Tuesday, July 17, 2018
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Irma: Gone but never to be forgotten

The hurricane may be no more, but it leaves behind a fascinating meteorological legacy.


Thankfully, the remnants of Hurricane Irma have largely dissipated over the eastern US, leaving behind a swathe of death and destruction across the Caribbean.

It will be many years before some communities will be able to return to anything approaching normal.

Meteorologically, too, Irma has left us quite a legacy. It was a cyclone which had a very interesting genesis, and an incredible development into one of the largest and most powerful hurricanes ever seen.

As with many hurricanes, Irma formed from a collection of storm clouds to the southwest of the Cape Verde islands, just off the coast of West Africa.

Most hurricane development is conditional on sea surface temperatures (SSTs) reaching at least 26.5C. But Irma had no problem developing over waters which were a relatively cool 23.5C.

Although the SSTs were low, there were other factors acting in favour of its formation.

First, the atmosphere was moist - always a good thing if storm clouds are to develop.

Second, and tied to the point about moist air, there is currently very little dry, dusty air blowing from the Sahara desert across the Atlantic.

Millions of tonnes of dust blow across the Atlantic every year. Some of it is deposited on the islands of the Caribbean to make them the lush tropical paradises they are.

In recent years there has been a lot of Saharan dust finding its way into the Atlantic. This dry air has a detrimental effect on any potential hurricane development. So its absence was another factor acting in Irma’s development.

Once it got going, there was no stopping Irma. It developed from a tropical wave to a hurricane within 24 hours on 30 August.

By 1 September it was a Category 2 hurricane, on the five-point Saffir-Simpson scale.

By 5 September it had reached category 5 status. Its sustained wind speeds of 298km per hour (kph) had approached the theoretical maximum for a hurricane, only topped by the 305kph of 1980s Hurricane Allen.

Even before it entered the Caribbean, Irma had rewritten the record books.

  • It was the strongest Atlantic hurricane outside of the Caribbean and Gulf of Mexico, with sustained winds of 298km per hour (kph). These records go back to 1851, and cover about 2,600 cyclones.
  • It also had the lowest central pressure ever recorded at 914 millibars.
  • Irma went on to be probably the strongest hurricane to ever hit the Leeward Islands.
  • It was then the first Category 5 hurricane to hit Cuba since 1924.
  • Irma was the longest-lasting Category 5 hurricane on record, keeping that status for over three days. Winds of 298kph were sustained for over a day and a half: also a record.

It is a little too early to be certain about how many records were broken after Irma made landfall in Florida, but rainfall totals of 404mm and wind gusts of 229kph will no doubt allow Irma to take its place in the US weather records.

Irma is perhaps a good illustration of the evidence that suggests that hurricane frequency is unlikely to change in years to come, but that warmer waters and a warmer atmosphere, as a result of global warming, allow for greater potential hurricane intensity.

With global temperatures rising towards the limit of 2C agreed upon in the Paris Agreement, it may not be long before Irma’s place in the record books is usurped by another product of our rapidly warming world.

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