Autumn heatwaves are often referred to as "Indian summers", but what does the term actually mean? Meteorologist Philip Eden explains.

Something strange has been happening to the way we perceive our climate over recent years.

Nowadays, when we get a bout of unusual weather, we ask what has gone wrong. There seems to be a growing disconnection between Brits and their very own climate.

But most spells of unusual weather are simply that - unusual. They have happened before and they will happen again.

One or two date records may have been broken during this spell, but date records are relatively easy to break. Taking a rather broader view, highs of 27-29C (80.6-84.2F) have happened before in late September.

The mercury has soared to 30C, or just above, on several occasions during the last week of the month. The highest October temperature ever recorded in the UK was 29.4C in the small market town of March, Cambridgeshire, on the first day of the month in 1985. Thus was coined meteorologists' favourite quiz question: "When was the October temperature record set in March?"

The idea there are particular times of the autumn in Britain when these warm spells might recur is a fanciful one

Autumn heatwaves such as this are frequently dubbed "Indian summers". But in the last few years the term seems to have become slightly less popular, possibly as a consequence of political correctness in the US. Somehow, "Native American summer" doesn't have quite the same ring to it, does it?

A late-19th Century Boston lexicographer named Albert Matthews made an exhaustive search of early American literature in an attempt to discover who coined the expression. The first reference he found dated from 1778, but from the context it was clearly already in widespread use.

The North American Indians - native Americans - who lived on the eastern seaboard used to depend on extended periods of fine, quiet, sunny weather at this time of the year to complete their harvest and to put together stores of food to see them through the long, cold winter.

The north-eastern US is well known for the combination of high temperatures and high humidity levels during the summer, often starting in June and not subsiding until September.

The early settlers quickly became aware of the contrast between the debilitating heat of high summer and the more comfortable conditions which prevailed during the autumn. During this time, long periods of hard work could be tackled without risk of heatstroke. They also noted that practically every autumn produced at least one such spell - many had two or three.

Some claim that an Indian summer cannot come until after the first damaging frost of autumn, or after a severely cold episode sometimes known as a "Squaw Winter".

Image caption A spell of autumnal warm weather was once linked to the church calendar

In the UK, weather observers knew of the American usage from the mid-19th Century onwards, but the expression did not gain wide currency until the 1950s. Famous 20th Century climatologists such as Ernest Bilham, Gordon Manley and Hubert Lamb did not use it in their extensive writings, except when referring to the American phenomenon.

Once established though, it quickly became linked to those spells of unseasonably warm weather which occur from time to time during the autumn.

The latest edition of the Meteorological Glossary defines it thus: "A warm, calm spell of weather occurring in autumn, especially in October and November." So, it is most commonly used in those two months, but the idea of such a spell starting in late-September is not necessarily precluded.

Before the middle of the last century, such a spell of fine weather would be linked to ancient weather lore and the church calendar.

In mid-October, for instance, it would have been called "St Luke's Little Summer" as the feast day of St Luke falls on 18 October, while in mid-November it would be "St Martin's Summer" as St Martin's feast day is 11 November.

Shakespeare also used the expression "All Halloween Summer" in Henry IV part I for a period of warm sunshine as October gives way to November. A more generic but now (sadly) politically incorrect idiom is "Old Wives' Summer".

All these expressions may still be heard in various parts of Britain, but chiefly in remote rural areas. Though they are naturally much less common than they were 60 or 70 years ago.

The idea there are particular times of the autumn in Britain when these warm spells might recur is a fanciful one. Detailed statistical analyses do not suggest that any one week is more favoured than any other, and in a few years autumn brings relentlessly disturbed weather with a progressive drop in temperature, and there is nothing remotely Indian summer-like at all.

The origin of all these sayings has, perhaps, more to do with keeping people's spirits up during the headlong rush into winter.