The atmosphere is composed of distinct horizontal layers. The lowest of these layers is the troposphere which extends from approximately 0-15 or 17 km into the atmosphere. It is the most important layer of the atmosphere (to us) and is the layer where turbulence and weather phenomena are most marked. The troposphere contains 75% of the molecular mass of the atmosphere. It contains nearly all the water and aerosols (particulate matter) in the atmosphere. Throughout this layer temperature decreases at a constant rate of 6.5 degrees celcius per km. This rate is known as the Environmental Lapse Rate (ELR). The ELR is limited by a layer of temperature inversion called the tropopause. Convective activity in the atmosphere are confined to the troposphere. Convection is the movement of air parcels from a place of higher temperature to a place of lower temperature. Convective activity in the torposphere usually occurs vertically. As air rises to the tropopause the surrounding air gets warmer.the air parcel condenses and moist air causes clouds. Clouds do not go beyond the tropopause because of the temperature inversion. Clouds spread on the boundaries of the tropopause in the direction of the wind. The height of the tropopause varies because of its correlation with sea level temperature and pressure. The height of the tropopause varies from 16 km in the tropics to 8 km at the poles.
The stratosphere extends from the tropopause upwards to about 50 km into the atmosphere. Most of the ozone in the atmosphere is contained in the stratosphere. In the stratosphere, temperature decreases all the way to the stratopause. The maximum temperature in the stratosphere occurs at the stratopause. This temperature is associated with the absorption of UV radiation in the stratosphere. Water vapour created by jet aircraft increases the stratosphere's ability to absorb heat.
Mesosphere
The mesosphere is a part of the upper atmosphere. The mean atmospheric temperature above the stratopause decreases to a minimum of -90 degrees celcius at 80 km into the atmosphere. Above 80 km temperatures begin to rise again with height. This is caused by temperature inversion at the mesopause. The reason for this inversion is the presence of molecular oxegen and ozone (seen as noctilucent clouds).
These clouds are caused by dust from meteorites around which ice crystals will form from trace water in the area. Pressure is extremely low in the mesosphere. At 90 km pressure is 0.01 mb because of the low weight of the atmosphere
Thermosphere
The atmospheric density of the thermosphere is extremely low and is not much different from space. The lower portion of the thermosphere is made up of nitrogen and oxygen. They occur mainly in their molecular and atonmic forms. At about 200 km into the atmosphere, the amount of atomic oxygen is greater than nitrogen. In general the temperature in the thermosphere rises with height. This rise in temperature is related to the absorption of extreme ultraviolet radiation (UVR). Anywhere above 100 km the atmosphere is affected by cosmic radiation, x-rays and U.V.R. These cause ionisation (electric charging) of the atmosphere by seperating the negatively charged electrons form the neutral oxygen atoms and nitrogen molecules.
Exosphere and Magnetosphere
The base of this layer is between 500 and 750 km. It is a tenuous layer (barely there) and contains some atomic oxygen, hydrogen and helium. Because of their low atomic weight, the helium and hydrogen easily extend into space.
