The storms of autumn

10.10.08 08:05

After some fine weather at the end of September, the torrential rains and strong winds that blighted much of the United Kingdom last weekend served as a reminder that we are well and truly moving into autumn.

At this time of year people sometimes start to talk of ‘equinoctial storms’ – in other words, storms that occur approximately at or soon after the time of the spring or autumn equinox, the latter being around September 22.

The definition of ‘storm’ is rather loose: it could be taken to mean a small-scale but powerful weather feature such as a cluster of thunderstorms or hailstorms or even a tornado.

Given the likely etymology of the phrase, however, it is generally accepted as meaning a large, deep, vigorous depression, bringing heavy rain or strong winds - or both - to a wide area. The storm of October 1987, for example, might come under the umbrella – pun intended – of ‘equinoctial storms’, even given that it occurred on the night of October 15-16.

The popular belief that these storms accompany the vernal and autumnal equinoxes is most likely to have arisen in the mid-18th century, when mariners in the Caribbean were battered by hurricanes that tend to reach their peak frequency near the end of September.

In temperate latitudes there is not much evidence that points towards exceptionally frequent stormy conditions around the equinox specifically, although what might be considered the first ‘severe’ storm of the autumn can often appear around this time as storms start to track at latitudes further south: a consequence of the northern hemisphere cooling.

However, it is interesting to note that the deluge last weekend was due in part to ex-Tropical Storm Laura, which had drifted north off the eastern seaboard of the USA then, caught in the jet stream, rushed east towards the UK and northern Europe.

Although not a tropical storm by the time it reached our shores, the subsequent depression still retained much of its energy and moisture. Such storms, some of which might previously been hurricanes, are occasionally, although by no means always, the cause of severe winds in the UK during early autumn.

This particular ex-tropical depression brought 20 to 50mm of rain to much of England and Wales within a 24 hour period, and more than 50mm to parts of Wales and to Cumbria; the most recorded was 92mm at Capel Curig, Conwy, and when the storm moved into the Netherlands it dumped a further 113mm.

The concept of equinoctial storms is familiar – perhaps even more so – in North America, and few people in the northeastern USA or eastern Canada could have been dissuaded of their existence when Hurricane Kyle blew past around September 28, making its first landfall, rather unusually, as far north as Nova Scotia.

Benjamin Franklin was among the first in the USA to formally remark on autumnal storms as far as they affect the northeast coast. In 1747 he noted that “we have frequently along this North American coast storms from the Northeast which blow violently, sometimes three or four days”.

Here he was describing the notorious ‘Nor’easter’, which later in the year can produce copious snow in the northeast. In Britain, the estimable Doctor Samuel Johnson even included a definition of ‘Equinoctial Gales’ in his dictionary in 1768.

While we now know that there is nothing particular or peculiar about the equinoxes that would engender storms of any magnitude, it is fair – even obvious - to say that late September marks an increase in the frequency and severity of strong winds while they gradually decrease during March and April.

By: Stephen Davenport