On Tuesday 5th September, Hurricane Irma grew into one of the Atlantic's most powerful storms. The category 5 hurricane - highest category storm on the Saffir-Simpson scale (a commonly used scale that attempts to measure potential property damage from storm winds) - had wind speeds around 185 mph, matching those of Wilma in 2005 which killed 87 people, costing billions in damage. As it moved through the Caribbean and towards southern Florida, Irma sustained winds of 185 mph for 37 hrs, making it the longest on record any tropical cyclone around the world has maintained this intensity.
It almost completely destroyed Barbuda as 185 mph winds, pummel 10 Caribbean islands and US territories - including St Martin, the British Virgin Islands, Haiti and the British territory of Turks and Caicos, and Cuba - before hitting Miami and blasting up the west coast of Florida at the weekend, with dangerous flooding along stretched of coast and 3.4 million homes without power. Irma has killed at least 28 people, left dozens injured and caused widespread damage.
Atlantic hurricanes begin life as stormy disturbances off the west coast of Africa – there are around 60 of these each year, but most peter out with only a few growing into hurricanes. Now for this to happen weneed 3 ingredients: warm ocean waters (at least 26.5°C) to fuel the growth of the storm, moisture in the mid-atmosphere (~5 km) and a lack (<23 mph) wind shear through the height of the atmosphere, as hurricanes need to maintain vertical structure. Whilst warm ocean waters are abundant across tropical Atlantic during the hurricane season (June-Nov), the last two ingredients are less consistent but when they are present, hurricanes may develop.
We use a Saffir-Simpson scale which ranges from 1 to 5 to categorise hurricanes – its a commonly used scale that attempts to measure potential property damage from storm winds. Irma is a category 5 storm which is extremely rare and unusually strong, with wind speeds exceeding 157 mph. These storms can cause complete destruction of building roofs, trees, power lines.
On average there are usually no more than about 2-3 hurricanes with a rating above 3 each season, which shows how unusual the present situation is.
How is Irma different from Harvey?
Irma and Harvey are quite different storms. Hurricane Harvey was a category 4 at its peak and bought catastrophic flooding to Texas and Louisiana – it became fairly stationary and dumped 33 trillion gallons of water onto the region.
Irma is a category 5 and one of the strongest ever recorded. But rather than torrential rainfall being the main issue, it is the wind speed and storm surges that are the main problems – unlike Harvey, it's moving faster through the Caribbean and it’s still strengthening meaning that the strong winds could destroy many buildings, trees, and power lines, whilst storm surges could inundate low-lying islands.
A succession of strong storms
First came Hurricane Harvey, which barreled into Texas and Louisiana in August,and now Irma, one of the most powerful hurricanes on record, is battering the Caribbean and has Florida in its sights. Jose and Katia – currently tropical storms out in the Atlantic - may well reach hurricane strength in the next few days. Katia is the fourth named storm in two weeks.
So is this unusual? We’re in peak hurricane season for the Atlantic - the formation of several storms in rapid succession is not uncommon at this time of the year, indeed, it’s when 95 percent of hurricanes form. However, what is rare is having a succession of two such strong hurricanes (category 4 and 5) in this space of time - in fact, since records began in 1851, the US has never been hit by two Category 4 or stronger hurricanes in the same season. Also, some of these storms are falling outside their normal range - for example, Irma is the easternmost on record.
There are possibly more to come, with tropical storms Jose and Katia potentially becoming major hurricanes the next few days.
Did Irma take people by surprise?
Not entirely. A third of the way into the 2017 Atlantic hurricane season it was looking like the region would likely experience "an above normal hurricane season" that "could be extremely active," with more named storms than previously expected—14 to 19 this season, with two to five major hurricanes.
Now, halfway through the season, this is being realised – so far there have been 12 named storms, four of which strengthened into hurricanes (with maximum sustained winds above 73 miles per hour).
The difficulty, whoever, is predicting precisely which of the stormy atmospheric disturbance flowing off the West African coast will peter out and which will turn into something stronger. And if they do strengthen, what path they will take.
What impact is climate change having?
Climate change does not ‘cause’ hurricanes, but it does have an impact. In a warming world we expect the likelihood of extreme weather events, such as Irma, to increase. Indeed, the intensity and duration of North Atlantic hurricanes, and the frequency of category 4 and 5 hurricanes, have increased since the early 1980s. However, attributing extreme weather events to climate change is a complex science; natural and human factors combine to determine how a hurricane develops and what impact it has.
As global temperatures continue to rise, the water cycle gets more active. There is more evaporation from the Earth’s surface and more condensation of water vapour into raindrops in the atmosphere. Together with warmer sea surface temperatures locally (surface temperatures in the eastern half of the tropical Atlantic Ocean were between 0.5°C and 1°C above average this summer), this can contribute to more major hurricanes (Category 3+) with higher rainfall rates, stronger winds and drastic impacts. On top of that sea levels have been rising (half a foot higher in recent decades) as the water gets warmer and expands, and ice melts, which will have an impact on storm surges.