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Aleman

Northolt, Heathrow, Temperature Records and Weather Station Siting

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5 hours ago, Aleman said:

 

Edit - I see Northolt was the hottest Met Station in the UK again yesterday.  But does that make the area around Northolt Airport the hottest or is it just that  Northolt station has become one of the most badly sited/encroached upon since it was built?

i used to drive past Northolt quite a lot as it was close to where i lived in London..not sure where the weather station is the but the RAF base is not built up per see and is an open expanse of green fields and its not over built up in the surrounding area either..the A40 does go past right next to it though but that's been there for 50 years and more

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Posted (edited)
1 hour ago, Quicksilver1989 said:

Heathrow and Northolt have always been in places that favour higher maximum temperatures.

Might be worth looking at how many times each of Heathrow and Northolt have recorded the highest temperatures on Summer days (going back to when the stations were established) so as to determine whether the frequency with which this happens has increased over time. Of course, other factors would have to be accounted for (e.g. number of stations present at each time). I would look into it myself but have no idea where to find such data!

Edited by Relativistic

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Posted (edited)
7 hours ago, Paul said:

Was actually heathrow yesterday 🙂 

There's a 6 lane road (Northern Perimeter) and huge tarmac carparks right next to the Heathrow enclosure, too. Google Maps link.

For airports, if they must have them there, this is the kind of site screens should be at - it's Southampton Airport's METAR AWS (exactly at that spot) but unfortunately, not a SYNOP station. There's nothing significant within a hundred metres on flat open grassland.

Edited by DaveL

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Posted (edited)
2 hours ago, Quicksilver1989 said:

Sorry and it wasn't specific to your post, it's just that when we see a record breaking heatwave there is always a spate of posts saying things like 'Ah but that French station is next to a motorway' yet the same people would never complain if there was record cold there... and there was also another 12 stations nearby that also broke the national record.

I may have misinterpreted your post in which case I do apologise. The analysis from BEST was primarily concerned with the potential impact of urbanisation and their series which focused on sites away from urban areas produced exactly the same anomalies as those from the already established land temperature datasets.

As for the quality of the stations, do you have any evidence that they are poorly maintained? I remember Heathrow was singled out during during the July 2015 heatwave but this was comprehensively debunked by the solar radiation data which showed a peak around 14.13 on July 1st when the record was broken.

https://www.carbonbrief.org/met-office-wind-data-dispels-doubt-about-cause-of-heathrow-high-temperatures

I think topography and geography may also play a big helping hand here as well, I just find it curious that everyone jumps onto these record values when the discussions have taken place many times beforehand. Heathrow and Northolt have always been in places that favour higher maximum temperatures.

Thanks for reviewing your comment and apologising. I am concerned about urbanisation on the temperature record and it might come across in the tone of my posts but at this time I am just concerned that a site that has as much encroachment as Northolt does not get disqualified from use as no longer being representative of its general surrounding area. There must be some days that it's practically just measuring the temperature of the A40.

 

 

2 hours ago, cheeky_monkey said:

i used to drive past Northolt quite a lot as it was close to where i lived in London..not sure where the weather station is the but the RAF base is not built up per see and is an open expanse of green fields and its not over built up in the surrounding area either..the A40 does go past right next to it though but that's been there for 50 years and more

 

You must not have looked at the links I gave. Check this one out again. I gather guidance is not to have development within 100 feet/30m of a station yet the A40 is just 10m or so away. It used to be further away but came closer when it expanded to 6 lanes and a cycle path from 4 previously (and only two lanes when the airport was just a grass field, though that might predate the weather station at that site). Again, I had not seen this blog but the but the photos were good and comment seems valid. (Note, the superimposed circles seem to be 10m, 30m and 100m as per classifications below.)

https://tallbloke.wordpress.com/2012/09/10/wmo03672-northolt/

 

 

7 hours ago, Paul said:

Was actually heathrow yesterday 🙂 

At the time of posting, Northolt edged Heathrow Airport in the 24hrs to 9am by 23.7C to 23.6C. And I remind you that I have read a post that suggested the new better USCRN stations  - with three temperature probes amongst other things - might have an acceptable error at +/- 0.2 to 0.3C so we don't know which was actually hottest.  (And that's nothing compared to error which people overlook in other fields. People would have a very different view of, say, GDP commentary, if they knew that the +0.2% intial estimate for Q1/2008 was revised over 5 years to -1.8%. Not including error can mislead the impression given. Just think how much guff you get from politicians over initial ("flash") GDP stats when they tell you next to nothing if you include error.)

