Hurricane!

Hurricanes are intense tropical cyclones, capable of delivering winds of extreme velocity and causing massive damage to a large area. The name hurricane is used specifically for tropical cyclones in the Atlantic Ocean, but similar intense depressions affect other areas of the topics as well. Typhoons affect the China Sea and the coasts of Japan, Philippines and China, willy-willies start in the south western Pacific and track towards Queensland and Papua New Guinea, Cyclones in the Indian Ocean can affect the Bay of Bengal (October - November) or swing south to Madagascar and the eastern coast of Africa from December to March.

Atlantic Ocean tropical cyclones (hurricanes) typically form from August to late October, although they can begin as early as June, and are the result of the northward motion of the inter tropical convergence zone (ITCZ) as a result of the seasonal migration of the overhead sun. Hurricanes will only form when a mix of conditions reach the right values:

• when sea temperatures are at 26°C or higher
• in autumn when sea temperatures are at their highest levels
• when there is a depth of warm water beneath the developing storm
• between latitudes 5° and 20° north or south of the Equator - within these limits water tends to be warmer and there is sufficient motion from the rotating Earth to start the storm spinning (Coriolis effect)

The ITCZ creates the conditions for air masses to converge at low levels. If this happens over a large area of warm water, where temperature, pressure and humidity are similar over large distances and anticyclonic conditions occur in the upper atmosphere, then intense low pressure and strong winds can develop close to the surface. The rotational motion of the earth means that this atmospheric disturbance tracks westwards, being flung towards the pole as it goes. In the northern hemisphere, hurricances migrate in a west north westerly direction, turning more northwesterly and even north as they go. Heat energy must be available to maintain the rising air. As the warm moist air rises from the surface, condensation occurs releasing massive amounts of latent heat energy. This drives the rotational winds, increasing speeds to the high velocities associated with true hurricanes (120 -160mph). The condensation creates the distinctive rotating clouds which show so well on satellite images and provide exceptionally high rainfall. It has been estimated that the energy released by a hurricane in a single day would be enough to power the United States for six months. The distinctive 'eye' of the hurricane only develops when the hurricane reaches maturity - at this stage the wall clouds surrounding the eye deliver immense rainfall with very high velocity winds swirling around the eye. As the eye itself passes over an observer, the wind will drop, temperatures will rise rapidly and clear skies will be seen. As the eye passes, the high velocity winds will return - but blowing from the opposite direction to before.

Once the hurricane moves over land, or over colder water, it loses its source of latent heat energy and rapidly decays. Atlantic hurricanes track across the Caribbean islands, towards the USA, crossing Florida and into the Gulf of Mexico or swinging north to the coast of Carolina. As the hurricane makes landfall it raidly loses energy and wind speeds drop - the hurricane is downgraded to a tropical storm and then to a tropical depression.

Hurricane hunt

Hurricanes can be tracked using satellite imagery and ground level observations. Early development can be seen from satellite - there are few remote observations stations in the equatorial Atlantic! Check the hurricane watch page for links to satellite images and remote buoy observations for Atlantic tropical cyclones.