Warm and cold spells don’t always correspond with the start and end of any particular month and that’s true of the monthly CET series devised by Gordon Manley. The daily series, which is maintained by the Met Office extends back to 1772, is even more useful, so I set about writing some code to look for the warmest and coldest 30 day spell (or any other arbitrary length period) from within the series. When I did that I found that the table was cluttered with overlapping periods that occurred just prior to and after the coldest 30 days. To do this I had to identify the most extreme spell for an event, and not just list multiple periods around it. That was easier said than done and took me a couple of days to figure out the best way in which I could thin out the set of results that I generated. Here are the results of my endeavours which make interesting reading.
The warmest 30 day spells since 1772
So the warmest mean temperature since 1772 was that of 22nd June to the 21st of July 1976, with an anomaly of 4.81°C above the LTA and just a whisker warmer than the July/August of 1995. Interestingly the recent warm spell of July 2018 only ranks at number six despite all the hype at the time.
The coldest 30 day spells since 1772
The coldest spell as you can see was the 30 days from the 28th of December 1813 to the 26th of January 1814, with a mean temperature 0f -3.33°C which was -7.1° below the 1961-1990 LTA. The cold January of 1963 ranked fourth coldest. Surprisingly to me at least was the December of 2010 which ranked thirteenth coldest.