https://www.ogimet.com/cgi-bin/gsynres?ind=03772&lang=en&decoded=yes&ndays=2&ano=2019&mes=07&day=04&hora=09

https://www.ogimet.com/cgi-bin/gsynres?ind=03672&lang=en&decoded=yes&ndays=2&ano=2019&mes=07&day=04&hora=09

 

I can't find the error reference I read about in a technical equipment assessment. I might look again later. I have found some more information on site classification (see page 6 onwards). From that and observing though the links I provided and Google maps and streetview, I gather Northolt would be a class 4 station on most parameters. It has least 4 failures within 30m (busy car park, busy road, tall trees, metal panel fence, and maybe the electrical outbuilding, too) and too many failures to list inside 100m. (Below, I've picked out the guidance just for temperature classification - NOTE THE INFERRED ERRORS!)

https://www1.ncdc.noaa.gov/pub/data/uscrn/documentation/program/X030FullDocumentD0.pdf

Class 1 – Flat and horizontal ground surrounded by a clear surface with a slope below 1/3 (<19º). Grass/low vegetation ground cover <10 centimeters high. Sensors located at least 100 meters from artificial heating or reflecting surfaces, such as buildings, concrete surfaces, and parking lots. Far from large bodies of water, except if it is representative of the area, and then located at least 100 meters away. No shading when the sun elevation >3 degrees.

Class 2 – Same as Class 1 with the following differences. Surrounding Vegetation <25 centimeters. Artificial heating sources within 30m. No shading for a sun elevation >5º.

Class 3 (error 1ºC) – Same as Class 2, except no artificial heating sources within 10 meters.

Class 4 (error ≥ 2ºC) – Artificial heating sources <10 meters.

Class 5 (error ≥ 5ºC) – Temperature sensor located next to/above an artificial heating source, such a building, roof top, parking lot, or concrete surface.

 

 

Maybe class 3 or 4 and error of 1 or 2C is acceptable to the Met. I think Northolt fails the common sense test for public or climate reference.

 

 

 

 

 

 

 

Edited by Aleman

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8 minutes ago, Aleman said:

At the time of posting, Northolt edged Heathrow Airport in the 24hrs to 9am by 23.7C to 23.6C. And I remind you that I have read a post that suggested the new better USCRN stations  - with three temperature probes amongst other things - might have an acceptable error at +/- 0.2 to 0.3C so we don't know which was actually hottest.  (And that's nothing compared to error which people overlook in other fields. People would have a very different view of, say, GDP commentary, if they knew that the +0.2% intial estimate for Q1/2008 was revised over 5 years to -1.8%. Not including error can mislead the impression given. Just think how much guff you get from politicians over GDP stats when they tell you next to nothing if you include error.)

 

24.5c was the highest temperature recorded at Heathrow yesterday. The Ogimet figures are an hourly reading, the Met Office are using more frequent readings.

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Posted (edited)

Paul. I used two sources. The station output links I gave (I wondered why it did not quote the 24-hour high for Heathrow, as is usually the case, though it did for Northolt) and this summary page link. So does this Ogimet source just quote hourly reading highs? I've oft quoted its highs and lows on here and nobody has pointed that out before!  (Again, being a bit misled by not undertstanding the weaknesses of what one is looking at!) Thanks.

https://eldoradoweather.com/climate/world-extremes/world-temp-rainfall-extremes.php?extremes=United%2BK

Does this just use hourly output because it would overload their updating to quote more frequently or is it a money/subscription thing to get higher or more timely detail?

 

 

 

Edited by Aleman

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Posted (edited)
2 hours ago, Relativistic said:

Might be worth looking at how many times each of Heathrow and Northolt have recorded the highest temperatures on Summer days (going back to when the stations were established) so as to determine whether the frequency with which this happens has increased over time. Of course, other factors would have to be accounted for (e.g. number of stations present at each time). I would look into it myself but have no idea where to find such data!

https://digital.nmla.metoffice.gov.uk/collection_86058de1-8d55-4bc5-8305-5698d0bd7e13/

UK daily weather reports may provide the answer, they go back to 1861 but I don't know if they have the daily maximums going that far, I know they go back to 2003.

Heathrow weather station has been in use since 1948, I don't know how far back Northolt goes. Daily data is available on tutiempo. I'll have a further look later.

