Hurricanes hardly happen in the Hebrides... or do they?

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The remains of ex-hurricane Gonzalo crossed Scotland on Tuesday, bringing heavy rain, strong winds and some disruption.

Although inconvenient and unpleasant for us, the amount of disruption caused was a far cry from that of the full-blown hurricanes which regularly threaten vulnerable tropical islands and coasts during the summer.

In the Atlantic Ocean (Gulf of Mexico and the Carribean Sea), the hurricane season lasts from June to December and up to a dozen or more hurricanes may form in any given year.

Occasionally (usually once or twice a year) one of these hurricanes swings north-eastwards into the North Atlantic Ocean and heads up towards Scotland, just as ex-Hurricane Gonzalo has done during this past week.

What are hurricanes?

Hurricanes are part of a family of small revolving storm systems in the Tropics, also known as Tropical Cyclones (in the Indian Ocean and Bay of Bengal) or Typhoons (in the Far East and China Sea).

Unlike our regular low pressure systems or “depressions“ in, tropical cyclones are found only in the tropical regions between about 5° and 25° N/S.

They are smaller than our deepest Atlantic lows, being only a few hundred kilometres across, but are considerably more severe and average windspeeds near to their centres can reach well over 120 knots (140 mph).

Such storms may also bring torrential rainfall of up to 500 mm/day (20 inches) and storm surges (sudden rises in sea-level) of up to 10 metres, devastating coastal regions.

About 80 tropical cyclones occur worldwide each year, but only about 20 of them reach “major” status.

There are a number of criteria that must be met before a hurricane can form. Most importantly is the pre-requisite that the sea temperatures must be greater than about 26.6°C (80°F) – which thankfully, is never achived near Scotland.

Only temperatures higher than value this allow for sufficient evaporation of water to sustain the storm system. If windspeeds do not vary greatly with height, this also favours their development, as it helps to avoid the system “tearing itself apart”.

This is contrast to our usual North Atlantic lows, in which windspeeds increase strongly with height.

How is a hurricane born?

In the case of the North Atlantic hurricane, the initial disturbance from which it grows is usually the remnants of a large thunderstorm moving offshore from North Africa.

As these disturbances track across the Atlantic from east to west, a tropical storm system may form. If average windspeeds within the storm reach 56 knots or more (64 mph), then the storm is classified as a tropical cyclone (hurricane).

Once formed they are remarkable self-sustaining systems, and it is not unknown for a tropical cyclone to meander around the tropics for two weeks or more, before eventually making landfall or decaying over an area of cooler sea temperatures.

The Eye

The most characteristic feature of a severe hurricane is the circular “eye” in the direct centre of the storm (see photo, courtesy NASA).

Typically the eye is a few tens of kilometres in diameter and is an area of eerily calm, clear and warm conditions, in complete contrast to the immense towering walls of cloud and raging tempest surrounding it on all sides.

Atmospheric pressure in the central eye has been known to drop to a low as 880 hPa (millibars).

Hurricane Force

A hurricane should not be confused with the term hurricane force, which refer to sustained winds of 64 knots or more (equivalent to Force 12 on the Beaufort Scale), no matter from what type of storm system they are generated.

Thus, the most severe winter windstorms in the Hebrides may be referred to as bringing winds of “hurricane force”, but strictly speaking they are not hurricanes.

Pack a Punch

But - every twenty years or so, one of ex-hurricanes reaching Scotland can still ‘pack a punch‘ and is very severe. Usually such ex-hurricanes heading towards Scotland lose their identity quickly and dissipate over the cooler seas of the North Atlantic, but the remaining large quantities of moisture and energy may become incorporated into our own separate low pressure systems, giving them a kind of “turbo boost”.

Such happened, for example, on 16 September 1961, when a low pressure system owing its origins to ex-Hurricane Debbie gave an extraordinary wind gust of 98 knots at Malin Head in county Donegal, Ireland, a record that stll stands to this day, and there was widespread havoc and disruption.

This week, we are having to deal with consequences of a very unsettled, albeit less severe ex-Hurricane Gonzalo.