, Air Force Reserve, Air National Guard), retirees and veterans, but this change is not retroactive to any earlier date; only decorations approved on or after 21 Oct 2004 are affected by this policy change.

The Air Force does not utilize numerals on the Air Medal. Subsequent awards are annotated with the traditional oak leaf clusters. Enlisted members are also awarded three points toward promotion.

Variants[link]

Air Medal ribbon with 2 Award Stars, Valor Device and Strike/Flight Numeral 3

The U.S. Navy and U.S. Marine Corps have two types of Air Medal awards: "individual" for singular meritorious acts and "strike/flight" for participation in sustained aerial flight operations. Bronze Strike/Flight numerals denote "strike/flight" awards. As of September 27, 2006, gold award numerals are used to denote "individual" Air Medals. This is a return to the standard used before November 22, 1989. In the interval between November 22, 1989 and September 27, 2006, bronze, gold, and silver award stars denoted "individual" Air Medals. A bronze star was used to denote a first award while gold stars were used for the second through the fifth, seventh through tenth awards, and so on. Silver stars were used in lieu of five gold stars, and denote the sixth and eleventh (and so on) awards. For "individual" Air Medals, the valor device may be authorized.

The United States Coast Guard issues the Air Medal with subsequent awards denoted by gold and silver stars. There are no additional devices authorized on the Coast Guard Air Medal.

During World War II, the Air Medal was also awarded to members of the Civil Air Patrol that had been participants in that organization's anti-submarine patrol program.

Design[link]

The medal's design is set forth by law.

Description: A Bronze compass rose 1 11/16 inches circumscribing diameter and charged with an eagle volant carrying two lightning flashes in its talons. A fleur-de-lis at the top point holds the suspension ring. The points of the compass rose on the reverse are modeled with the central portion plain for engraving the name of the recipient.

Ribbon: The ribbon is 1 3/8 inches wide and consists of the following stripes:

1. 1/8 inch Ultramarine Blue 67118;
2. ¼ inch Golden Orange 67109;
3. center 5/8 inch Ultramarine Blue;
4. ¼ inch Golden Orange; and
5. 1/8 inch Ultramarine Blue.

Components: The following are authorized components of the Air Medal and the applicable specifications for each:

• a. Decoration (regular size): MIL-D-3943/23. NSN for decoration set is 8455-00-269-5747. For replacement medal NSN 8455-00-246-3837.

• b. Decoration (miniature size): MIL-D-3943/23. NSN 8455-00-996-5002.

• c. Ribbon: MIL-R-11589/7. NSN 8455-00-252-9963.

• d. Lapel Button: MIL-L-11484/17. NSN 8455-00-257-4308.

Designer: Walker Hancock. Hancock had competed for the medal design as a civilian, but prior to the award of the competition had been inducted into the army.^[1]

Notable recipients[link]

References[link]

Sammy Lee Cole

External links[link]

Wikimedia Commons has media related to: Air Medal

• Air Medal - Description, Components, and History
• Medal Description and History at U.S. Army site

v t e Inter-service awards and decorations of the United States military Federal military decorations Medal of Honor  · Silver Star  · Legion of Merit  · Distinguished Flying Cross  · Bronze Star Medal  · Purple Heart  · Meritorious Service Medal  · Air Medal DoD decorations Defense Distinguished Service  · Defense Superior Service  · Defense Meritorious Service  · Joint Service Commendation  · Joint Service Achievement  · Joint Meritorious Unit Federal service medals National Defense Service Medal  · Armed Forces Expeditionary Medal  · Humanitarian Service Medal  · Outstanding Volunteer Service  · Armed Forces Service Medal  · Armed Forces Reserve  · POW  · Antarctica Service  · Korean Service  · Korea Defense Service  · Vietnam Service  · Southwest Asia Service  · Kosovo Campaign  · Afghanistan Campaign Medal  · Iraq Campaign Medal  · GWOT Expeditionary Medal  · GWOT Service Medal

Blue light is scattered more than other wavelengths by the gases in the atmosphere, giving the Earth a blue halo when seen from space.

Limb view, of the Earth's atmosphere. Colours roughly denote the layers of the atmosphere.

