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Aleman

Northolt, Heathrow, Temperature Records and Weather Station Siting

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On 17/07/2019 at 21:05, Quicksilver1989 said:

Class 1 - the highest

• T1) Ground within a 10m radius ‘mostly’ covered with lawn or natural and low vegetation (generally less than 10cm) representative of the region. Probably a fail as the path covers a good deal of the 10m radius 
o Open patches of bare ground acceptable if natural and representative of the region.

• T2) Generally flat area within 10m of sensor. This area surrounded by generally open space with a slope of less than 1:3 (19°) that is considered to be representative of the large scale area. PASS, its flat

• T3) No irrigation or routine lawn watering system operational within 30m radius. Pass

• T4) Sensor located at more than 100m from significant heat sources or reflective surfaces (buildings, roads, concrete surfaces, car parks, unnatural exposed ground areas, etc.). This also includes expanses of water even if only seasonal. Fail

o Open water can be excluded if representative of the region such as open coastal or large scale inland water/swamp/muskeg (ie. 25% of 100km radius, approximately equal in most directions).

A ‘significant’ source of heat (or expanse of water) is defined to have an impact if it comprises more than 10% of the horizontal surface within a 100m radius surrounding the sensor, comprises more than 5% of the horizontal surface within 30m radius or comprises 1% of the horizontal surface within a 10m radius.
10% of 100m radius circle = 3142 sq.m (32,212 sf)
5% of 30m radius circle = 141 sq.m (1368 sf)
1% of 10m radius circle = 3 sq.m (33 sf)

• T5) Sensor away from any subjectively determined heat source such as nearby infrastructure (ie. electronics boxes, metal mounting structures, etc.) that will have any affect on sensor. Minimum distance from sensor to nearby other objects at least 2x the height of the object away (2:1). The fence around it probably doesn't fall into this class of objects, so PASS?

• T6) Site not subjected to any jet blast, prop wash or helicopter downwash. No routine significant aircraft ground movement/parking or significant paved public highways within 100m. Fail, it's a class two.

• T7) Sensor in a ventilated solar radiation shield or enclosure. PASS

• T8) Sensor mounted between 1.2m and 2.0m above ground level. PASS

• T9) The maximum anticipated (subjective and non-extreme) snowdepth will remain at least 1m below the bottom surface of sensor shield or enclosure. Maybe a fail if a 1947 style winter came along.

• T10) Sensor radiation shield away from all significant shade when the Sun is higher than 5 deg. above local geographic horizon. Fail due to the shrubbery behind the sensor.

More detailed info on classification systems can be found here...


https://www.wmo.int/pages/prog/www/IMOP/SitingClassif/Canada/Siting Classification System7 - Sep 2012.pdf

I'd say its a class 2 station. If everywhere was a class 1 station how limited would we be with regards to potential site placements? These sites have been there for many decades (St James Park 1903 I think). The main thing is that the temperature data is homogenous for long term climate studies. The readings may not be top notch perfect from an open station point of view but surely they are representative of the overall area they are in.

Great link. Very helpful. Thanks.

It fails Class 2 on the last bit of this:

T4) Sensor located at more than 30m from significant heat sources or reflective surfaces (buildings, roads, concrete surfaces, car parks, unnatural exposed ground areas, etc.). This also includes expanses of water even if only seasonal. o Open water can be excluded if representative of the region such as coastal or large scale inland water/swamp/muskeg (ie. 25% of 100km radius, approximately equal in most directions). A ‘significant’ source of heat (or expanse of water) is defined to have an impact if it comprises more than 10% of the horizontal surface within a 30m radius surrounding the sensor, "comprises more than 5% of the horizontal surface within 10m radius or comprises 1% of the horizontal surface within a 5m radius".

