Hurricanes

Bringing geographical news stories alive in the classroom is an exciting aspect of teaching geography as it reaffirms its importance in society and needless to say its relevance in secondary education. By raising awareness of current affairs, young geographers are encouraged to learn about world issues and events. The devastation caused by Hurricane Irene has been reported in recent news stories, however, the media often avoid revealing the science behind such natural phenomena. Therefore, the aim of this blog is to gain a better understanding of how hurricanes form before investigating the effects of Hurricane Irene.

What are hurricanes?

A hurricane is the American classification for a ‘tropical cyclone’. These a referred to differently, depending on where in the world they arise – Typhoons in the South Asian Pacific, Cyclone in the Indian Ocean and the Willy Willies in Australia.

Tropical cyclones are born from low-pressure systems emerging over tropical or sub-tropical waters where organised convection (thunderstorm activity) and surface winds circulate.

This map published by 'The Rough Guide to Weather' (Robert Henson, 2007),
identifies the major zones where tropical cyclones develop and the typical
direction in which they travel.

An aerial view of a hurricane

Why do they form?

> Key atmospheric and oceanic conditions are the catalyst for their formation.

At the tropics there is a vast area of low pressure reaching north and south of the equator called the Inter-tropical Convergence Zone (ITCZ). North-east trade winds flow in the northern hemisphere tropics and south-east trade winds flow in the southern hemisphere. These winds facilitate the transfer of energy within parcels of convection currents (air that has been heated) causing thunderstorms. When thunderstorms cluster together, a flow of substantially warm, moist, rapidly rising air leads to the development of a nucleus of low pressure, or depression, at the surface.

Coriolis force is at its strongest at the tropics the rotation the column of rising air is easily initiated. When winds reach 74 m.p.h the tropical storm is re-categorised as a hurricane. The helical winds accelerate inwards and upwards, releasing energy and moisture in the process.

Main factors influencing hurricane development:

1. Warm surface waters exceeding 26 °c between August and October to supply sufficient latent heat energy from condensation (average of 60% humidity levels)
2. Convergence of surface winds causing air to rise and clouds to collide.
3. Low wind shear (conformity of wind speed with height) to enable clouds to ascend vertically.
4. Sufficient Coriolis force to create spin.

The structure of a hurricane:

·        In their infancy they are known as ‘tropical depression’ – this is when they are at their weakest. As they intensify it becomes a tropical storm, then a hurricane/typhoon.

·        At the core, a small area of horizontal surface winds can travel in excess of 100 m.p.h.

·        Above this huge cumulonimbus clouds extend high into the sky like great towers of spiralling air.

·        6 miles above the surface the clouds merge into a thick blanket and change direction – moving away from the nucleus or ‘eye’ of the storm.

·        Air descends at the eye of the storm – this is characteristically a funnel of dry, cloudless air.

> Once hurricanes travel over land they dissipate (die) for they loose their source of moisture.