This image shows the moon at centre, with the limb of Earth near the bottom transitioning into the orange-coloured troposphere, the lowest and most dense portion of the Earth's atmosphere. The troposphere ends abruptly at the tropopause, which appears in the image as the sharp boundary between the orange- and blue- coloured atmosphere. The silvery-blue noctilucent clouds extend far above the Earth's troposphere.

The atmosphere of Earth is a layer of gases surrounding the planet Earth that is retained by Earth's gravity. The atmosphere protects life on Earth by absorbing ultraviolet solar radiation, warming the surface through heat retention (greenhouse effect), and reducing temperature extremes between day and night (the diurnal temperature variation).

Atmospheric stratification describes the structure of the atmosphere, dividing it into distinct layers, each with specific characteristics such as temperature or composition. The atmosphere has a mass of about 5×10¹⁸ kg, three quarters of which is within about 11 km (6.8 mi; 36,000 ft) of the surface. The atmosphere becomes thinner and thinner with increasing altitude, with no definite boundary between the atmosphere and outer space. An altitude of 120 km (75 mi) is where atmospheric effects become noticeable during atmospheric reentry of spacecraft. The Kármán line, at 100 km (62 mi), also is often regarded as the boundary between atmosphere and outer space.

Air is the name given to atmosphere used in breathing and photosynthesis. Dry air contains roughly (by volume) 78.09% nitrogen, 20.95% oxygen, 0.93% argon, 0.039% carbon dioxide, and small amounts of other gases. Air also contains a variable amount of water vapor, on average around 1%. While air content and atmospheric pressure vary at different layers, air suitable for the survival of terrestrial plants and terrestrial animals is currently only known to be found in Earth's troposphere and artificial atmospheres.

Composition[link]

Composition of Earth's atmosphere. The lower pie represents the trace gases which together compose 0.039% of the atmosphere. Values normalized for illustration. The numbers are from a variety of years (mainly 1987, with CO₂ and methane from 2009) and do not represent any single source.

Mean atmospheric water vapor

Air is mainly composed of nitrogen, oxygen, and argon, which together constitute the major gases of the atmosphere. The remaining gases are often referred to as trace gases,^[1] among which are the greenhouse gases such as water vapor, carbon dioxide, methane, nitrous oxide, and ozone. Filtered air includes trace amounts of many other chemical compounds. Many natural substances may be present in tiny amounts in an unfiltered air sample, including dust, pollen and spores, sea spray, and volcanic ash. Various industrial pollutants also may be present, such as chlorine (elementary or in compounds), fluorine compounds, elemental mercury, and sulfur compounds such as sulfur dioxide [SO₂].

Structure of the atmosphere[link]

Principal layers[link]

In general, air pressure and density decrease in the atmosphere as height increases. However, temperature has a more complicated profile with altitude. Because the general pattern of this profile is constant and recognizable through means such as balloon soundings, temperature provides a useful metric to distinguish between atmospheric layers. In this way, Earth's atmosphere can be divided into five main layers. From highest to lowest, these layers are:

Exosphere[link]

The outermost layer of Earth's atmosphere extends from the exobase upward. It is mainly composed of hydrogen and helium. The particles are so far apart that they can travel hundreds of kilometers without colliding with one another. Since the particles rarely collide, the atmosphere no longer behaves like a fluid. These free-moving particles follow ballistic trajectories and may migrate into and out of the magnetosphere or the solar wind.

Thermosphere[link]

Temperature increases with height in the thermosphere from the mesopause up to the thermopause, then is constant with height. Unlike in the stratosphere, where the inversion is caused by absorption of radiation by ozone, in the thermosphere the inversion is a result of the extremely low density of molecules. The temperature of this layer can rise to 1,500 °C (2,700 °F), though the gas molecules are so far apart that temperature in the usual sense is not well defined. The air is so rarefied that an individual molecule (of oxygen, for example) travels an average of 1 kilometer between collisions with other molecules.^[3] The International Space Station orbits in this layer, between 320 and 380 km (200 and 240 mi). Because of the relative infrequency of molecular collisions, air above the mesopause is poorly mixed compared to air below. While the composition from the troposphere to the mesosphere is fairly constant, above a certain point, air is poorly mixed and becomes compositionally stratified. The point dividing these two regions is known as the turbopause. The region below is the homosphere, and the region above is the heterosphere. The top of the thermosphere is the bottom of the exosphere, called the exobase. Its height varies with solar activity and ranges from about 350–800 km (220–500 mi; 1,100,000–2,600,000 ft).^{[citation needed]}