Edited by Quicksilver1989

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35 minutes ago, Aleman said:

Paul. I used two sources. The station output links I gave (I wondered why it did not quote the 24-hour high for Heathrow, as is usually the case, though it did for Northolt) and this summary page link. So does this Ogimet source just quote hourly reading highs? I've oft quoted its highs and lows on here and nobody has pointed that out before!  (Again, being a bit misled by not undertstanding the weaknesses of what one is looking at!) Thanks.

https://eldoradoweather.com/climate/world-extremes/world-temp-rainfall-extremes.php?extremes=United%2BK

Does this just use hourly output because it would overload their updating to quote more frequently or is it a money/subscription thing to get higher or more timely detail?

 

 

 

The script in use pulls data in to a cache and will only update after a set number of minutes. I use the same script on my website and it shows different values than the site you provided.  There's also a lag time from Ogimet too.

Sometimes values don't even match MetOffice ones..

https://warehamwx.co.uk/extremes.php

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Posted (edited)
48 minutes ago, Aleman said:

Paul. I used two sources. The station output links I gave (I wondered why it did not quote the 24-hour high for Heathrow, as is usually the case, though it did for Northolt) and this summary page link. So does this Ogimet source just quote hourly reading highs? I've oft quoted its highs and lows on here and nobody has pointed that out before!  (Again, being a bit misled by not undertstanding the weaknesses of what one is looking at!) Thanks.

https://eldoradoweather.com/climate/world-extremes/world-temp-rainfall-extremes.php?extremes=United%2BK

Does this just use hourly output because it would overload their updating to quote more frequently or is it a money/subscription thing to get higher or more timely detail?

Ogimet does quote the max at 18Z and min at 06Z as here, going back in this case for 10 days http://www.ogimet.com/cgi-bin/gsynres?lang=en&ind=03772&decoded=yes&ndays=10&ano=2019&mes=07&day=04&hora=18

The Country Summary at those times also reports the max and min respectively http://www.ogimet.com/cgi-bin/gsynres?lang=en&state=United+K&osum=no&fmt=html&ord=REV&ano=2019&mes=07&day=04&hora=18&ndays=1&Send=send

They are usually correct, in my experience but for the 3rd, at Heathrow, there appear to be some missing values including the Tmax, so it just uses the highest hourly report.

Edited by DaveL

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1 hour ago, Aleman said:

 

You must not have looked at the links I gave. Check this one out again. I gather guidance is not to have development within 100 feet/30m of a station yet the A40 is just 10m or so away. It used to be further away but came closer when it expanded to 6 lanes and a cycle path from 4 previously (and only two lanes when the airport was just a grass field, though that might predate the weather station at that site). Again, I had not seen this blog but the but the photos were good and comment seems valid. (Note, the superimposed circles seem to be 10m, 30m and 100m as per classifications below.)

https://tallbloke.wordpress.com/2012/09/10/wmo03672-northolt/

A40 has been 6 lanes for a long as i can remember its not a recent development.

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Posted (edited)

Thanks to all, especially DaveL, who seems to have picked out my problem in his last sentence - that would explain why Heathrow was hotter but my usual summary table got it wrong.

 

Here's Northolt Airport, 1954 (John Stroud Collection). It looks very likely that the "cactus" in the grass just down from the entrance from the dual carriageway is probably the unfenced weather station. It was twice as far to the smaller road road back then and had no access path, so had only grass up to 20-25m - VERY MUCH unlike today!

http://www.ukairfieldguide.net/airfields/Northolt

(photo John Stroud Collection)

 

.image.thumb.png.3231cc80545a938264406f75a4882f8f.png-

 

 

 

 

image.png

Edited by Aleman

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Though there may be another, much more prosaic explanation for why that wee corridor so often sees our highest maxima, geography: the zone in question just happens to be far-enough north, to be outwith the cooling effect of the English Channel -- and, at same time, it's so far south that it benefits from the warmest 850hPa temps?

And, anyway, if urbanisation/UHI were the cause, wouldn't nighttime minima also be anomalously high?

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Posted (edited)

How many times do I have to say? I'm not looking at urbanisation/UHI. I'm looking at local encroachment of station siting boundaries.

