Weather World goes to Cambridge

An interesting bit in the December Weather World from the BBC about the UK record maximum temperature of 38.7°C recorded at the Botanic Gardens in Cambridge on the 25th of July, with Nick Miller posing questions to Phil Johnson of the Met Office. I spent a number of years as an observer, and before the advent of ERT we had to walk out to the enclosure, which at some locations could mean a long walk to take the wet and dry bulb readings each hour of the day and night. It never really occurred to me that opening the screen door for 10 seconds or so might affect the reading that much although I’m pretty sure it never really did. Not so with the remote temperature sensors we have today according to Phil.

Courtesy of the BBC

I personally think that the daily extremes derived from the much more sensitive ERT sensors of today and those manual readings taken from mercury or alcohol in glass thermometers in the past are much larger than is thought.

Nick Miller’s questions are far from probing about the suitability of the location of the screen, but he obviously recognises that people do have legitimate concerns about it.

Courtesy of the BBC

Phil Johnson goes on to defend how the site and the screen meets all the requirements for a climate temperature site as laid down in the WMO regulations. Despite the site having changed radically in the last 20 years because of the galloping urbanisation that’s gone on in the immediate vicinity, he can find no problem with it. Here’s Phil with the new Sainsbury Laboratory building behind, the roof of which is covered in glass skylights and solar panels just over 40 metres to the NW of the enclosure.

Siting classifications for surface observing stations on land
Courtesy of WMO

The site at Cambridge almost certainly can’t be classified as WMO class 1 for temperature, with buildings extending from the west through to north easily exceeding the 10% allowance within a 100 metre radius. On the face of it, it does meet the more relaxed WMO class 2 classification, which allows for an AWS being 30 metres from any heat source. The top of their screen does look rather thin and in poor shape, in my experience the top of a Stevenson screen was always covered in a sheet of zinc. I still wonder why we didn’t get to have a peek inside.

Courtesy of Google Maps

This is how the site at the Botanic gardens Cambridge looks at the moment. If the Met Office are serious about how accurately they measure temperature across the UK, why didn’t they just ask the Botanic gardens there to relocate the screen away from the buildings?

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