Air: The earth's atmosphere is made up of a mixture of gases:

This layer of gases varies in thickness according to altitude and location. Gases change as they are warmed or cooled, or as their pressure is changed. Our weather, seasons and climate are all dependent on the way the cycle of the gases that make up the atmosphere work.

The lowest layer in the atmosphere is the troposphere; this is the warm layer of air next to the earth's surface and differs in thickness from the poles to the equator. Most of the cloud systems we are in this layer. Above the troposphere is the stratosphere that reaches to about 50 km, here the temperature remains constant. The next layer is the mesosphere, then above that the ionosphere which extends to about 1,000 km. All play an important part in the overall behaviour of the air in which we live.

The lower atmosphere (troposphere) in which we live and breath is a giant convection system powered by solar energy, most of which reaches the earth at the equator. As warm air rises it cools and then descends so flowing over the earth. however this rise and fall of air is interrupted by the seas and land surfaces the air crosses.

These interruptions create air masses. Polar and arctic air masses are cold, tropical air masses are warm. Continental air masses are dry having travelled over land and maritime air masses are moist having crossed oceans. As air travels across oceans, seas, and large bodies of water it collects water vapour. The warmer the air the more moisture it can hold. Air blown across large land masses tends to be dry.

Emissions from human activity are added to the flows of air and water vapour collecting over industrial areas, towns and cities is often polluted and eventually falls as acid rain.

Because the earth rotates every twenty-four hours, the flows of air are generally driven by east - west convection, rather than north/south. This creates the surface winds we know as 'easterlies' and 'westerlies' and are the prevailing weather systems.

The conditions in the atmosphere dictate both weather and climate these are closely linked to human activity, they determine how and where we produce crops. Climate change such as global warming affects the seasons and our ability to grow food. human activity is changing the composition of the gases in the atmosphere such as greenhouse gases that in turn causes changes in the weather systems. Increased rainfall causing extreme flooding, droughts, extended El Nino cycles are just some of the ways changes in the atmosphere affect the weather systems.

The air in the troposphere is the air we breathe and that all living things are dependent upon for life. Pollutants in the atmosphere inhibit the way plants and animals are able to exchange gases, such as oxygen and carbon dioxide. This 'breathing' is called respiration. All living things respire by taking in oxygen to produce energy for food. During daylight hours plants take in carbon dioxide, one of the greenhouse gases and produce oxygen, at night this is reversed. tropical rainforests such as the Amazon Basin are known as the 'lungs of the world' for their ability to assimilate carbon dioxide so cleaning it out of the atmosphere. The destruction of these vast forests reduces the amount of CO2 being removed and increases the build up of greenhouse gases.