Mesosphere[link]

The mesosphere extends from the stratopause to 80–85 km (50–53 mi; 260,000–280,000 ft). It is the layer where most meteors burn up upon entering the atmosphere. Temperature decreases with height in the mesosphere. The mesopause, the temperature minimum that marks the top of the mesosphere, is the coldest place on Earth and has an average temperature around −85 °C (−120 °F; 190 K).^[4] At the mesopause, temperatures may drop to −100 °C (−150 °F; 170 K).^[5] Due to the cold temperature of the mesosphere, water vapor is frozen, forming ice clouds (or Noctilucent clouds). A type of lightning referred to as either sprites or ELVES, form many miles above thunderclouds in the troposphere.

Stratosphere[link]

The stratosphere extends from the tropopause to about 51 km (32 mi; 170,000 ft). Temperature increases with height due to increased absorption of ultraviolet radiation by the ozone layer, which restricts turbulence and mixing. While the temperature may be −60 °C (−76 °F; 210 K) at the tropopause, the top of the stratosphere is much warmer, and may be near freezing^{[citation needed]}. The stratopause, which is the boundary between the stratosphere and mesosphere, typically is at 50 to 55 km (31 to 34 mi; 160,000 to 180,000 ft). The pressure here is 1/1000 sea level.

Troposphere[link]

The troposphere begins at the surface and extends to between 9 km (30,000 ft) at the poles and 17 km (56,000 ft) at the equator,^[6] with some variation due to weather. The troposphere is mostly heated by transfer of energy from the surface, so on average the lowest part of the troposphere is warmest and temperature decreases with altitude. This promotes vertical mixing (hence the origin of its name in the Greek word "τροπή", trope, meaning turn or overturn). The troposphere contains roughly 80% of the mass of the atmosphere.^[7] The tropopause is the boundary between the troposphere and stratosphere.

Other layers[link]

Within the five principal layers determined by temperature are several layers determined by other properties:

• The ozone layer is contained within the stratosphere. In this layer ozone concentrations are about 2 to 8 parts per million, which is much higher than in the lower atmosphere but still very small compared to the main components of the atmosphere. It is mainly located in the lower portion of the stratosphere from about 15–35 km (9.3–22 mi; 49,000–110,000 ft), though the thickness varies seasonally and geographically. About 90% of the ozone in our atmosphere is contained in the stratosphere.
• The ionosphere, the part of the atmosphere that is ionized by solar radiation, stretches from 50 to 1,000 km (31 to 620 mi; 160,000 to 3,300,000 ft) and typically overlaps both the exosphere and the thermosphere. It forms the inner edge of the magnetosphere. It has practical importance because it influences, for example, radio propagation on the Earth. It is responsible for auroras.
• The homosphere and heterosphere are defined by whether the atmospheric gases are well mixed. In the homosphere the chemical composition of the atmosphere does not depend on molecular weight because the gases are mixed by turbulence.^[8] The homosphere includes the troposphere, stratosphere, and mesosphere. Above the turbopause at about 100 km (62 mi; 330,000 ft) (essentially corresponding to the mesopause), the composition varies with altitude. This is because the distance that particles can move without colliding with one another is large compared with the size of motions that cause mixing. This allows the gases to stratify by molecular weight, with the heavier ones such as oxygen and nitrogen present only near the bottom of the heterosphere. The upper part of the heterosphere is composed almost completely of hydrogen, the lightest element.
• The planetary boundary layer is the part of the troposphere that is nearest the Earth's surface and is directly affected by it, mainly through turbulent diffusion. During the day the planetary boundary layer usually is well-mixed, while at night it becomes stably stratified with weak or intermittent mixing. The depth of the planetary boundary layer ranges from as little as about 100 m on clear, calm nights to 3000 m or more during the afternoon in dry regions.

