Hurricane has wreaked havoc on Caribbean islands and is heading for US mainland
Superstorm Irma: warmer climate means devastating storms have more 'fuel in the engine'
Hurricane season in the Caribbean peaks from the middle of August to early September, so now is the time when severe weather events are to be expected.
But Hurricane Irma is out of the ordinary — it is the Atlantic basin’s second-most powerful hurricane ever — and a host of factors have had to combine to create it.
With sustained wind speeds of 185mph, this tropical cyclone (an intense, low-pressure hurricane formed over an ocean) is easily classed as a Category 5 hurricane, meaning it could cause devastation that leaves an area uninhabitable for months.
Since forming off west Africa a week-and-a-half ago, Irma has travelled south and then north-west towards the Caribbean, gaining energy as it did so from water vapour evaporated from the warm ocean. As the water vapour condensed to form clouds, it released heat, further warming the air around it.
For a hurricane to grow to Irma’s power, very light winds were needed in the upper part of the atmosphere.
“It seems strange you would need light winds to have something so powerful on the surface … If you have stronger winds in the upper part of the atmosphere, it blows the top off the thunderstorm at the centre of the hurricane and weakens it,” said Chris Bell, a weather forecaster with a UK-based forecasting company, Weatherquest.
It was crucial for there to be little vertical wind shear, which is variation in wind speed or direction. High wind shear weakens a hurricane by dispersing its heat; in its absence, the storm retains energy and gains strength.
Winds spiralled inwards and picked up more water vapour from the ocean, which had to be at a temperature of at least around 27C for such a severe hurricane to form. Atlantic surface temperatures have been above average this year.
Irma has also been able to strengthen because it has not faced large areas of dry air, which would have prevented heat transfer and stifled its development.
Another factor has been Irma’s slow rate of movement, averaging little more than 15mph, which has maximised the energy it has picked up from the deep warm water it has travelled over.
Irma has yet to be weakened by hitting a major landmass, which would have cut off its moisture supply. It is expected to reach the United States mainland on Sunday.
When severe weather events occur, questions are often asked about whether climate change is a factor.
“When you’re talking about climate change, you’re talking about a change to the base average weather conditions over a long-term period, over a 10 or 20-year period. A single hurricane for a week and a half does not have a direct relationship to climate change,” said Mr Bell, who is originally from Texas and has experienced tropical cyclones first hand.
“But if you have a warmer climate, that gives the atmosphere more of an ability to hold more water. You could end up having more powerful hurricanes. There’s more fuel in the engine, so to speak.”