Hurricane Irma is already blasting its way into the record books, but what causes destructive storms like it to build?

As recovery begins on the islands of the Caribbean and Florida braces itself for an onslaught, Irma has stormed into the record books as the most powerful hurricane ever recorded in the Atlantic Ocean.

But such weather systems are far from a rarity. Hurricanes are a type of storm known as a tropical cyclone, which build from warm, moist air over the oceans.

Water evaporates, leaving behind a region of low pressure, which is then filled by surrounding air, explains Adrian Champion, an expert in climate systems from the University of Exeter. This causes more “new” warm air to rise into the cyclone, he explains. As this warm, moist air cools, it forms clouds, releasing energy back into the cyclone, helping it to develop.

Such systems form in the tropics as the water is warmer than elsewhere, therefore more easily evaporated. The “spin” of the hurricane is down to the rotation of the Earth affecting the movement of the air: since Irma formed north of the equator, it spins anticlockwise.

Many different factors can cause the system to build. “What causes it to intensify [beyond a standard cyclone] is either that the ocean is unusually warm or there is further instability [such as strong winds] in the atmosphere,” said Champion.

Depending on where these systems strike, they are given different names. “If they hit the Americas then they are called hurricanes, if they hit Asia they are called typhoons and if they hit Africa and Australia they are called cyclones,” said Champion.

But it is when these severe storms head towards land that interest generally gathers pace – a factor that has led to hurricanes and other tropical cyclones being given specific monikers to avoid confusion and help the public and the media to share information. The names themselves are allocated based on a system devised by the World Meteorological Organization – if a tropical cyclone is particularly deadly, as in the case of Hurricane Katrina, its handle will not be used again.

The strength of a hurricane is marked by its category on what is known as the Saffir-Simpson scale, which reflects the wind speeds – and hence the level of damage expected once the cyclone strikes land.

While category 1 hurricanes are serious, expected to damage rooftops and bring down trees with wind speeds between 74 and 95mph, Hurricane Irma was initially at the top end of the scale, meeting criteria for a category 5 hurricane: winds of 157mph or more, catastrophic damage, power outages that could last for many weeks, and areas of land rendered uninhabitable for months.



But drastic though Irma is, one thing is for sure: the storm will pass.

“The big thing is [hurricanes] only have this energy supply when they are over the oceans,” said Champion. “When they hit landfall they weaken and die away.”