Yachting World — November 2017

›

48 November 2017

Irma had reached an almost indescribable level of fury

for those in its path with absolutely nothing to stop it.

Barbuda received a direct hit from Irma at her absolute

peak. A central pressure of 914mb was observed and the

result was complete devastation to the island. Parts of St

Maarten suffered similarly.

Far from finished

The British Virgin Islands remained in the direct path of

the most ferocious winds and sea surge. My beloved

Soggy Dollar Bar webcam went off air before Irma made

landfall and the pictures that followed explained why.

Still a Category 5 storm, Irma’s eye wall passed just to

the north of Puerto Rico and as a result there was

comparatively little damage to the island. On her way to

the Turks and Caicos and Bahamas, Irma was briefly

downgraded to a Category 4 before 30-32°C water

temperatures fuelled the storm. Irma reintensified to

Category 5 status as the southern eye wall decimated the

northern shore of Cuba.

But Irma was far from finished. Now a Category 4 storm,

she rounded the south-west periphery of the subtropical

ridge steering her and headed toward the US Keys. Irma’s

long predicted turn came later than some forecasters

Anatomy of a Hurricane

■ Rising aiR: in the eye wall

and rain bands, warm, moist air

rises. Low pressure at the ocean

surface in the heart of the hurricane

draws in surrounding air. These

spiralling winds pick up speed as

they approach the eye, pulling more

heat and moisture from the ocean

surface.

■ EyE: a cloud-free area of sinking

air and light winds, usually 10-40

miles (15-65km) in diameter. Some

air is forced towards the centre of

the hurricane, where it converges.

As this air sinks, it compresses

and warms, forming a cylinder of

relatively sedate air that makes the

eye the calmest part of the storm.

■ EyE wall: a ring of tall

thunderstorms that produce heavy

rains and very strong winds. The

most destructive section of the

storm is in the eye wall on the side

where the wind blows in the same

direction as the storm’s forward

motion. So in a hurricane that is

moving west, the most intense

winds would be found on the

northern side of the storm.

■ Rain bands: Non-annular

hurricanes typically have curved

bands of clouds that trail away in

a spiral fashion. Rain bands are

capable of producing heavy bursts

of rain and wind, perhaps one-half

or two-thirds the strength of winds

in the eye wall.

annUlaR sTRUCTURE:

Irma developed an

atypical annular structure

with little or no rain bands,

and a symmetric eye.

COld ClOUds: Irma developed very cold, very high,

powerful thunderstorms on its southern, eastern and

northern sides. Storms with cloud tops reaching very low

temperatures can generate the heaviest rainfall.

expected. A carefully orchestrated evacuation had taken

Florida’s primarily east coast population and shifted it

either north or west or both. The Category 4 hurricane had

the southern Keys in its sights. The eastern eye wall hit

Marathon Key on the morning of 10 September and early

reports showed there is little left there. Key West on the

western eye wall fared much better than the central Keys.

Irma then marched in a northerly direction up Florida’s

western coast. Naples Island airport measured a gust of

142mph near the eye of the storm. It wasn’t until 11

September that Irma lost hurricane status and was

reclassified as a Tropical Storm.

Irma went on record as the strongest Atlantic hurricane

ever recorded. Caribbean Sea hurricanes have rivalled her,

and a few have had a lower barometric pressure, but Irma

will rank among the most destructive.

What made Irma so unique? Several things. A lack of

upper level low pressure systems in the central Atlantic

allowed Irma to not only form but to morph into a major

hurricane in less than one day. Given the relatively cool

waters of the mid-Atlantic this is indeed unusual.

A stronger than normal high pressure ridge to the north

of Irma forced her to the south-west. This gave Irma

warmer waters and better insulated her from dry

Hurricanes

Right: The

2017 hurricane

season has been

unusually active.

Harvey caused

widespread

damage to Texas

in august, irma

was followed by

Maria, which also

made landfall

in the leeward

islands, and Jose,

which passed

further north

illustration Weather Underground