Outline the main causes of air movement. Discuss the application and limitations of the tri-cellular model.

Rossby waves are bands of air that flow in troughs and ridges and can be found in each hemisphere, with approximately three to six waves in each. Within these waves there is fast flowing air also known as jets streams which are strong regular winds in the upper atmosphere that can be found between the poles and tropics, about 10 Km above the surface. Two jet streams can be found in each hemisphere causing air to move at high speeds of 100-300 Km/hr. One is found between 30-50degrees which is the polar front jet stream that moves eastwards the other is the sub tropical jet stream which is between 20-30degrees and moves westwards. Jet streams are a result of air temperature differences that are found at different cells of the tri-cellular model: polar (Polar cell), subtropical (Ferrel cell) and tropical (Hadley cell). The air travels at varying speeds as it is warmer and faster travelling towards the poles and therefore colder and slower moving away from the poles. Where this air slows down, it builds up and the weight causes it to subside to the ground causing high pressure. Therefore the air that speeds up is sucked up from the ground level to then replace the gap, creating low pressure. This process then causes alternate areas of high and low pressure creating anticyclones and depressions, this explaining why the usual path of the polar front jet stream towards the North East to the UK brings frequent wet and windy weather.

The Hadley cell which is at the equator involves the meeting of trade winds from the inter-tropical convergence zone, where the sun is directly overhead and therefore creates equatorial heating. The air rises due to convection which then causes low pressure and afternoon thunderstorms explaining why tropical rainforests and conditions are found at the equator. The rising air cools to the surrounding environmental temperature so uplift ceases and moves away from the equator. As the air continues to cool the density increases causing it to subside at the subtropical high pressure belt leaving clear skies and dry stable conditions creating desert climates. At the surface some of this air returns to the Equator as North East trade winds.  The rest of the air then moves pole wards into the Ferrel cell forming south westerlies. Then once the warm winds meet the cold Arctic air at the polar front at about 60degrees North it is uplifted creating low pressure and unstable conditions like heavy rainfall whilst some of the rising air then returns to the tropics. As the rest of the air continues to rise pole wards to the Polar cell it loses heat and therefore descends which makes high pressure and dry, stable conditions. The air returning to the polar front is also known as cold easterlies.

The friction of the earth’s surface is not taken into account with the tri-cellular model as this decreases the wind speed and then modifies its direction. The coriolis and centripetal force also limit the model as they affect the air movement. The coriolis force is the deflection of wind due to the Earth’s rotation, it is deflected to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. However it doesn’t exist at the equator, it only increases towards the poles. The centripetal force is the air moving towards a low pressure area in a deflected path that then increases towards the centre of low pressure.