The gathering storms

Three hundred years ago today - just before midnight on 26 November 1703 - a fast-moving Atlantic hurricane hammered into Britain from the west. It arrived entirely without warning, but it was to make its presence felt over the next six hours as it battered its way eastwards through the night. By dawn the following morning it had moved on to Scandinavia, but it left behind a 300-mile wide trail of devastation across England and Wales, all the way from Cornwall to the Wash. Some 8,000 people were killed, thousands more injured, and Britain looked and felt as if it had been transformed overnight into a war-torn country with a battlefield in every village and town. The author and journalist Daniel Defoe was in London at the time, and he used his own experiences of "this terrible Providence" as material for what became his first full-length book, The Storm, which he published the following year.

Defoe's account offers an unparalleled insight into what is still the worst storm in British history, and the only true hurricane ever to have arrived on our shores at maximum strength. We have had plenty of severe storms since 1703, most notably on the night of 15-16 October 1987, but we have never played host to another hurricane.

The meteorological definition of a hurricane is a tropical storm with sustained wind speeds of at least 74mph, originating in the western Atlantic. Given the rapidity with which our weather patterns are changing, however, and the increased number of damaging storms which we have suffered in recent years, how long will it be before Britain feels the force of another visiting cyclone like Defoe's? We are not just talking of a belter of a storm depression veering up from the Bay of Biscay, as happened in October 1987, but a full-blown hurricane flailing in from the tropics "like an Army of Terror in its furious March", as Defoe so memorably described it.

The question is difficult to answer with any degree of certainty, but since we know that the oceans are getting warmer every year, that a storm of such severity over Britain is viewed as a once-in-every-300-years event, and that the hurricane season for 2004 has been predicted to be unusually active by the US government's Hurricane Research Division, it appears that the question may well answer itself sooner rather than later.

The rise in ocean temperatures is the crucial factor here. Hurricanes have their origins in the warm waters of the Atlantic Ocean east of the Caribbean, where they are generated by the collision of warm and cold air masses along lines of convergence known as fronts. Large volumes of cold, heavy air drift south from the polar regions, to encounter large volumes of warm tropical air drifting northwards from the equator. The buoyant, moisture-laden tropical air rises above the heavier polar air, causing a drop in atmospheric pressure along the line of their convergence. The greater the difference in temperature between the air masses, the steeper the pressure gradient between them will be, and the deeper the resulting depressions (lows) that form along their border.

The tropical air is full of warm water vapour, which it picks up through evaporation from the surface of the sea, but as it is forced up over the colder air, the water vapour it contains condenses quickly into cloud, releasing an enormous amount of energy in the form of latent heat. This freed-up energy, excited and unstable, begins to drive the winds around an available low-pressure centre and, as the system builds, it begins to move, and what began as a small frontal wave at the border of a pair of colliding air masses is fast turning into a storm.

This process occurs all year round, but it is intensified during the hurricane season, which lasts typically from July to September, due to the higher temperature of the water at that time of year. The warmer the water, the more moisture it loses to evaporation, and the more latent energy will be released later on when the water vapour condenses into cloud. Warmer oceans lead to bigger storms. Hurricanes can form only above water with a surface temperature of 26C (79F) or more, so an overall rise in ocean temperatures may well encourage more storms, in more places, to develop into hurricanes in the future.

The distinction between a storm and a hurricane is more than a matter of meteorological semantics, despite what many people felt after the notorious "no hurricane" TV weather forecast of October 1987. The October storm was not a hurricane, since its origins were in the eastern Atlantic and it featured no sustained wind speeds of more than 74mph; psychologically, however, it felt like one, and that it seemed to take the Met Office by surprise only added to the sense of alarm.

In fact the Met Office was aware of the 1987 storm, but thought that it would pass along the Channel to France rather than move inland to Britain, which it did, suddenly and without warning, at around 1am on 16 October. The BBC weatherman Michael Fish, who said in that evening's television forecast that "actually the weather will become rather windy", was guilty of understatement rather than error. Listeners to that evening's Radio 4 Shipping Forecast, incidentally, would have been far better informed about the situation, since it carried unambiguous severe gale warnings for the entire southern coastal region. Those who live near the coast will already be aware that the Shipping Forecast is the only bulletin for them, since its detail and precision is unmatched by anything that television weather reporting can offer.

Damaging and disturbing as it was, the storm of October 1987 cannot begin to be compared with a devastating hurricane, which is in another order of magnitude altogether. In October 1998, for example, hurricane Mitch rampaged through Central America, its 180mph winds destroying everything in its path. Some 10,000 people lost their lives in the carnage, many of them in mudslides which transformed the landscapes of Honduras and Belize into what looked like the battlegrounds of the Somme.

If the British Isles were to be hit by a monster such as Mitch, we would quickly discover, once and for all, the difference between a hurricane and a storm. Bearing in mind that the Atlantic as a whole is heating up through global warming, and that an increase in hurricane numbers and severity is expected as a result, the right conditions may well be emerging for just such a lesson to be delivered.

It has happened before, after all. The hurricane of 26-27 November 1703 arrived at the end of an unusually warm and wet year, which suggests that there would have been temperature anomalies lurking around the mid-Atlantic regions. Perhaps they were the tail ends of earlier dying hurricanes, or the result of an exceptionally strong jet stream caused by frontal wave activity off the North American coast, but whatever their origins, they clearly contained enough latent heat to "feed" the hurricane on its long journey eastwards, boosting its passage with little bursts of energy, like the refuelling of an aircraft in mid flight. In other words, it would have arrived in Britain as fresh as the day it was made, due to the lingering and dangerous effects of the unusually warm hurricane season that preceded it.

Three hundred years on, the oceans are heating up, and we are faced with the possibility of a return visit. Forget the 1987 storm - that was not the 300-year event. Such storms may themselves be products of global warming, but it's a hurricane that we need to look out for, heading towards us over the western horizon. "No Pen can describe it, no Tongue can express it, no Thought conceive it," wrote Defoe in 1703, and he prayed to God that nothing like it would ever happen again.

'The Storm' by Daniel Defoe, edited with an introduction by Richard Hamblyn, is published today by Penguin, £14.99