The average temperature of the atmosphere at the surface of Earth is 14 °C (57 °F; 287 K)^[9] or 15 °C (59 °F; 288 K),^[10] depending on the reference.^{[11] [12][13]}

Physical properties[link]

Comparison of the 1962 US Standard Atmosphere graph of geometric altitude against air density, pressure, the speed of sound and temperature with approximate altitudes of various objects.^[14]

Pressure and thickness[link]

The average atmospheric pressure at sea level is about 1 atmosphere (atm) = 101.3 kPa (kilopascals) = 14.7 psi (pounds per square inch) = 760 torr = 29.92 inches of mercury (symbol Hg). Total atmospheric mass is 5.1480×10¹⁸ kg (1.135×10¹⁹ lb),^[15] about 2.5% less than would be inferred from the average sea level pressure and the Earth's area of 51007.2 megahectares, this portion being displaced by the Earth's mountainous terrain. Atmospheric pressure is the total weight of the air above unit area at the point where the pressure is measured. Thus air pressure varies with location and weather.

If the atmosphere had a uniform density, it would terminate abruptly at an altitude of 8.50 km (27,900 ft). It actually decreases exponentially with altitude, dropping by half every 5.6 km (18,000 ft) or by a factor of 1/e every 7.64 km (25,100 ft), the average scale height of the atmosphere below 70 km (43 mi; 230,000 ft). However, the atmosphere is more accurately modeled with a customized equation for each layer that takes gradients of temperature, molecular composition, solar radiation and gravity into account.

In summary, the mass of the earth's atmosphere is distributed approximately as follows:^[16]

• 50% is below 5.6 km (18,000 ft).
• 90% is below 16 km (52,000 ft).
• 99.99997% is below 100 km (62 mi; 330,000 ft), the Kármán line. By international convention, this marks the beginning of space where human travelers are considered astronauts.

By comparison, the summit of Mt. Everest is at 8,848 m (29,029 ft); commercial airliners typically cruise between 10 km (33,000 ft) and 13 km (43,000 ft) where the thinner air improves fuel economy; weather balloons reach 30.4 km (100,000 ft) and above; and the highest X-15 flight in 1963 reached 108.0 km (354,300 ft).

Even above the Kármán line, significant atmospheric effects such as auroras still occur. Meteors begin to glow in this region though the larger ones may not burn up until they penetrate more deeply. The various layers of the earth's ionosphere, important to HF radio propagation, begin below 100 km and extend beyond 500 km. By comparison, the International Space Station and Space Shuttle typically orbit at 350–400 km, within the F-layer of the ionosphere where they encounter enough atmospheric drag to require reboosts every few months. Depending on solar activity, satellites can still experience noticeable atmospheric drag at altitudes as high as 700–800 km.

Density and mass[link]

Temperature and mass density against altitude from the NRLMSISE-00 standard atmosphere model (the eight dotted lines in each "decade" are at the eight cubes 8, 27, 64, ..., 729)

The density of air at sea level is about 1.2  kg/m³ (1.2 g/L). Density is not measured directly but is calculated from measurements of temperature, pressure and humidity using the equation of state for air (a form of the ideal gas law). Atmospheric density decreases as the altitude increases. This variation can be approximately modeled using the barometric formula. More sophisticated models are used to predict orbital decay of satellites.

The average mass of the atmosphere is about 5 quadrillion (5×10¹⁵) tonnes or 1/1,200,000 the mass of Earth. According to the American National Center for Atmospheric Research, "The total mean mass of the atmosphere is 5.1480×10¹⁸ kg with an annual range due to water vapor of 1.2 or 1.5×10¹⁵ kg depending on whether surface pressure or water vapor data are used; somewhat smaller than the previous estimate. The mean mass of water vapor is estimated as 1.27×10¹⁶ kg and the dry air mass as 5.1352 ±0.0003×10¹⁸ kg."

Optical properties[link]

Solar radiation (or sunlight) is the energy the Earth receives from the Sun. The Earth also emits radiation back into space, but at longer wavelengths that we cannot see. Part of the incoming and emitted radiation is absorbed or reflected by the atmosphere.

Scattering[link]

When light passes through our atmosphere, photons interact with it through scattering. If the light does not interact with the atmosphere, it is called direct radiation and is what you see if you were to look directly at the Sun. Indirect radiation is light that has been scattered in the atmosphere. For example, on an overcast day when you cannot see your shadow there is no direct radiation reaching you, it has all been scattered. As another example, due to a phenomenon called Rayleigh scattering, shorter (blue) wavelengths scatter more easily than longer (red) wavelengths. This is why the sky looks blue; you are seeing scattered blue light. This is also why sunsets are red. Because the Sun is close to the horizon, the Sun's rays pass through more atmosphere than normal to reach your eye. Much of the blue light has been scattered out, leaving the red light in a sunset.

