I heard on the TV weather yesterday that yesterday, the 7th of December 2020, had been the coldest day in the UK at Santon Downham in Norfolk, where presumably the daytime maximum never exceeded -0.7°C. It seems to me that the term “coldest day” is in itself a little misleading. Why? Because the coldest day could be seen as the place with either the lowest minimum temperature or it could be seen as the place with the lowest maximum temperature. Not only that there is the time the maximum and minimum temperatures are read to consider. In a SYNOP observations daytime extremes are reported for the period between 06 and 18 UTC, but for climate observations the period is between 09 and 21 UTC. Because Santon Downham is not a SYNOP station and is one of the stations on the secret Met Office climate list they must be referring to the 0900-2100 maximum temperature although they don’t state that. That makes it impossible to verify if this was indeed the coldest day since 2018, without dipping into the MIDAS temperature dataset which is not updated in real time and also presupposes that Santon Downham is in it.
Strangely, using the available UK SYNOP observations that I download, I find the lowest 06-18 UTC maximum in the last year of -1.3°C occurred just the day before at Castlederg in Northern Ireland.
Here are the hourly temperatures for the 6th of December at Castlederg, and as you can see the temperature never rose higher than -1.3°C from 09 UTC on the 6th to 09 UTC on the 7th.
Getting back to the Met Office tweet which specifically states that the -0.7°C refers to Santon Downham and not the whole of the UK, but of course tweets can be edited later, and I suspect that this is what happened to this one. Why would anyone in the press team go to all the trouble of publishing a tweet to say that the lowest day maximum temperature for a single station? I’m guessing that this tweet started life saying that the -0.7°C had been the coldest for the whole UK (and which one of the TV weathercasters had picked up on and has now been deleted from iPlayer), but was changed slightly once they realised the lowest daytime maximum since 2018 had actually occurred the day before at Castlederg!