 

I hadn't looked at Heathrow. It's very nearly as bad with increasing encroachment. The same blogger that did Northolt covered Heathrow in 2012 and has it as a class 3. Since then, looking at Google maps and streetview, it looks like the southern fence has been upgraded from just mesh to part-panelled - within 10 meters! That would downgrade it to class 4 and increase roughly inferred error from 1C+ to 2C+. That's a crazy thing to do to a weatherstation! Plus the building within 30m to the west has had its roof go from white to pitch black. It strikes me this station is not fit for purpose - not because of planes or runways or UHI but because of building more and more heating souces around it within the class 2 exclusion range of 30m. How on earth can anybody defend it? It's just not good enough.

https://tallbloke.wordpress.com/2012/08/30/wmo03772-heathrow/

I had no idea our weather stations were so bad - and after so much controversy about Heathrow, too. You'd think it would be sorted. I think it's appalling that it's got worse..

Edited by Aleman

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14 minutes ago, Aleman said:

How many times do I have to say? I'm not looking at urbanisation/UHI. I'm looking at local encroachment of station siting boundaries.

Well, last time I checked, the main reason for 'encroachment', as you so deftly call it, was (unless I've lost the plot?) urbanisation?:oldgrin:

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Building stuff within the exclusion zone around a weather station has nothing to do with urbanisation/UHI. That would would still be taking place and being measured at Heathrow Airport even if it was correctly sited with only grass for 30m or 100m. Heathrow Airport could still be a grass field in the middle of nowhere and putting such heat sources within the weather station's exclusion zones would still make it unfit for purpose.

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2 minutes ago, Aleman said:

Building stuff within the exclusion zone around a weather station has nothing to do with urbanisation/UHI. That would would still be taking place and being measured at Heathrow Airport even if it was correctly sited with only grass for 30m or 100m. Heathrow Airport could still be a grass field in the middle of nowhere and putting such heat sources within the weather station's exclusion zones would still make it unfit for purpose.

Says you. But it seems those with the responsibility for such matters don't necessarily agree...Such is life, I suppose; I guess not all opinions found within the blogosphere can be entirely trusted? :oldgrin:

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Posted (edited)
3 hours ago, Aleman said:

Building stuff within the exclusion zone around a weather station has nothing to do with urbanisation/UHI. That would would still be taking place and being measured at Heathrow Airport even if it was correctly sited with only grass for 30m or 100m. Heathrow Airport could still be a grass field in the middle of nowhere and putting such heat sources within the weather station's exclusion zones would still make it unfit for purpose.

I agree with this. We already know the extent of the UHI in London, it's about +1.7°C at night, and around +0.4°C on maxima in late Spring/early summer, though the latter is reported to be not statistically significant - and some earlier studies were a bit higher on max. https://rmets.onlinelibrary.wiley.com/doi/full/10.1002/asl.896

UHI effects can be adjusted for in climate modelling but the question is, and I think this is your argument, that the Northolt and Heathrow sites are not representative of the wider surrounding urban areas - due mainly to adjacent areas of concrete or tarmac, and buildings or trees acting as wind breaks. I haven't yet read any studies into how that translates into temperature anomalies but I am sure there are such studies, I think you may have some figures on that.

Another thing to mention is that annual and monthly anomalies such as produced in the UHI study I linked don't tell us what the daytime anomaly is on specific days and in specific weather conditions - these would vary according to wind direction, speed and amount/brightness of sunshine on any particular day. That's something I'd like to get to the bottom of for sites like Northolt and Heathrow - and even ostensibly rural sites like Hurn which, while remote from the runway, is close to some airport buildings. I linked earlier to Southampton Airport where I know the exact METAR AWS screen location - that is a good example of a representative site, albeit not a SYNOP station - compare it with any of the three sites I mentioned above, it's far better sited and should be upgraded IMO. Southampton Airport METAR AWS.

Edited by DaveL

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Perhaps an e mail to the Met Office asking for their view on these suggestions might just get an official answer. If you don't try then you will not get an official reply. Years ago I was officially involved in this kind of discussion, a complex issue I assure you and siting is of very serious concern to UK Met.

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pia23148-1041.jpg?itok=TDQhKFrP

I found this quite staggering in the extent of differential between urbanisation and rural. Note the airports are marked and show significant heat early morning.

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With regards to Heathrow, surely if there were station issues... maxima would be increasing more then its surroundings or something along those lines.

I compared the annual maxima in Heathrow and Benson since 1974 (minima has some zero values in heathrow so not looked at that). Data is from Tutiempo.

What does it show?

image.thumb.png.cfcd35b740119d7a9fa86911e057432e.png

Maxima is almost identical! and both show an upwards trend! A few minor differences due to the fact in earlier years Benson doesn't have weekend data.

