The mixture of gases in the air today has had over 4.5 billion years in which to evolve. The earliest atmosphere must have consisted of volcanic emanations mainly. Gases that erupt from volcanoes today are basically a mixture of water vapor, carbon dioxide, sulphur dioxide, and nitrogen, it has almost no oxygen. If this is the same mixture that existed in the early atmosphere then various processes would have had to operate to produce the mixture we have today. One of such processes was condensation. As the air cooled then much of the volcanic water vapour condensed to fill the earliest oceans. Chemical reactions would also have occurred on the earth. Some carbon dioxide would have reacted with the rocks of the Earth's crust to form carbonate minerals and even some would have become dissolved in the new oceans. Later, as primitive life was capable of photosynthesis evolved in the oceans, new marine organisms began producing oxygen. Almost all the free oxygen in the air today is believed to have formed by photosynthetic combination of carbon dioxide with water. This was the way oxygen was produced. About 570 million years ago the oxygen content of the atmosphere and oceans became high enough to permit marine life capable of respiration, hence they prospered. Later, some 400 million years ago, the atmosphere contained enough oxygen for the evolution of air-breathing of land animals.

The water-vapour content of the air varies considerably, depending on the temperature and relative humidity. With 100 per cent relative humidity the water-vapour content of air varies from 190 parts per million (ppm) at -40° C (-40° F) to 42,000 ppm at 30° C (86° F). Minute quantities of other gases, such as ammonia, hydrogen sulphide, and oxides of sulphur and nitrogen, are temporary constituents of the atmosphere. Oxides and other pollutants added to the atmosphere by factories and vehicles have become a major concern only because of their damaging effects in the form of acid rain. In addition, the strong possibility exists that the steady increase in atmospheric carbon dioxide, mainly as the result of fossil-fuel combustion over the past century, may affect the Earth's climate through the process known as the greenhouse effect.

Similar concerns are posed by the sharp increase in atmospheric methane. Methane levels have risen 11 per cent since 1978. About 80 per cent of the gas is produced by decomposition in rice paddies, swamps and even the intestines of grazing animals. Besides adding to the greenhouse effect, methane reduces the volume of atmospheric hydroxyl ions, thereby impairing the atmosphere's ability to cleanse itself of pollutants. This can be very harmful.

The study of air samples shows that up to at least 88 km (55 mi) above sea level the composition of the atmosphere is substantially the same as at ground level; the continuous stirring produced by atmospheric currents counteracts the tendency of the heavier gases to settle below the lighter ones. In the lower atmosphere a gas called ozone which is a form of oxygen with three atoms in each molecule, is normally present in extremely low concentrations. The layer of atmosphere from 19 to 48 km up contains more ozone, produced by the action of ultraviolet radiation from the Sun. Even in this layer the percentage of ozone is only 0.001 by volume. Atmospheric disturbances and downdrafts carry varying amounts of this ozone to the surface of the Earth. Human activity adds to ozone in the lower atmosphere, where it becomes a pollutant that can cause extensive damage to crops .

The ozone layer became a subject of concern in the early 1970s when it was found that chemicals known as chlorofluorocarbons (CFCs), or chlorofluoromethanes, were rising into the atmosphere in large quantities because of their use as refrigerants and as propellants in aerosol dispensers. The concern centred on the possibility that these compounds, through the action of sunlight, could photochemically attack and destroy stratospheric ozone, which protects the Earth's surface from excessive ultraviolet radiation. As a result industries in industrialized countries have tried to replace chlorofluorocarbons in all.

The atmosphere may be divided into several layers. In the lowest one, the troposphere, the temperature as a rule decreases upwards at the rate of 5.5° C per 1,000 m .This is the layer in which most clouds occur. The troposphere extends up to about 16 km in tropical regions (to a temperature of about -79° C, or -110° F) and to about 9.7 km in temperate latitudes (to a temperature of about -51° C, or -60° F). Above the troposphere is the stratosphere. In the lower stratosphere the temperature is practically constant or increases slightly with altitude mainly over tropical regions. Within the ozone layer the temperature rises more rapidly, and the temperature at the upper boundary of the stratosphere, almost 50 km above sea level, is about the same as the temperature at the surface of the Earth. The layer from 50 to 80 km is called the mesosphere and is characterized by a marked decrease in temperature as the altitude increases.

From investigations of the propagation and reflection of radio waves, it is known that beginning at an altitude of 80 km rays such as ultraviolet radiation, X-rays, and showers of electrons from the Sun are able to ionize several layers of the atmosphere which causes them to conduct electricity. These layers reflect radio waves of certain frequencies back to Earth because of the relatively high concentration of ions in the air above 80 km this layer is an extending to an altitude of 640 km is called the ionosphere. It is also termed the thermosphere because of the high temperatures in this layer .The region beyond the ionosphere is called the exosphere, which extends to about 9,600 km the outer limit of the atmosphere.

The density of dry air at sea level is about 1/800 the density of water while at higher altitudes it decreases rapidly as it is being proportional to the pressure and inversely proportional to the temperature. Pressure is measured by a barometer and is expressed in torrs, which are related to the height of a column of mercury that the air pressure will support where 1 torr equals 1 mm of mercury. Normal atmospheric pressure at sea level is 760 torrs, that is, 760 mm of mercury. At about 5.6 km it is 380 torrs , half of all the air in the atmosphere lies below this level. The pressure is again approximately halved for each additional increase of 5.6 km in altitude. At 80 km the pressure is 0.007 torr .

The troposphere and most of the stratosphere can be easily explored directly by means of sounding balloons equipped with instruments to measure the pressure and temperature of the air and with a radio transmitter to send the data to a receiving station at the ground. Rockets carrying radios that transmit meteorological-instrument ,readings have explored the atmosphere to altitudes above 400 km . Study of the form and spectrum of the aurora gives information to a height possibly as great as 800 km.

Atmospheric Pressure

Atmospheric Pressure is also called Barometric Pressure, force per unit area exerted by an atmospheric column (that is, the entire body of air above the specified area). Atmospheric pressure usually is measured with a mercury barometer (hence the commonly used synonym barometric pressure), which indicates the height of a column of mercury that exactly balances the weight of the column of atmosphere the base of which coincides with that of the mercury column. Also, it may be measured using an aneroid barometer, in which the sensing element is one or more hollow, partially evacuated corrugated-metal disks supported against collapse by an inside or outside spring; the change in the shape of the disk with changing pressure can be recorded using a pen arm and a clock-driven revolving drum.

Atmospheric pressure is expressed in several different systems of units: inches (or millimetres) of mercury, pounds per square inch (psi), dynes per square centimetre, millibars (mb), atmospheres, or kilopascals. Standard sea-level pressure, by definition, equals 29.92 inches (760 mm) of mercury, 14.70 pounds per square inch, 1013.25 ´ 103 dynes per square cm, 1013.25 millibars, one atmosphere, or 101.35 kilopascals. Variations about these values are quite small; for example, the highest and lowest sea-level pressures ever recorded are 32.01 inches (in the middle of Siberia) and 25.90 inches (in a typhoon in the South Pacific). The small variations in pressure that do exist largely determine the wind and storm patterns of the Earth.

Near the Earth's surface the pressure decreases with height at a rate of about 3.5 mb for every 30 m (100 feet). The pressure at 270,000 m (10-6 mb) is comparable to that in the best man-made vacuum attainable. At heights above 1,500 to 3,000 m (5,000 to 10,000 feet), the pressure is low enough to produce mountain sickness and severe physiological problems unless careful acclimatization is undertaken.