There has been lots of interest in the media and weather community recently on various topics such as Hurricane Michael and Storm Callum. One of these many events is the unusual warmth the UK is going through here during mid-October 2018. In this article, I’ll be analysing how unusual this sort of warm weather is through October with historical occurrences of such.
First, we need to clarify on the term ‘Indian Summer’ because it does get misused and thrown around a lot. The exact origins of the term are uncertain but theories show that it was so-called because it was first noted in regions that were inhabited by Native Americans with warm and hazy conditions occurring in the Autumn season when Native Americans hunted. The US National Weather Service (NWS) classify an Indian Summer as a period of clear and sunny conditions with above average temperatures taking place in May to mid-June or late September to mid-November, after a frost occurred. There is a large division on how we classify an Indian Summer here in the UK.
My own definition for an Indian Summer is a period in October or November where temperatures are well above average after the first widespread frosts of the season have passed. The latter part of September, under a high pressure system, brought ground frost widely with air frosts recorded for quite a large portion of the UK including an air minimum of -3.6°C at Katesbridge, Northern Ireland; the lowest September minimum temperature in the UK since 2012. Further frost occurred on October 6th with a minimum temperature of -4.3°C at Aboyne, Aberdeenshire. This was followed by a notably warm period for mid-October with a maximum temperature of at least 70f (21.1°C) somewhere in the UK each day from the 9th-13th October 2018. The highest temperature of the spell when writing this article has been 25.7°C at Weybourne, Norfolk on the 13th; making it the warmest October 13th on record in the UK.
The table below shows the maximum temperature records for each date of October in the UK. Two years particularly stand out here, 2011 and 1921. 1st October 2011 recorded the UK’s highest October temperature on record of 29.9°C at Gravesend, Kent beating the previous record of 29.4°C at March, Cambridgeshire ending the ever popular pub quiz question of “When was the UK October maximum temperature set in March?”. This was not an Indian Summer but for October warmth, this was incredible. Both 2011 and 1985 had very warm nights for October too including the UK record of 19.4°C at Aber, Gwynedd on 1st October 1985.
The October 1921 warm spell was exceptional and very forgotten. A maximum temperature of at least 25.6°C (78f) was recorded somewhere in the UK every day from October 4th-10th. To this day, 1921 still holds the maximum temperature records for each of those dates bar the 4th and equalling with 1995 on the 8th as you can see from the table. Until 1969 (and later 2001), this was the warmest October on record for the UK with persistent southerly winds bringing up these unseasonably warm temperatures. It was the sunniest October on record and still is to this day. The fact that this occurred within the driest year on record too with an extremely dry February, June and July makes 1921 a very remarkable year and is unfortunately forgotten. This one wasn’t an Indian Summer either going by the definition I gave but again for October warmth, this is one that really stands out.
October 8th/9th 1995 both recorded temperatures around 25°C in the south of the UK making it the highest temperatures this late in the year at the time since 1978 including a maximum of 25.6°C at Prestatyn which equals the 1921 record for the 8th October.
Ten years later, the end of October 2005 brought tropical southwesterly winds from unusually low latitudes according to Philip Eden. It was largely cloudy but the exception was the 27th where it was notably sunny and a maximum of 23.6°C at Aber, Gwynedd and this would become the highest temperature on record in the UK for so late in the year until Halloween 2014’s incredible warmth led to a maximum of 23.6°C too then and now 31 October 2014 holds that title.
These are just some of the many examples of October warm spells or periods that have occurred in history. I could be here all day telling you about the different ones that occurred such as 1869, 1969, 1978, 1990, 2001 and 2017. However, these are some of the most notable ones that I have highlighted that I think are worth noting.
So the question is, how unusual is a period like the one we’re experiencing right now? For the very beginning of October, it is not too unusual and occurs every 4-5 years on average in terms of the maximum temperatures being recorded but for mid-October, this is far more unusual and has a return period of more than 10-20 years.