Atmosphere Contents

6. Secondary Circulation

The exact mechanisms for production and growth of hurricanes are not well understood

It is known that hurricanes are formed when the ITCZ is displaced about 5° north or south of the equator (the trade wind areas). The coriolis force is zero at the equator but displacement away from the equator ensures that a sufficient rotational component can be achieved about an area of low pressure formed within the ITCZ. The cyclones are therefore formed by the convergence over a vast area of surface winds, in tropical waters where the sea surface temperature exceeds 27°C, providing sufficient water and evaporation for the development of the tropical storm. Heat released through condensation of water vapour to form precipitation during convection feeds the system to develop into a hurricane. The storm systems are approximately 650 km in diameter with a pressure at sea level of typically 950 mb. They follow a well-defined path travelling along an anti-clockwise course in the northern hemisphere and clockwise in the southern. From their source region they move westwards having enough energy to cut across the poleward limb of the Hadley Cell and move into the mid-latitudes, where due to the westerlies they move north-eastwards. Once over land they tend to lose energy as they are cut off from the warm water supplied by the ocean.

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How are hurricanes formed?