THE equinoxes, over the centuries, have acquired an unsavoury reputation for violent storms. This year's autumn equinox occurs "tomorrow, and it remains to be seen if the dreaded gales appear on schedule.
The notion of equinoctial gales suggests that storms are more frequent around the equinoxes than they are at other times of the year.
But this is patently not so statistics show that gales in the North Atlantic are most frequent in January and December.
At Malin Head, for example, the average number of days with gale force winds increases from one in August to nearly four in September, but climbs gradually through the remainder of the autumn to reach 10 in December. Indeed, the equinoctial myth was spotted as long ago as 1772 by Dr John Rutty, a Dublin physician who took a close interest in these matters.
"As to storms that according to vulgar tradition should accompany the equinoxes," he wrote, "it appears from my history of storms during the space of above 60 years, that out of 131 storms which happened in that period of time, only one happened on the equinox and six within about three days before or after it; from whence we see how far these vulgar traditions will stand the test of solid fact and observation."
What the statistics do show, however, is that there is and abrupt increase in the frequency of high winds during the second half of September. It is not uncommon, therefore, for late September to provide the final real gale of the season, and this may account to some extent for the "guilt by association" suffered by the equinox.
Another factor has enhanced the reputation of the autumn equinox as a very windy period. Long after the winds of a Caribbean hurricane have died away, a quantity of unusually warm and very moist air frequently survives, concentrated into a relatively small region of the upper atmosphere to be carried northwards into mid Atlantic. If this warm humid air is absorbed into the circulation of an ordinary North Atlantic depression, it may add enough energy to cause a dramatic intensification of the low.
Some of our worst September storms over the years have had their origins in this process, the so called Hurricane Debbie that hit Ireland on September 16th, 1961, being perhaps the best remembered. Although the storms containing the warm, humid residues of Hurricanes Isodore and Gustav in September, 1990, tracked well north of Ireland, they do did their bit to perpetuate the equinoctial myth.