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The region where the trade winds of the two hemispheres meet is known as the Inter‐Tropical Convergence Zone (ITCZ). There, warm, moist air is forced upward. When it cools, large amounts of water vapor condense, leading to massive cloud formation and high rainfall almost every day. Tropical cyclones, Earth's most powerful and destructive storms are also associated with the ITCZ (Figures 3.11 and 3.12).

Huge amounts of energy are transferred by these short‐lived storms as they form over the tropical oceans, then move westward before heading towards higher latitudes and decaying. Most of them are born between June and October, when the surface temperatures of the oceans exceed 27°C. (They are rare close to the equator, since the Coriolis effect is weak and rapid rotation is less likely.) The number varies, but on average there are 40–50 tropical cyclones each year, with an equal number of less intense tropical storms.

These storms are given different names by the local inhabitants, but the causes are the same.⁶ They begin as the result of rapid upward movement of air caused by divergence and an absence of strong winds at high altitude. This causes surface pressure to fall rapidly.

Figure 3.12 Katrina was the first Category 5 storm of the 2005 Atlantic hurricane season. One of the most powerful hurricanes ever recorded, it formed over the Bahamas on August 23, 2005, then crossed southern Florida before strengthening rapidly over the warm waters of the Gulf of Mexico (orange‐red). A storm surge from Katrina caused catastrophic damage and flooding along the coastlines of Louisiana, Mississippi, and Alabama.

(NASA)

Warm, moist air flows inwards towards the developing center of low pressure. As the surface air pressure plummets, the wind speed increases dramatically. The strong updraft causes towering convectional clouds to form, causing heavy rain. The storms are also fed by an additional source of heat, which is released during the rapid condensation process (conversion of water vapor to water droplets). This is known as “latent heat.”

A fully fledged storm can develop in less than a week. At its height, wind speeds around the central “eye” may exceed 120 km/h, occasionally producing sustained winds of 250 km/h in a Category 5 storm. Indeed, everything about tropical cyclones is extreme. The circulating system of strong wind and dense cloud may measure up to 1,000 km across. Spiral bands of convectional cloud tower 12 km or more above the ocean surface.

A well‐developed tropical cyclone may pick up as much as two billion tonnes of water a day through evaporation and sea spray; 1,250 mm of rain may be dropped over any location along its track. As a result, flooding and mudslides are often more of a problem than damage caused by strong winds.

A central pressure as low as 880 mb also causes the ocean surface to bulge upwards. When combined with huge waves whipped up by the wind and the torrential rainfall, the outcome is a storm surge, which can inundate low‐lying coastal areas.

An average tropical cyclone transfers a huge amount of energy. The daily energy release from a cyclone is equivalent to the output of the entire United States' electricity grid over six months.

The tracks followed by tropical cyclones are very hard to predict. However, the average storm moves east to west along a curved path, traveling at 15–50 km/h. They rapidly lose their potency when deprived of their supply of warm, humid air, so they usually die out when they move inland. Others lose intensity when they head away from the tropics and move over cooler water. However, downgraded hurricanes may still bring strong winds and heavy rainfall to temperate regions, particularly if they merge with other low‐pressure areas or depressions.