Jet streams

    The warm south westerly winds from the equator meet cold Arctic air at the Polar Front. At the Polar Front air is uplifted to form an area of low pressure and this forms the rising limb of the Polar and Ferrel cell. Surplus heat is transferred either by rising air returning to the tropics or some air travels towards the poles where having lost heat it descends to form a stable area of high pressure. Air returning to the Polar Front does so as cold easterlies.

    This three cell model is however, simplistic as the reality is much more complex.  In theory if the world did not rotate and was composed entirely of land or water there would be one large convectional cell in each hemisphere on Earth. Surface winds would therefore blow parallel to the pressure gradient i.e. high to low pressure areas would be present. However, due to the Coriolis force air is deflected to the right in the Northern hemisphere. This is affected by the Earth’s rotation.

    The Coriolis force influences the surface and upper air circulation as rising air, warmed at the Equator loses heat to space as there is less cloud cover to retain it. Moving air is also deflected to the right in the Northern hemisphere and to the left in the Southern hemisphere. This is the effect of the Coriolis force. The Earth’s rotation through 360° every 24 hours means that the wind blowing in a northerly direction in the northern hemisphere appears to have been diverted to the right. Therefore, the prevailing winds from the tropical high pressure zone approach from the south west rather than the south. Winds in the upper troposphere, unaffected by friction with the Earth’s surface, show there is a balance between the forces exerted by the pressure gradient and the coriolis deflection. The result is the geostrophic wind which blows parallel to isobars. Friction caused by the Earth’s surface upsets the balance between the pressure gradient and the Coriolis force by reducing the effect of the latter.

    Winds result from differences in air pressure which in turn can be caused by differences in temperature and the force exerted by gravity as pressure decreases rapidly with height. As an increase in temperature causes air to heat, expand, become less dense and rise creating an area of low pressure below. Conversely, a decrease in temperature produces an area of high pressure. Differences in pressure are shown by isobars which are lines joining places of equal pressure. The pressure gradient is the difference in pressure and therefore, the higher the pressure gradient, the stronger the winds.

          Higher in the atmosphere, less friction is evident than at ground level. As a result, the winds are stronger and are deflected more by the Coriolis force. The outcome is that air circulation is more directly east-west than at the surface. In the tropics, the main movement is from east to west but in higher latitudes there are stronger air flows from west to east. Within these flows, there are flows of very high winds called jet streams. The average speed of the high altitude westerly is 125km/h however, maximum speeds can reach 500km/h/ Jet streams are formed where there is a rapid temperature gradient. A polar jet stream is evident at the tropopause above the Polar Front and a sub tropical jet stream at about 25°-30° latitude also exists at the boundary between the Hadley and Ferrel cells.

   The winds in the upper troposphere do not flow in parallel lines around the Earth. Winds in the higher latitudes have a wave like pattern called Rossby waves. These waves change shape and wave length, creating a series of ridges and troughs. The movement of these Rossby waves and the polar front jet stream affects the weather in temperate latitudes. Settled conditions as well as changeable conditions are due to complex changes in the upper troposphere. The causes of Rossby wave patterns are not understood well although surface features eg. mountain ranges and oceans are thought to have an influence.

     In conclusion, factors such as Rossby waves, jet streams and the coriolis force all influence upper air circulation. These all deflect the wind from its original course if the Earth was perfectly smooth. On the surface pressure and friction from land results in winds being deflected. Therefore, surface and upper air circulation is never perfect as a number of factors influence the course and strength of the winds.