Gartocharn: Portnellan Farm DCNN6223 – Interesting details of instrumentation changes.

56.04804 -4.56724 Met Office CIMO Assessed Class 5 Archived temperature records from 1/6/1993

Portnellan Farm weather station sits at the southern end of Lough Lomond and ranks as one of the most picturesque sites the Met Office has. The site owner is clearly a dedicated meteorologist with the observation record being excellent. The equipment maintenance is of such a high standard that even the site owner’s photographs online show the Stevenson Screen being cleaned. Unfortunately no amount of good equipment maintenance can make up for poor siteing, and this site is exceptionally poorly sited. This review studies those site shortcomings but also uncovers some interesting features of instrumentation changes.

The screen is located alongside the driveway to a holiday let and is next to the car parking area. Cars can have their engines running to warm up on cold days or cool down via air- con on hot ones within a few feet of the screen. Such possibilities are the principal reason that weather stations should be in their own access restricted enclosure rather than these low grade, almost anything goes sites.

If this seems an extreme view, then consider this image below from the site owners own facebook page. The screen is to the rear of the car’s exhaust just above the cow in the image.

Leaving aside the motorised vehicles, the slope down to the Loch is clearly shown as are all the surrounding trees and shrubbery. The Met Office assesses the site as meeting no regulatory standards jointly set by the World Meteorological Organisation and International Standards Organisation at all, and it is not hard to see why. Despite nice views, diligent observations and well kept equipment it is still a very poor site that is only ever going to supply unrepresentative readings. Some (but not many) meteorologists may feel these sites are acceptable but I doubt the general public realises how almost primitive the Met Office’s data collection points actually are. An almost archaic and unregulated cottage industry seems to be the standard of modern climate science – good enough to declare a “climate emergency”?

However, in researching this manual reporting site I discovered a partial answer to a question of mine the Met Office had refused to answer – when was each manual site converted from a traditional Liquid in Glass Thermometer (LIGT) to a Platinum Resistance Thermometer (PRT)? I had been told this was not retained information by individual sites – i.e. a deflection from the actual question I had asked. Every thermometer has to be originally calibrated and rechecked at preset intervals. For the Met Office to claim this was not “retained information” was clearly untrue, but, of course they simply claimed such a “list” that I had requested was not available which probably was true. Re-asking the question slightly differently to avoid them playing their silly avoidance games ended up hitting their “vexatious” defence and refusing to answer any more questions. It would appear that the dates of instrumentation change are a matter of “National Security” and obviously “Top Secret”. Pity they did not let their manual station operators into the need for such secrecy.

One thing I had noted about manual stations was that around 2017 there seemed to be a major transition programme from LIGT to PRT. Curiously sites daily maximum temperature readings inexplicably stopped for a lengthy period despite all other readings (including daily minimum temperature readings) continuing to be taken. I made an educated guess this was the changeover period but could not explain this maximum reading shutdown for several weeks, even months. Then Portnellan Farm publicised their system change which was far more comprehensive than I realised.

The image below is from a “Facebook” post dated 8/12/2017 showing the not only the installation of the PRT but also the entire change of the screen and all instrumentation. Clearly the ground debris indicates removal of much overgrown vegetation.

Note the old redundant screen now on the ground. Below is what the old wooden (thus painted) screen looked like with its white painted INSIDE, wooden (teak?) instrument mounts, traditional alcohol filled thermometers but importantly no inlet holes to the base (floor) of the unit.

Contrast that aged wooden model with the BLACK powder coated inner louvres to the similarly powder coated metal casing and white mounting instrument racks. A significant set of changes by any measure.

The 2017 temperature archives show maximum readings stopping 5th October and only resuming on the Facebook indicated switchover date of 8th December.

This seems remarkably strange. The changeover took just one day yet there are 64 days maxima not taken. This absence of readings must have been either under instruction not to take them in some way or they were taken but not recorded – why?

Dave Woolcock offers great assistance to the Surface Stations Project in analysing numbers. At an early stage he was able to point out to me (from his looking at Met Office data) when stations were converted from manual to automatic reporting. In these circumstances the two differing sets of instruments (from LIGT to PRT) run simultaneously with an overlap period seemingly to confirm correlation between the different instrumentation types and reporting media. Dave identified the phenomenon of recording “Treblications” where single 24 hour former manual readings were mixed with twice daily 12 hourly automatic readings for the same days. He also noted discrepancies in the transition phase which often seemed to extend this period. I also noted this very extended transition at sites like Shirburn Model Farm extending into years.

This former historical correlation process above contrasts starkly with the more recent instrumental changes at manual stations which apart from being deliberately covert seem to almost avoid any possible comparisons. The old LIGT was visible on changeover but no data for the previous two monthss from it exist.

Further explanation of the manual station reading process is given by this youtube clip made by the observer at the Floors Castle Weather station.

https://www.facebook.com/watch/?v=1893457610801591

It is worth noting several important points regarding these instrument changes at manual sites. Firstly, there are no electronic transmission benefits as these sites remain manually reporting. Secondly, there are no real accuracy improvements – both systems, LIGT and PRT, report to the 1st decimal place. Thirdly, only the maximum readings are affected with minima continuing (for the time being) to be taken by slower reacting liquid thermal expansion/contraction rather than rapid reacting electrical conductivity changes driven by temperature change.

In my follow up parts to the Reification of Averages report I will further highlight how siting, siting changes and structured instrumentation changes can significantly affect meteorological averaging in “Playing the System”

In summary Portnellan it is a well maintained and observed site but one of unacceptable siting standard that has experienced questionable instrumentation change practises in common with most other manual recording sites.

Source: https://tallbloke.wordpress.com/2025/07/29/gartocharn-portnellan-farm-dcnn6223-interesting-details-of-instrumentation-changes/