Northolt WMO 03672 – The worst it can possibly get?

51.54873 -0.41691 Met Office CIMO Assessed Class 5S Archived temperature records from 1/1/1948

The lowest normal CIMO rating for a weather station is Class 5 with an additional inaccuracy due to siting of up to +/- 5 °C – but then there is also the addition of an “S” indicating also subject to the effects of shading. Northolt successfully plumbs the lowest possible standard and then some. Despite this atrocious siting (fully acknowledged by the Met Office themselves) it is still used as a comparative location yardstick against which other sites’ readings are verified.

The Northolt Stevenson screen sits between the frequently gridlocked A40 Western Avenue to the south and the outer service road for the main aircraft concourse to the north. The large trees to the east are the cause of the extensive shading and there is a local electrically operated sewage pump housed in a partially hidden building under them. There is both extensive car and trailer parking to the north west. All in all this is a classic urban weather station site prone to all the Urban Heat Island effect London has to offer plus an active military and civilian airport. The Street View image brings an even more startling compromising effect into close view.

The perimeter security fencing also includes metal profiled sheeting (modern day corrugated iron) which I have been advised is probably for the purposes of shielding car headlights from dazzling onto the airfield. {If anyone can confirm or deny that I would be interested to know}. From a meteorological viewpoint this grey sheeting will rapidly warm in sunlight and likely store heat between itself and the nearby screen. There is no visible wind mast at the enclosure (not uncommon – they are often separated from other instrumentation) and I suspect there may well be another more aviation focused set of recorders elsewhere on the airfield. Tim Channon implied in his original 2012 review that he had located an earlier screen location but did not specify where. I have not been able to 100% confirm this exact former location but all circumstantial evidence suggests this location below close to the Control Tower from where staff would be able to readily take hourly readings.

This original site appears to have operated from 1948 to 1954 whereupon historic data was taken from the road side site. However, there are indications the old side carried on functioning independently to the mid 1990s for distinct aviation purposes and may even still be used. The relocation distance was about 840 metres /2,750 feet. Normally I would consider this an issue but, in the case of Northolt, both original and new locations are so equally bad in climate reporting terms that the entire historic dataset is really not worth considering at all. Including such a known corrupted site in a historic temperature record is irresponsible.

The real concern with this site is the way its readings are used to justify readings at other west London sites notably Heathrow. A Met Office mantra seems to have occurred that bad sites are justified by comparison with other nearby bad sites. The basis of this being it was X degrees at Heathrow but Northolt was just as hot so it must be correct. A variation of “great minds think alike” attracting the obvious “fools seldom differ” response.

There is a remarkable similarity to Heathrow with the now remote Met Office Northolt instrumentation almost certainly ignored for aviation purposes, and the so called “climate station” being unsuitable for those latter purposes as well. To sum up it seems there is a right way to do things and a wrong way to do things but a poor fool can do neither – Northolt in a nutshell.