Equinoctial gales

Ex-hurricanes blow our way
by Philip Eden

Many Septembers bring settled late-summer weather during the first half of the month, giving way to much more disturbed conditions with rain and rough winds during the second half. If this changeover should happen within, say, a week of the autumnal equinox (on or around the 22nd) mariners will talk knowingly of "equinoctial gales". Both the spring and autumn equinoxes have long been associated with stormy winds, and over many centuries seafarers came to fear violent gales in British coastal waters during the latter parts of March and September.

Early in the present century these general observations became perverted into a belief by some that gales occurred more frequently at the equinoxes than at any other time of the year. The statistics do not support such an idea. What the records do show is that there is quite an abrupt increase in the frequency of high winds in British waters during the second half of September, and a more gradual decrease in late-March and early-April. Meteorologists suspected way back in the 19th century that some autumn windstorms were somehow linked to tropical hurricanes that had been reported several days earlier on the other side of the Atlantic. But they could not be certain because there was insufficient observational data over the ocean to enable the production of proper daily weather charts which would have allowed such a storm to be tracked from day to day.

We now know that a true hurricane can only maintain its power when it is located over the warmest parts of the planet's oceans - where surface waters are at 26C or more. It will dissipate rapidly after crossing a coastline and moving inland, and it will lose energy more gradually if it strays into a region of cooler water. However, even when such a hurricane has "died", there frequently remain extensive remnants of very warm and moist air in the upper atmosphere. If this warm and humid air is absorbed into the circulation of a travelling Atlantic temperate-latitude depression it may provide sufficient added energy to cause a dramatic intensificiation of that depression, and this is undoubtedly the cause of some (but by no means all) of the severe September gales that have swept northwest Europe over the years. This is why weather forecasters sometimes refer to "ex-hurricane A" or "the remnants of hurricane B". As we have seen, this is strictly speaking incorrect, but "the Atlantic depression containing the residual warmth and moisture of hurricane C" is an awful mouthful!