Rupert Holmes examines tropical revolving storms and their aftermath
Tropical revolving storms form in the doldrums, typically between 7-15 degrees of latitude where there’s sufficient evaporation of surface water from the sea to provide an initial energy source.
They are seeded from tropical disturbances, clusters of cloud and thunderstorms. Most of these die away, but a few develop into tropical storms which reach hurricane status if sustained wind speeds exceed 63 knots. This only happens if the water temperature is high enough – usually 28C or more – to provide sufficient evaporation.
They tend to be concentrated weather features, with wind 100 miles from the storm centre often not exceeding Force 8. However, closer to the eye wall the wind increases rapidly to 70 knots for a category 1 hurricane to more than 150 knots for one of category 5.
In the northern hemisphere young storms move to the west-northwest at around 10-15 knots until a latitude of roughly 25 degrees is reached. At this point storms recurve to the north-northeast, accelerating to 20-25 knots. Storms are divided into navigable and dangerous semicircles. The former is on the equatorial side of the storm, where recurvature will take it away from a vessel, and (in the northern hemisphere) a fast broad reach on starboard gybe also takes the boat towards less intense winds.
However, recurvature takes storms closer to vessels in the dangerous semicircle. Traditionally it wasn’t possible to out-run a tropical storm by crossing ahead of it, although some of today’s fastest boats have potential to do so. Otherwise sail close-hauled on starboard until the wind is too strong to continue and survival tactics must be employed.
In the north Atlantic and eastern portion of the north Pacific, hurricane season lasts from June to November, with activity peaking in September. They can form as far west as the Cape Verde islands and in 2005 one of these recurved exceptionally early, to go ashore in Portugal. Hurricanes are unknown in the south Atlantic, where there is too much vertical wind shear for their formation, apart from one that went ashore in Brazil in 2004.
On the west side of the south Pacific, hurricane season is December to April, while typhoons may occur at any time of the year in the west side of the north Pacific, although the main season is June to November. In the Arabian Sea and Bay of Bengal storms occur most frequently when the monsoon changes.
Tropical revolving storms can end in one of two ways. If a storm’s track takes it overland it loses the evaporating sea water that fuels it and will rapidly dissipate, although it can first cause havoc in coastal areas, as well as significant rainfall further inland.
Alternatively, as a storm moves away from the tropics over the ocean, evaporation slows and the deep convection that characterises a hurricane will be lost. At this stage it expands into a deep extra-tropical depression, such as hurricane Katia at the end of last summer, that can wreak destruction on European shores.
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