Atmosphere of Earth :
Gaseous envelope of the Earth, or any other celestial object. The Earth’s atmosphere is made up of nitrogen (78 per cent), oxygen (21 per cent), argon (0.9 per cent), carbon dioxide (0.03 per cent), varying amounts of water vapour, and trace amounts of hydrogen, ozone, methane, carbon monoxide, helium, neon, krypton, and xenon.The atmosphere is divided into several layers. The lowest one, the troposphere, extends up to about 16 km (10 mi) in tropical regions and to about 9.7 km (6 mi) in temperate latitudes. Most clouds occur in this layer. Above the troposphere is the stratosphere, which has an upper boundary of almost 50 km (30 mi). The layer from 50 to 80 km (30 to 50 mi), called the mesosphere, is characterized by a marked decrease in temperature as the altitude increases. At an altitude of 80 km (50 mi), ultraviolet radiation, X-rays, and showers of electrons from the sun ionize several layers of the atmosphere, causing them to conduct electricity. Because of the relatively high concentration of ions, this layer, extending to an altitude of 640 km (400 mi), is called the ionosphere. The region beyond the ionosphere is called the exosphere; it extends to about 9,600 km (6,000 mi), the outer limit of the atmosphere.Air 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. Normal atmospheric pressure at sea level is 760 torrs. At about 5.6 km (3.5 mi), it is 380 torrs; half of all the air in the atmosphere lies below this level.
