Why Winter Night time Minimums are increasing.

A widely noted issue regarding the “claims” made by the Met Office of rapid recent mean temperature rise has been overnight minimum temperatures not being as low as they formerly were. The above graph shows a rapid rise in winter readings from levels barely any different to the 1920s starting around 2010. These winter increments have had a major effect in annual means. The absurd meteorological averaging system detailed here operates solely by extremes rather than norms and hence magnifies even minor variations – almost instantaneous global warming with no CO2 required. Minimums are nearly always a nightimes phenomenon so is this winter overnight warming real or are there other explanations?

Since restarting the Surface Station Project I have had podcast interviews with both Tom Nelson and James Corbett which prompted a number contacts from skilled professionals in both meteorology and climate science. A more recent interview with Maryann Gebauer produced several more contacts who offered even more insight into issues regarding data recording and one very unusual aspect as I will explain below.

Urban Heat Island effect is well known and results in overnight temperatures in towns and cities holding up higher than rural areas. Both the closure of many rural weather stations (as I highlighted with Elmstone) and urban encroachment around many other sites has unquestionably contributed to this rise. However, the graph indicates a very sudden incremental onset that is unlikely to be solely UHI.

Another contributory cause is the Met office’s persistence in use of the outdated Victorian technology of Stevenson Screens. These are known to over record temperatures (euphemistically put “a design that is not immune from problems“) in low wind speeds due to Aitken Effect as identified by researchers at Reading University . They stated “We estimated that across Europe about 4% of maximum temperatures will be affected in this way, and about 12% of recorded minimum temperatures.” A potential incremental incidence of Aitken Effect can be due to lack of maintenance to vegetation growth and general windbreak effects surrounding screens as demonstrated at very many sites – notable examples being Lentran and Glasgow Bishopton. Most other national meteorological authorities are now moving to artificially aspirated screen as per World Meteorological Organisation guidelines to overcome this problem whilst the Met Office are not. Yet again, though, this seems unlikely to fully explain such a sudden start to the winter rise.

A plausible explanation could be the transition from manual reporting to automation of weather stations. This process started with the first unit in 1976 which then proceeded relatively slowly at first, gaining momentum as technology advanced particularly with the use of solar photovoltaic panels and batteries as the power supply. This also required a swap out from LIGT instruments to electronic PRT instruments with ultimately all weather stations (even those continuing to report manually) being switched to PRTs. Could this be a reason for the increases given that this improved instrumentation is more rapidly responding? This also seems unlikely to cause a sudden change – PRTs might record heat spikes but they should not, on their own, cause any elevation to night time minimum readings. Or do they indirectly? Examining the way temperature readings are taken and reported needs careful examination. This video taken by the Floors Castle station observer on 20th January 2021 gives real world evidence and is essential viewing.

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

Floor Castle reports manually at 09:00 GMT daily. That timing is very important to note. Equally noteworthy is that the site has both a PRT with a continuous recording data logger plus a LIGT minimum recording unit. The data logger records readings every minute of the day thus the minimum LIGT is technically redundant and no longer required. If this site were automated the PRT/Data Logger would supply both maximum and minimum readings. Below are direct quotes from the observer.

“That’s our current temperature outside…..sometimes it changes as we open the box, erm…. obviously ‘cos once you let a normal bit of air inside its cosy little box it changes” ……..This is a classic Aitken Effect. Floors Castle is a walled kitchen garden deliberately designed to avoid any wind chill. Warmer air from the day time has been retained inside the insulated box and over recorded, which is exactly why the WMO recommends artificially aspirated instrumentation. {Ed note: a walled kitchen garden micro-climate should never be used for weather readings – typical Met Office Class 5 junk.}

The observer then goes on to read the LIGT for minimum and resets the “rider” (the little iron slider that registers the lowest reading) before going to the PRT Data Logger and tracking back its “history” to find the maximum reading. “This allows me to go back to yesterday’s weather. One day behind” The image clearly shows all readings are available including the minimum that has been ignored in favour of the LIGT.

This is what those readings for 20/1/2021 ultimately looked like in the archives showing the 1.5°C minimum. Note that there is just one set of readings per day (columns I and J) with column D showing “24” indicating a once daily reading.

The relevent point to note is that on the day of that reading, sunrise (Kelso) was at 08:22 GMT so the reading was taken just 28 minutes later, the coldest time of the day.

The meteorological winter runs from 1st December to 28/29th February with sunrise in the Borders region being from 08:14 to 07:30 over that period with the latest at 08:28. Why is this at all relevant? Well consider the same days readings from the nearby Charterhall automatic reporting site.

The first obvious difference is that there are now two reading sets per day with column D showing “12” indicating readings for the 12 hourly periods up to 09:00 and 21:00 GMT each day. This makes a huge difference. In the Floors Castle case the maximum reading is attributed to the day before with the minimum attributed to the day of reading. With the minimum reading being taken at the likeliest coldest time of day and the rider then reset, even if the temperature increases for the next 24 hours, that reading will still be there to be recorded for the following day. One colder night is effectively frequently double attributed to two days not one.

In the Charterhall example the minimum is effectively electronically “reset” at the 21:00 reading and the earlier low reading will NOT get recorded the following day. Although in the specific example day above, the double recording did not definitely occur it is worth noting that the following days minima were markedly different at each site with Floors Castle colder.

The conclusion here is that the transition from manual daily observations to automated twice daily archived figures has eliminated previously false lower readings. Rather than purely reflecting warming the progressive change has simply started to correct a prior error. The overall number of manual once only every 24 hour reporting stations is declining (they now represent under a third of all official weather stations) and this is an ongoing process.

The obvious question in all this though is, why are the LIGT minimum instruments being retained in manual sites when not required? Automated sites do not have them, nor require them, so what is it about manual sites that requires LIGT retention? I asked a recent new contact from my latest podcast who is, shall I say, “is in a position to know the reason”. I was astonished to be advised “automatic stations very rarely have the screen door opened”. The Floors Castle observer’s comment came back to me…… “That’s our current temperature outside…..sometimes it changes as we open the box, erm…. obviously ‘cos once you let a normal bit of air inside its cosy little box it changes“

There are many reasons for the “observed” recent daily “mean” temperature increases, how much of it is actually increased temperature seems to be open to question.

Source: https://tallbloke.wordpress.com/2025/09/15/why-winter-night-time-minimums-are-increasing/