Absorption[link]

Different molecules absorb different wavelengths of radiation. For example, O₂ and O₃ absorb almost all wavelengths shorter than 300 nanometers. Water (H₂O) absorbs many wavelengths above 700 nm. When a molecule absorbs a photon, it increases the energy of the molecule. We can think of this as heating the atmosphere, but the atmosphere also cools by emitting radiation, as discussed below.

Rough plot of Earth's atmospheric transmittance (or opacity) to various wavelengths of electromagnetic radiation, including visible light.

The combined absorption spectra of the gases in the atmosphere leave "windows" of low opacity, allowing the transmission of only certain bands of light. The optical window runs from around 300 nm (ultraviolet-C) up into the range humans can see, the visible spectrum (commonly called light), at roughly 400–700 nm and continues to the infrared to around 1100 nm. There are also infrared and radio windows that transmit some infrared and radio waves at longer wavelengths. For example, the radio window runs from about one centimeter to about eleven-meter waves.

Emission[link]

Emission is the opposite of absorption, it is when an object emits radiation. Objects tend to emit amounts and wavelengths of radiation depending on their "black body" emission curves, therefore hotter objects tend to emit more radiation, with shorter wavelengths. Colder objects emit less radiation, with longer wavelengths. For example, the Sun is approximately 6,000 K (5,730 °C; 10,340 °F), its radiation peaks near 500 nm, and is visible to the human eye. The Earth is approximately 290 K (17 °C; 62 °F), so its radiation peaks near 10,000 nm, and is much too long to be visible to humans.

Because of its temperature, the atmosphere emits infrared radiation. For example, on clear nights the Earth's surface cools down faster than on cloudy nights. This is because clouds (H₂O) are strong absorbers and emitters of infrared radiation. This is also why it becomes colder at night at higher elevations. The atmosphere acts as a "blanket" to limit the amount of radiation the Earth loses into space.

The greenhouse effect is directly related to this absorption and emission (or "blanket") effect. Some chemicals in the atmosphere absorb and emit infrared radiation, but do not interact with sunlight in the visible spectrum. Common examples of these chemicals are CO₂ and H₂O. If there are too much of these greenhouse gases, sunlight heats the Earth's surface, but the gases block the infrared radiation from exiting back to space. This imbalance causes the Earth to warm, and thus climate change.

Refractive index[link]

The refractive index of air is close to, but just greater than 1. Systematic variations in refractive index can lead to the bending of light rays over long optical paths. One example is that, under some circumstances, observers onboard ships can see other vessels just over the horizon because light is refracted in the same direction as the curvature of the Earth's surface.

The refractive index of air depends on temperature, giving rise to refraction effects when the temperature gradient is large. An example of such effects is the mirage.

Circulation[link]

An idealised view of three large circulation cells.

Atmospheric circulation is the large-scale movement of air through the troposphere, and the means (with ocean circulation) by which heat is distributed around the Earth. The large-scale structure of the atmospheric circulation varies from year to year, but the basic structure remains fairly constant as it is determined by the Earth's rotation rate and the difference in solar radiation between the equator and poles.

Evolution of Earth's atmosphere[link]

Earliest atmosphere[link]

The outgassings of the Earth were stripped away by solar winds early in the history of the planet until a steady state was established, the first atmosphere. Based on today's volcanic evidence, this atmosphere would have contained 60% hydrogen, 20% oxygen (mostly in the form of water vapor), 10% carbon dioxide, 5 to 7% hydrogen sulfide, and smaller amounts of nitrogen, carbon monoxide, free hydrogen, methane and inert gases.^{[citation needed]}

A major rainfall led to the buildup of a vast ocean, enriching the other agents, first carbon dioxide and later nitrogen and inert gases. A major part of carbon dioxide exhalations were soon dissolved in water and built up carbonate sediments.

Second atmosphere[link]

Water-related sediments have been found dating from as early as 3.8 billion years ago.^[17] About 3.4 billion years ago, nitrogen was the major part of the then stable "second atmosphere". An influence of life has to be taken into account rather soon in the history of the atmosphere, since hints of early life forms are to be found as early as 3.5 billion years ago.^[18] The fact that this is not perfectly in line with the 30% lower solar radiance (compared to today) of the early Sun has been described as the "faint young Sun paradox".

