The Earth’s Atmosphere

The earth is surrounded by a layer of air, which is called the atmosphere. It reaches to 560 kilometres from the surface of the Earth, so we are only able to see what occurs fairly close to the ground. The atmosphere consists  about 77% nitrogen, 21% oxygen, with traces of argon, carbon dioxide and water. The atmosphere of the Earth can be divided into several distinct zones, as shown in the figure.

The troposphere is where all weather takes place; and extends from 8 to 14.5 kilometres above the surface of the Earth. The air pressure at the top of the troposphere is only 10% of that at sea level. Above the troposphere is the stratosphere and extends to 50 kilometres, where air flow is mostly horizontal. The thin ozone layer in the upper stratosphere has a high concentration of ozone. This layer primarily important as it absorbs the ultraviolet radiation from sunlight. The mesosphere is above the stratosphere and extends to 85 kilometres. Above the mesosphere is the thermosphere (or ionosphere) and extends to 600 kilomettres, where many atoms are ionized. The ionosphere is  very thin, but it is where aurora takes place, and is also responsible for absorbing the most energetic photons from the sun. the exosphere starts at the top to the thermosphere and continues until it merges with interplanetary gases, or space.