Westonbirt DCNN 4987 & 4989 – A long standing weather station with confusing provenance.

51.60691 -2.22091 Met Office CIMO Assessed Class 4 Originally from 1888, Met Office Claims 1976, Temp records from 1983

Westonbirt weather station lies in the Westonbirt National Arboretum alongside the “Smokery” of the same name. It is approximately 26 km/ 16 miles northeast of Bristol and 31 km/20 miles west northwest of Swindon. At first glance this appears to be a potentially good candidate for a long term temperature history reconstruction. Unfortunately, detailed examination uncovers a very disappointing and confused provenance that renders the site’s long term value too dubious to be of value.

The Met Office’s CEDA archives indicate a station start date of 1/1/1976 with temperature records only in digital form from 1/1/1983. This seems at odds with the widely publicised history of this site as having originally started almost 100 years prior in 1888. A brief history of the site as below.

https://www.fowa.org.uk/blog/moving-up-the-stations-by-sally-day-database-and-records-officer/

The headline location image for this site indicates a reasonably open site but the ground imagery displays a completely different set of characteristics. The Met Office Class 4 assessment will be determined by both the sloping nature of the site and the unquestionable levels of both shadow and wind shade from the mature trees of the surrounding arboretum. Whilst not a highly confined clearing, the enclosing nature of the trees will create a micro-climate unrepresentative of the surrounding area. This rarely seems to overly concern the Met office who have many such clearing type sites – notable other examples being High Beach and Alice Holt Lodge

Whilst the above image indicates a well maintained site, the below example indicates the highly variable nature of the area immediately around the enclosure with vegetation having an intrusive effect. Any low level breeze will be further dampened by this growth coming up the height of the base of the screen (though outside the fenced in area) and increasing the frequent likelihood of inadequate ventilation to the screen. {NOTE: The issue of inadequate ventilation to Stevenson Screens will be discussed further in a dedicated post on the subject of Aitken Effect}

Looking further into the archives and aerial imagery brings in the confusing issues. The archive shows 2 separate District County Network Numbers – DCNN 4987 and 4989 as identifiers. This is important as it is positive proof of a significant relocation. The Met Office Surface Stations Users Guide expressly states:

“Over time certain instruments, or the whole enclosure, may be relocated some distance away from the original site. Where the distance moved is small, the observations obtained from the new site may have exactly the same climatological characteristics as previously and it makes sense to regard them as coming from the same source or station distinguished by certain identifiers. “Where the distance moved is large, or, where the exposure at the new site is sufficiently different that a detectable impact on the measured climatology is judged likely, it is appropriate that observations from the new site are labelled by a different set of identifiers.” (My bold)

From 1/1/1983 to 30/4/1999 the record claims manual observations from DCNN4989. From 1/5/1999 to 24/9/2009 manual observations transferred to DCNN4987 when the station was automated with readings from 25/9/2009 onwards. When such relocation occurs the two sites reading were retained for comparison of concurrent readings to draw comparisons. The comparative readings are shown as below for the early part of the transition.

Column C denotes the DCNN numbers showing both sites separately. Columns I and J are maximum and minimum respectively. The two sets of readings are seen to be consistently between 0.3°C and 0.6°C apart positively showing the different climatologies of the two locations. This is rather frustrating, anyone simply interrogating the database by name (i.e. Westonbirt) would not naturally be drawn to the fact that two different sites have been bonded together from 2/12/1999 onwards when DCNN4989 was shut down, . Whilst an “expert” may know what has occurred it would be very easy to demonstrate “climate change” from such sites to the general public without disclosing the site changeover.

If this suggestion of data set bonding seems a bit “conspiracy theorist” it is worth noting that the change over from manual (Liquid in Glass Thermometer) observation to automatic (electronic Platinum Resistance Thermometer) in 2009 is NOT shown with overlap reference data to allow scrutiny of comparability.

All that can be seen from the above period archive is that the frequency of readings recording changed from 24 hourly to 12 hourly (column D). The public would not know this indicated the significant change of instrumentation and recording protocol which, due to overcoming the former Time of Observation bias, will naturally elevate daily minimums notably in winter and spring.

The issue of where the screen actually was before its current location is one I have not yet fathomed out and I no longer feel there is any need to ascertain it. The dubious data provenance throughout its digital archived period from 1983, as well as the far from best modern siting, is enough to rule it out of contention for accurate historic recording. One issue I did verify from historic images is that, although having records from 1888, it certainly was not anywhere in this 1945 frame showing its current site.

Perhaps, though, the most striking contradiction of this verifiable long term site of 148 years claimed observational record is that it is not included as a “climate averages station” in the Met Office Location Specific Long Term Averages listing.

In lieu of any data from the automatic Westonbirt station, the alternative nearest climate stations offered are: – manually reporting with dubious averaging Cirencester, former RAF (now MOD) Lyneham (yet to be reviewed), Filton CLOSED DOWN 15/10/2018, Bath, Beechen Cliff School CLOSED DOWN 1/4/2005 and Cheltenham CLOSED DOWN a mere quarter of a century ago on 31/1/2001. I make no further comment!

The original Westonbirt screen – such a pity.

Source: https://tallbloke.wordpress.com/2026/04/11/westonbirt-dcnn-4987-4989-a-long-standing-weather-station-with-confusing-provenance/