The geological record however shows a continually relatively warm surface during the complete early temperature record of the Earth with the exception of one cold glacial phase about 2.4 billion years ago. In the late Archaean eon an oxygen-containing atmosphere began to develop, apparently from photosynthesizing algae which have been found as stromatolite fossils from 2.7 billion years ago. The early basic carbon isotopy (isotope ratio proportions) is very much in line with what is found today,^[19] suggesting that the fundamental features of the carbon cycle were established as early as 4 billion years ago.

Third atmosphere[link]

Oxygen content of the atmosphere over the last billion years. This diagram in more detail

The accretion of continents about 3.5 billion years ago^[20] added plate tectonics, constantly rearranging the continents and also shaping long-term climate evolution by allowing the transfer of carbon dioxide to large land-based carbonate storages. Free oxygen did not exist until about 1.7 billion years ago and this can be seen with the development of the red beds and the end of the banded iron formations. The Earth had a lot of iron in the beginning, and higher amounts of oxygen was not available in the atmosphere until all the iron had ben oxidized. This signifies a shift from a reducing atmosphere to an oxidising atmosphere. O₂ showed major ups and downs until reaching a steady state of more than 15%.^[21] The following time span was the Phanerozoic eon, during which oxygen-breathing metazoan life forms began to appear.

The amount of oxygen in the atmosphere has gone up and down during the last 600 million years. There was a peak 280 million years ago, when the amount of oxygen was about 30 %, much higher than today. The cause of changes in the atmosphere is two main processes: Plants converts carbon dioxide into the bodies of the plants, which emits oxygen into the atmosphere, and break down of pyrite rocks cause sulphur to be added to the oceans. Volcanos cause this sulphur to be oxidized, reducing the amount of oxygen in the atmosphere. But volcanos also emit carbon dioxide, so that plants can convert this to oxygen. The exact cause of the variation of oxygen in the atmosphere is not known. Periods with much oxygen in the atmosphere are believed to cause rapid development of animals. Even if the atmosphere today has only 21 percent oxygen, today is still regarded as a period with rapid development of animals because of a high amount of oxygen in the atmosphere.^[22]

Currently, anthropogenic greenhouse gases are increasing in the atmosphere. According to the Intergovernmental Panel on Climate Change, this increase is the main cause of global warming.^[23]

Air pollution[link]

Air pollution is the introduction of chemicals, particulate matter, or biological materials that cause harm or discomfort to organisms into the atmosphere.^[24] Stratospheric ozone depletion is believed to be caused by air pollution (chiefly from chlorofluorocarbons).^{[citation needed]}

See also[link]

	Atmosphere portal
	Environment portal

References[link]

External links[link]

Wikimedia Commons has media related to: Earth's atmosphere

• NASA atmosphere models
• NASA's Earth Fact Sheet
• American Geophysical Union: Atmospheric Sciences
• Outreach of the GEOmon project See how Earth atmosphere is observed and monitored by a European project that combines many approaches.
• Stuff in the Air Find out what the atmosphere contains.
• Layers of the Atmosphere
• Answers to several questions of curious kids related to Air and Atmosphere
• The AMS Glossary of Meteorology
• Paul Crutzen Interview Free video of Paul Crutzen Nobel Laureate for his work on decomposition of ozone talking to Harry Kroto Nobel Laureate by the Vega Science Trust.
• Slides describing the Earth's modern atmosphere

v t e Atmospheres Major Sun Venus Earth Mars Jupiter Saturn Titan Uranus Neptune Triton Pluto HD 209458 b Tenuous Mercury Moon Io Europa Ganymede Callisto Enceladus Dione Rhea See also Stellar atmosphere Coma (cometary) Extraterrestrial atmospheres

Medal distributed by Cecilia Gonzaga to political allies, a common practice in Renaissance Europe. Designed by Pisanello in 1447.