However since the 1970s we can say that there is no issue with the data here after comparing an urban site and one that is considerably less urban.

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On 05/07/2019 at 18:34, Quicksilver1989 said:

With regards to Heathrow, surely if there were station issues... maxima would be increasing more then its surroundings or something along those lines.

I compared the annual maxima in Heathrow and Benson since 1974 (minima has some zero values in heathrow so not looked at that). Data is from Tutiempo.

What does it show?

image.thumb.png.cfcd35b740119d7a9fa86911e057432e.png

Maxima is almost identical! and both show an upwards trend! A few minor differences due to the fact in earlier years Benson doesn't have weekend data.

However since the 1970s we can say that there is no issue with the data here after comparing an urban site and one that is considerably less urban.

Thanks for that. Is that unadjusted station output? I'd be curious to see minima values even with an odd gap. My own analysis of the last 30 days shows Heathrow Airport over 1C warmer tha n Benson for maxes and over 2.5C warmer for mins:

HIghs and low were taken from Ogimet tables here:

https://www.ogimet.com/cgi-bin/gsynres?lang=en&ind=03772&ano=2019&mes=7&day=7&hora=16&min=0&ndays=30

https://www.ogimet.com/cgi-bin/gsynres?lang=en&ind=03658&ano=2019&mes=7&day=7&hora=16&min=0&ndays=30

 

I then subtracted Benson from Heathrow to show how much warmer Heathrow was (in degrees C) - all in my head so hope no errors.

Hi   Low

July 7th +0.2 +0.4

+0.7 +6.3

+0.7 +4.0

+1.6 +4.7

+1.1 +5.4

+1.9 +6.2

+3.2 +2.2

+0.8 +1.3

+1.2 +2.8

-0.9 +3.8

+0.6 +1.5

+2.6 +1.3

-1.5 +1.3

-1.5 +3.0

+1.5 +2.3

+2.0 +5.1

+2.4 +3.4

+0.9 +3.2

+0.6 +0.4

+1.3 +3.2

+2.6 +0.9

+1.6 +1.7

+1.8 +2.6

+0.9 +1.8

+0.3 +0.9

+5.1 +0.6

-0.1 +3.3

+0.3 +3.4

June 8th +1.6 +0.1

Average +1.17 +2.57

I'm not saying this proves anything. So Heathrow stays warm at night this time of year, maybe significantly more on an odd occasion.  (At a quick glance,  the winter night difference looks even bigger but could be bothered to do more.) Maybe Heathrow has more residual night heat from UHI or maybe from encroachment. How do we know how much of each? Kew has marginally higher highs (annual ave. +0.2C from Wiki) but cooler lows (-0.7C)  What does that say?  It's central London but low encroachment in a nice green site with lots of trees at 30m+. I'm not saying this proves anything. It just raises more questions than answers about encroachment for me.

https://en.wikipedia.org/wiki/Climate_of_London#Kew_-_Weather_station_in_South_West_London,_adjacent_to_the_River_Thames

Check old pictures and Heathrow and Northolt have obviously fallen in WMO classfications (link below) due to significant encroachment. How much encroachment do you tolerate before you say enough is enough? I can't find any evidence of where the Met Office draw the line, or even if they do. I'd have thought that common sense would have seen these two booted out. I'm just asking questions here first about what is and is not acceptable, as many Netweather posters seem very knowledgeable.

http://www.wmo.int/pages/prog/www/IMOP/SitingClassif/CIMO_Guide_2014_en_I_1-2_Annex_1B.pdf

 

(I don't pretend to be an expert. I'm a novice. As it happens, I'm just trying to find the best site in my garden for a budget station now and it makes you think a lot about it. I was surprised what I saw when I thought to check Northolt and Heathrow on the back of following the Heatwave High competition.)

 

And thanks to Paul for moving this into a new thread.

 

 

 

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Benson is a known frost hollow though - it’s regularly one of the UK cool spots for minima, especially in winter. You’d expect it to have much lower minima than Heathrow.

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15 hours ago, Aleman said:

Thanks for that. Is that unadjusted station output? I'd be curious to see minima values even with an odd gap. My own analysis of the last 30 days shows Heathrow Airport over 1C warmer tha n Benson for maxes and over 2.5C warmer for mins:

HIghs and low were taken from Ogimet tables here:

https://www.ogimet.com/cgi-bin/gsynres?lang=en&ind=03772&ano=2019&mes=7&day=7&hora=16&min=0&ndays=30

https://www.ogimet.com/cgi-bin/gsynres?lang=en&ind=03658&ano=2019&mes=7&day=7&hora=16&min=0&ndays=30

 

I then subtracted Benson from Heathrow to show how much warmer Heathrow was (in degrees C) - all in my head so hope no errors.

