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46 November 2017
HURRICANES
The eye of Hurricane Irma was bigger than Barbuda, and passed directly over itPuerto RicoVirgin IslandsAnguillaBarbudaBoats ride out the storm in
Miami. Irma was a Category 4
as it hit the Florida Keys with
130mph winds. Right: yachts
in Nanny Cay, on the BVIsrma was designated a Category 5 hurricane
on 5 September, making her the most
dangerous kind of tropical cyclone. The
following day she smashed into the
Leeward Islands in the Caribbean,
decimating Barbuda and causing vast damage to some of
the world’s most popular sailing destinations, including St
Maarten and the British Virgin Islands before tracking
onwards to Florida, where 130mph winds and a large
storm surge pounded the Florida Keys.
Wider than the state of Florida, Irma caused the largest
evacuation in American history of around six million
people (nearly a third of the state’s population), led to
around 70 deaths, and left untold damage to coastal
communities with hundreds of yachts destroyed.
Hurricane Irma began as a vigorous tropical wave
exiting the coast of Africa and marching through the Cape
Verde islands on 29 August. The National Hurricane
Center in Miami placed a ‘watch’ on the wave. Irma
became the ninth named tropical storm of 2017.
SHIPS (Statistical Hurricane Intensity Prediction
Scheme) models initially suggested a moderate
development of the storm, taking into account dry
Saharan dust that had been present in this area of the
Atlantic for several previous hurricane seasons. However,
water vapour satellite imagery told a different story, with
the very dry area to the north of Irma. Outfl ow (the fl ow of
air outwards from a storm system, critical to its growth)
began and in a period of just 12 hours Irma went from a
70mph tropical storm to a major hurricane with Category
3 status and winds of up to 130mph.
Increase in intensity
A subtropical ridge to the north of Irma strengthened on
2 September, pushing the storm to the west-south-west,
and into an even more favourable environment. The
central Atlantic does not have ideal sea surface
temperatures (SSTs) for hurricane formation and it is
unusual for storms to increase much in strength until
they are on a fi nal approach to the Leeward Islands and
warmer water. While ferocious and devastating,
hurricanes are fairly delicate when it comes to upper level
wind shear. Shear can take a violent Category 5 storm and
reduce it to a Category 1 or lower in as little as 24 hours if
the shear in the upper atmosphere is strong enough.
Irma was now set up in a near perfect environment to
increase her intensity. Under low shear and 29°C water
temperatures she grew into a strong Category 4 hurricane
and as she approached the northern Leeward Islands grew
into a large diameter Category 5 monster.
Then something unusual occurred: Irma started to take
on an annular hurricane structure – a giant, symmetric
eye with a thick ring of intense winds around it. With
almost no traditional banding present, the incredible
force of the hurricane’s core winds grew to 185mph
sustained, with gusts in the 225mph range.
NOAA