The atmosphere has several different layers

The Earth's atmosphere has several distinct layers, some of which are shown in the image above. The layer nearest the Earth's surface is called the troposphere. The troposphere is usually approximately 16 km (ten miles) thick and is the layer that contains most of the weather. Above the troposphere is the stratosphere, which stretches from ten miles to 30 miles high. The stratosphere contains the ozone layer, which protects the Earth's surface from dangerous ultraviolet radiation from the Sun. The ionosphere is found above the stratosphere. This layer contains many free electrons and ions, which reflect radio waves. This reflection allows radio waves to travel much farther than they would without the ionosphere. The highest layer of the atmosphere is the exosphere. Here the air is extremely thin and some of the air molecules are lost to space.


Credit: NASA Goddard DAAC

For more info: http://daac.gsfc.nasa.gov/CAMPAIGN_DOCS/ATM_CHEM/atmospheric_structure.html