Hi   Low

July 7th +0.2 +0.4

+0.7 +6.3

+0.7 +4.0

+1.6 +4.7

+1.1 +5.4

+1.9 +6.2

+3.2 +2.2

+0.8 +1.3

+1.2 +2.8

-0.9 +3.8

+0.6 +1.5

+2.6 +1.3

-1.5 +1.3

-1.5 +3.0

+1.5 +2.3

+2.0 +5.1

+2.4 +3.4

+0.9 +3.2

+0.6 +0.4

+1.3 +3.2

+2.6 +0.9

+1.6 +1.7

+1.8 +2.6

+0.9 +1.8

+0.3 +0.9

+5.1 +0.6

-0.1 +3.3

+0.3 +3.4

June 8th +1.6 +0.1

Average +1.17 +2.57

I'm not saying this proves anything. So Heathrow stays warm at night this time of year, maybe significantly more on an odd occasion.  (At a quick glance,  the winter night difference looks even bigger but could be bothered to do more.) Maybe Heathrow has more residual night heat from UHI or maybe from encroachment. How do we know how much of each? Kew has marginally higher highs (annual ave. +0.2C from Wiki) but cooler lows (-0.7C)  What does that say?  It's central London but low encroachment in a nice green site with lots of trees at 30m+. I'm not saying this proves anything. It just raises more questions than answers about encroachment for me.

https://en.wikipedia.org/wiki/Climate_of_London#Kew_-_Weather_station_in_South_West_London,_adjacent_to_the_River_Thames

Check old pictures and Heathrow and Northolt have obviously fallen in WMO classfications (link below) due to significant encroachment. How much encroachment do you tolerate before you say enough is enough? I can't find any evidence of where the Met Office draw the line, or even if they do. I'd have thought that common sense would have seen these two booted out. I'm just asking questions here first about what is and is not acceptable, as many Netweather posters seem very knowledgeable.

http://www.wmo.int/pages/prog/www/IMOP/SitingClassif/CIMO_Guide_2014_en_I_1-2_Annex_1B.pdf

 

(I don't pretend to be an expert. I'm a novice. As it happens, I'm just trying to find the best site in my garden for a budget station now and it makes you think a lot about it. I was surprised what I saw when I thought to check Northolt and Heathrow on the back of following the Heatwave High competition.)

 

And thanks to Paul for moving this into a new thread.

 

 

 

be interesting to see the difference in temps between Heathrow and Northolt?..i would say if anything Heathrow would be effected more by urban environment due to its size, air traffic and esp being right next to the M25

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4 minutes ago, cheeky_monkey said:

be interesting to see the difference in temps between Heathrow and Northolt?..i would say if anything Heathrow would be effected more by urban environment due to its size, air traffic and esp being right next to the M25

I'll put a comparison together. 

In terms of Benson, it's really no shock that rural Oxfordshire is cooler than urban west London! The bigger surprise is that the difference isn't larger, which perhaps suggest that the theories about heathrow/northolt being badly sited and giving iffy readings are bogus. 

I think we have to be careful of buying into some of these theories presented by bloggers who maybe have alterior motives - such as climate change denial and the like. 

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8 hours ago, mb018538 said:

Benson is a known frost hollow though - it’s regularly one of the UK cool spots for minima, especially in winter. You’d expect it to have much lower minima than Heathrow.

But what defines a frost hollow, though? That usually means a dip in the topography or something sheltered otherwise from wind. That seems to be just what Benson is not. It looks well sited in an airport. I grant that could cause an unrepresentative dip due to lower humidity, though. How many other stations are well sited? Could it be the cold winter ones were well sited and remain so while the others have encroachment of things that might keep temperature up at night - paths, fences, car parks, (heated) buildings, vents, drainage. Again, the US has gone for a new set of stations to get rid of these types of arguments. It would be nice if we could divert a bit of those supercomputer budgets to have some well sited stations designed to be well-sited and consistent and remain that way.

 

And I think interference from encroachment might show in summer temperatures to some degree but is more likely to observable in winter lows. To avoid UHI effects, you'd have to compare free and encroached rural ones (and then maybe free and encroached urban ones, if possible to choose otherwise similar).

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