Expert comment: September heat breaks 100 year record

The University of Reading’s weather observers recorded the hottest September day in over 100 years last week.

With the welcome return of more normal September temperatures, Dr Stephen Burt, of the University of Reading’s Department of Meteorology, has delved into the archives to provide a short summary of last week's exceptional heatwave.

Dr Burt said: “The maximum temperature on Saturday, 9 September, was 31.4 °C, making it both the hottest day of the heatwave and the hottest day of the year, surpassing 30.9 °C recorded on 10 June.

“Based on the university's records back to 1908, this is the latest date in the year to have become the hottest day of the year - the previous 'latest hottest day in the year' being in 1929, when 31.2 °C was recorded on 4 September. In over 100 years of records, only twice before - 1929 (4 Sept) and 1954 (1 Sept) - has the hottest day of the year occurred in September.

“Saturday was the hottest September day on the university's records since 31.7 °C was recorded on 8 September 1911. That record was made at the more sheltered and slightly warmer London Road site, and had records still been made there, the 1911 record would almost certainly have been surpassed.

“Within the past 100 years, only two spells with temperatures reaching at least 29 °C every day have lasted longer than last week's six-day spell, those being in June-July 1976 and August 2003. The longest such spell on our records was one of 16 days, 23 June to 8 July 1976, when the highest temperature reached was 34.0 °C.

“Nights were warm, too, also at near-record levels. The warmest nights were Thursday, 7 September and Monday 11 September, when the overnight temperatures fell no lower than 17.5 °C. Only two September nights have been warmer since records commenced at the Whiteknights Atmospheric Observatory site in 1968, the highest being 18.0 °C on 5 September 2006. As of this morning, the temperature has not fallen below 15 °C since last Wednesday (6 September) morning.

“The high temperatures by night and by day were exacerbated by very light winds (wind speeds at peak temperatures were often only 1-2 m/s) and very high dew points, which made the heat feel oppressively humid.”