Equinoctial galesMeteorological Glossary
The implication contained in this term, which is in fairly wide popular use, that GALES are either more frequent or more severe near the EQUINOXES than at other times, is not supported by observations. In all parts of the British Isles, for example, the peak frequency of moderate or severe gales is near the winter solstice and the minimum frequency near the summer solstice.
I don’t know who and when someone coined the phrase equinoctial gales, but each equinox I can’t help being on the lookout for any potential severe low that could bring gales to the start of the spring or autumn season. Some years they do occur to coincide with the change in season, but most years they don’t or miss the mark by coming early or late. So is there any truth to the term Equinoctial Gales and do gales peak at the time of each equinox? Well if you look at the gale index in the objective LWT data that’s maintained by the Climate Research Unit at University of East Anglia the short answer is no they don’t.
It’s true that in spring the daily mean MSLP does fall from 1013.2 hPa on the 12th of March to a minimum of 1011.3 hPa on the 30th. At the same time there is also a small peak in the frequency of gales days on the 18th (16.5%), but that falls, albeit rather erratically through the rest of the spring. These figures could be used to support the notion that March comes in like a lion and goes out like a lamb.
In autumn mean daily air pressure falls for the whole of September, from ~1014.2 hPa at the start of the month to 1013.0 hPa by the end of it. Gale frequency on the other hand increases throughout September, from close to 4% at the start to 10% by the end of the month, with no obvious peak at the time of the equinox.
I am not quite sure what statistics the late Philip Eden was referring to in his short article that he wrote about equinoctial gales, but he seems to support the idea that there is a peak in Autumn gales due to the fact that IONA is in the firing range of any long lived ex-hurricane. If you examine the graph of gale frequency at the end of September it does paek which would coincide with the height of the tropical cyclone season on the 12th of September.
For a more definitive answer than mine, which although I say it myself I think is pretty sound, you’ll have to subscribe to the Royal Met Society or fork out $7 for a quick read to find out what conclusion Jill Austin came to about the whole subject. My apologies to her for pinching the title of her article.