Medallion received by Dr. Chandra Sarma for proficiency in Anatomy from Berry-White School of Medicine in 1930-31, Dibrugarh, Assam, India

A medal, or medallion, is generally a circular object that has been sculpted, molded, cast, struck, stamped, or some way rendered with an insignia, portrait, or other artistic rendering. A medal may be awarded to a person or organization as a form of recognition for athletic, military, scientific, academic, or various other achievements. Medals may also be created to commemorate particular individuals or events, or even as works of artistic expression in their own right; artists who focus their talents on the creation of medals or medallions are termed a medalist. There are also devotional medals which may be worn as a matter of religious faith. Medals are popular collectible items either as a form of exonumia or of militaria phaleristics. Medallions are occasionally referred to as "table medals" because they are too large to be worn and can only be displayed on a table top, desk or cabinet. Medals may also be produced in a rectangular shape, though these would more correctly be described as a plaque, and a smaller version as a plaquette. In colloquial use, "medallion" is sometimes improperly used to refer to a pendant of a necklace. It can also be used as a symbol to dignify greatness, especially when awarded on the collegiate level.

Etymology[link]

First attested in English in 1578, the word medal is derived from the Middle French médaille, itself from Italian medaglia, and ultimately from the post-classical Latin medalia, meaning a coin worth half a denarius. The word medallion (first attested in English in 1658) has the same ultimate derivation, but this time through the Italian medaglione, meaning "large medal". There are two theories as for the etymology of the word medalia: the first is that the Latin medalia itself is derived from the adjective medialis meaning "medial" or "middle";^[1] the second is that medaglia, comes from the Vulgar Latin metallea (moneta) meaning "metal (coin)" and that from Latin metallum,^[2] which is the latinisation of the Greek μέταλλον (metallon), "a mine".^[3][4][5][6]

Features[link]

Various prize medals with the obverse designs, suspension rings and ribbons typical of medals intended to be draped over the head and hung from the neck

The main or front surface is termed the obverse, and may contain a portrait, pictorial scene or other image along with an inscription. The reverse, or back surface of the medal, is not always used and may be left blank or may contain a secondary design. It is not uncommon to find only an artistic rendering on the obverse, while all details and other information for the medal are inscribed on the reverse. The rim is found only occasionally employed to display an inscription such as a motto, privy mark, engraver symbol, assayer’s marking or a series number.

Medals that are intended to be hung from a ribbon also include a small suspension piece at the crest with which to loop a suspension ring through. It is through the ring that a ribbon is run or folded so the medal may hang pendent. Medals pinned to the breast use only a small cut of ribbon that is attached to a top bar where the brooch pin is affixed. Top bars may be hidden under the ribbon so they are not visible, be a plain device from which the ribbon attaches or even decorative to complement the design on the medal; some top bars are elaborate and contain a whole design unto themselves.

Bronze has been the most common material employed for medals, due to its fair price range, durability, ease with which to work when casting and the ample availability, but a wide range of other media have also been used. Rarer metals have been employed, such as silver, platinum and gold, when wishing to add value beyond the mere artistic depiction, as well as base metals and alloys such as copper, brass, iron, aluminum, lead, zinc, nickel and pewter. Medals that are made with inexpensive material might be gilded, silver plated, chased or finished in a variety of other ways to improve their appearance. Medals have also been made of rock, gemstone, ivory, glass, porcelain, terra cotta, coal, wood, paper, enamel, lacquerware, and plastics.

History[link]

Bronze medal of Isabella d'Este, princess and patron of Renaissance humanists, distributed as a gift.

Silver Geuzen medal commemorating the Capture of Brielle in 1572 by the Sea Beggars; this was produced commercially

The first known instance of a medal being awarded comes from the historian Josephus who, writing long after the event, accounts that in the fourth century BCE the High Priest Jonathan led the Hebrews in aid of Alexander the Great, and that in return for this, Alexander "sent to Jonathan... honorary awards, as a golden button, which it is custom to give the king's kinsmen." The Roman emperors used both military awards of medals, and political gifts of medallions that were like very large coins, usually in gold or silver, and die-struck like coins.^[7] Both these and actual gold coins were often set as pieces of jewellery, worn by both sexes.

The bracteate is a type of thin gold medal, usually plain on the reverse, found in Northern Europe from the so called "Dark Ages" or Migration Period. They often have suspension loops and were clearly intended to be worn on a chain as jewellery. They imitate, at a distance, Roman imperial coins and medallions, but have the heads of gods, animals, or other designs.^[8] The Liudhard medalet, produced around 600AD in Anglo-Saxon England, is an isolated example, known from a single copy, of a Christian medal, featuring an inscription naming Liudhard (or "Saint Letard"), the first priest among the Anglo-Saxons, and probably presented to converts. The surviving example is mounted for wearing as jewellery.