The earth's atmosphere is a mixture of gases including about 78 percent nitrogen, 21 percent oxygen, 0.93 percent argon, and trace amounts of carbon dioxide, water vapor, neon, helium, methane, krypton, hydrogen, nitrous oxide, xenon, ozone, iodine, carbon monoxide and ammonia.
The atmosphere can be divided into five distinct layers based on temperature. The lowest of these, the troposphere, extends to a height of six to ten kilometers above the earth's surface and contains about 75 to 80 percent of the total mass of the atmosphere. It also contains 99 percent of the water vapor in the atmosphere, with the greatest concentration in the tropical regions, as well as most of the earth's weather systems. The air temperature decreases with altitude because the air becomes thinner as its elevation rises.
Above this is the stratosphere, which has a thickness of about 35 kilometers. It contains about 19 percent of the atmosphere's total mass as well as the ozone layer, which shields life below from harmful ultraviolet radiation. In contrast to the stratosphere, the air temperature in the stratosphere gradually rises with altitude. Commercial jet aircraft fly mainly in the lower stratosphere to avoid turbulence that is common in the troposphere, while light aircraft and helicopters usually fly in the troposphere.
Above the stratosphere is the mesosphere, which extends from about 50 to 85 kilometers above the earth's surface. As with the troposphere and in contrast to the stratosphere, temperatures decrease with the elevation, and it is the coldest layer of the atmosphere. Most meteors and other items coming from above burn up due to air friction in this layer.
The thermosphere is located from about 85 to 500 kilometers above the earth's surface. It absorbs much of the shortest wavelengths of solar radiation, including much of the extreme ultraviolet radiation and x-rays, thereby resulting in very high temperatures, reaching more than 1,700 degrees centigrade, and also helping protect life. The International Space Station and low earth orbit satellites are in this layer.
The outermost layer is the exosphere, which extends from about 600 kilometers to roughly 10,000 kilometers above the earth. It is composed mostly of extremely low densities of hydrogen, helium and several heavier elements including nitrogen, oxygen and carbon dioxide. The atoms and molecules of these elements are so far apart that they can travel hundreds of kilometers without colliding with each another, and thus this layer is essentially a vacuum. Because of the very low gravity, these atoms and molecules can escape into space. Most satellites orbit in this layer because they can move easily with virtually no air resistance.
Despite its enormous size, the atmosphere is fragile. It is being damaged by human activity, and, if continued, this will have disastrous effects on humans and other forms of life. The level of carbon dioxide has increased by about 40 percent from 1750 as a result of the large-scale burning of coal and other fossil fuels, and most of this increase has occurred during the past 50 years. Such burning is the main cause of the greenhouse effect and the consequent climate change. Another major type of damage has been the destruction of the ozone layer, through the release of chlorofluorocarbons and other chemicals.