The hot day of 25 July 2019

Thursday the 25th of July was an exceptionally hot day over many parts of the UK. By the end of it we thought we had set a new record for the hottest July day in the UK of 38.1°C, rather surprisingly this was not at the usual hotspot of Heathrow Airport, but at the National Institute of Agricultural Botany [NIAB] in Cambridge.

Fig 1 – Courtesy of the Met Office
Four days later

The Met Office sprang into action, and Just four days later they managed to pull another climate station out of their hat which had reported a maximum of 38.7°C (101.7°F). This reading came from the nearby Botanic gardens in Cambridge, trumping both the 38.1°C from the NIAB and the 38.5°C of the old record from Faversham of August 2003, to make July the 25th 2019 the hottest day on record in the UK rather than just for July.

You may have read that the site of the enclosure in the Botanic Gardens is far from perfect (fig 2) with large glass covered buildings to the north, but the Met Office has now given the site and the reading it’s official blessing validating it (fig 3). How much that blessing is worth from the same organisation that see’s nothing really wrong with the met enclosure at Heathrow Airport I’ll leave you to judge.

Fig 2 – Courtesy of Google Maps
Fig 3 – Courtesy of the Met Office

Here is a graph of extreme temperatures at the Botanic Garden for this year from the MIDAS record (fig 4), and as you can see the record is far from perfect, with multiple breaks throughout the year, some of them ten up to days in length. This doesn’t particularly inspire you with confidence about the veracity of their record keeping, although in these days it’s more likely to do with a dodgy comms port on their data logger.

Figure 4 – MIDAS data courtesy of the CEDA Archive

And here is a table and map of the highest temperatures also from the MIDAS database for 0900-0900 UTC on that day. (fig 5 & 6). As you can see the maximum temperature is only read once at 09 UTC on the following day at the Botanic Gardens.

Figure 5 – MIDAS data courtesy of the CEDA Archive
Figure 6 – MIDAS data courtesy of the CEDA Archive

Finally, here’s a look at the plotted chart from 15 UTC on the 25th to give you an idea of the situation (fig 7). I never did find out why the Met Office removed the temperature readings from the nearby AWS at Bedford, which at lunchtime looked well placed to exceed 100°F.

Fig 7 – Data courtesy of OGIMET
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