Benson WMO 03658 – Conflicting claims and close comparisons.

51.62027 -1.09867 Met Office CIMO assessed Class 2 Installed 1/1/1951

RAF Benson is one of the Met Office’s best weather stations. It is currently rated as Class 2 though until recently it was Class 1. When Tim Channon made his original brief report he estimated Class 1 and I am rather surprised the Met Office have since marked this site down. Tim went on to supply a more extensive technical review on diurnal radiation that focused on Benson which I recommend reading for further background.

https://tallbloke.wordpress.com/2014/08/30/diurnal-radiation-three-uk-sites-compared/

The headline image includes a 100 metre radius encircled area. There is very little hard standing at all within the area. Aviation there is now almost entirely helicopters and the runways and taxiways are distant from the screen. The grass is impeccably well kept as it so often is at military sites. Readings from this site should be reliable, however, they are frequently played down by many meteorologists with suggestions that they are unrepresentative of the wider environment. This implication of inaccuracy due to siting runs directly opposite to its CIMO rating and warranted investigation.

The text reads from the Channel 4 TV weatherman and meteorologist Liam Dutton reads –

“The well known frost hollow, Benson in Oxfordshire, is already down to -8.7C at 5pm. A frost hollow is a dip in the topography where cold, dense air sinks, making it markedly colder and more frost-prone than surrounding areas.”

This somewhat intrigued me, 6,000 feet long runways are not usually in a “dip in the topography.” Others puzzlingly seemed to agree with Liam’s opinion. The Met Office themselves acknowledge the low CIMO classification (i.e. 1 or 2 ) means Benson represents a wide area and their own specifications specifically declare such frost hollow sites as “undesirable” as below.

A second opinion of what constitutes a “frost hollow” was in order. This article contained a good description with case studies.

Looking at the Ordnance Survey sheet of the Benson area does not specifically reveal any particular “dip” The screen is at a noted elevation of 57 metres amsl and the airfield is especially flat as would be expected, in fact the exact site is possibly at the high point rather than any dip. Perhaps the most important point is that there is also a Met Office site at Wallingford just 2,150 metres distant (all downhill) which at 47 metres amsl is 10 metres/33 feet lower by the Thames………and coincidentally the same altitude as the village of Benson noted for parts of it being in a frost hollow.

Is the airfield flat site some distance and elevation away from the village being tarred with the same brush whilst Wallingford is seemingly not? I am not a meteorologist and would welcome any further advice from qualified readers. Personally though I find it hard to accept that the Benson screen location is in some way adversely affected by any very localised effect. What I can see is a site in reasonably open countryside that is not subject to any artificial or anthropogenic effects and is probably reading correctly in a manner representative of the natural wider area.

The relevance is that there have been many sites similar to RAF Benson closed over recent decades. Even since Tim did his epic review of all synoptic stations in double quick time in 2012, it is noticeable that many he reviewed are no longer operational. The replacement sites over the intervening years have a worrying habit of being of much lower classifications than their predecessors. Is this another example of “site selection” slewing climate averages?

More topically I recently reviewed a not dissimilar setting not far away at South Newington recording “cooler” and referenced Benson also showing a lower temperature. Then yesterday 14/3/2025 this happened.

Is RAF Benson really an unnaturally cool site or is it simply one of the better ones recording the reality of inland England away from large urbanisation effects?

Source: https://tallbloke.wordpress.com/2025/03/15/benson-wmo-03658-conflicting-claims-and-close-comparisons/