It fails Class 3 on the last bit of this:

T4) Sensor located at more than 10m from significant heat sources or reflective surfaces (buildings, roads, concrete surfaces, car parks, unnatural exposed ground areas, etc.). This also includes expanses of water even if only seasonal. o Open water can be excluded if representative of the region such as coastal or large scale inland water/swamp/muskeg (ie. 25% of 100km radius, approximately equal in most directions). A ‘significant’ source of heat (or expanse of water) is defined to have an impact if it comprises "more than 10% of the horizontal surface within a 10m radius surrounding the sensor or comprises more than 5% of the horizontal surface within 5m radius."

It passes Class 4

T4) Sensor located at more than 10m from significant heat sources or reflective surfaces (buildings, roads, concrete surfaces, car parks, unnatural exposed ground areas, etc.). This also includes expanses of water even if only seasonal. o Open water can be excluded if representative of the region such as coastal or large scale inland water/swamp/muskeg (ie. 25% of 100km radius, approximately equal in most directions). A ‘significant’ source of heat (or expanse of water) is defined to have an impact if it comprises more than 50% of the horizontal surface within a 10m radius surrounding the sensor or comprises more than 30% of the horizontal surface within 5m radius.

 

I'm not sure how the thicker black rails would count but I still think set up in this case looks ill-advised when compared to other site perimeters. I'd be surprised if they did not have a slight influence.

 

I'm not sure about the record being homogenous. Maybe, but it's still a class 4 site with some other minor variables to take in that might well have varied over time.

 

Going from memory, your more detailed Canadian source looks to probably make Northolt a Class 3, down from a Class 2 when the road was expanded. And Heathrow looks worse that St James - probably still Class 4 rather than 5 - but having lots of changes that have made it worse over time.  St James looks reasonably consistent over time but Northolt and Heathrow don't.

I stand by my previous thoughts that these three stations do not meet the Met Office's own siting guidelines, although they are vague. The latter seem to tie in with Class 2 guidelines from WMO and Meteo France and USCRN. I don't think any of these stations would meet Class 2. I haven't even looked at measurement sensors other than temperature, which looks like they would throw some fails at Class 2 or 3.

 

 

Edited by Aleman

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On 17/07/2019 at 19:42, Aleman said:

Great effort! But could that just be down to Kew having significatly more moisture in the air thanks to tree transpiration and shading?  It would not heat as much in summer or cool as much in winter.

There is nothing but short grass (and a low enclosure fence) within 30 metres of the Kew Gardens site (map here) - (photo here) and beyond that, the trees and shrubs are reasonably spread out until you get to 40 metres (on part of one side only), with any buildings or large concrete etc. areas other than paths over 90 metres distant and only on the NW side.

I haven't been through the official site classification regulations but if Kew doesn't meet class 1 on temperature, I would be surprised if anywhere else in London does, unless I have missed something. That's why I did the Kew - Heathrow comparison - perhaps the best versus the worst, but we shall see.

I will be doing  more work on this, mean monthly max and min comparisons for example, and hopefully eventually I can compare changes over the years but it's not straightforward, given the way the daily temperature data is presented by the CEDA website ("Met Office MIDAS Open").

Edited by DaveL

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On 19/07/2019 at 13:32, Aleman said:

Great link. Very helpful. Thanks.

It fails Class 2 on the last bit of this:

T4) Sensor located at more than 30m from significant heat sources or reflective surfaces (buildings, roads, concrete surfaces, car parks, unnatural exposed ground areas, etc.). This also includes expanses of water even if only seasonal. o Open water can be excluded if representative of the region such as coastal or large scale inland water/swamp/muskeg (ie. 25% of 100km radius, approximately equal in most directions). A ‘significant’ source of heat (or expanse of water) is defined to have an impact if it comprises more than 10% of the horizontal surface within a 30m radius surrounding the sensor, "comprises more than 5% of the horizontal surface within 10m radius or comprises 1% of the horizontal surface within a 5m radius".

It fails Class 3 on the last bit of this:

T4) Sensor located at more than 10m from significant heat sources or reflective surfaces (buildings, roads, concrete surfaces, car parks, unnatural exposed ground areas, etc.). This also includes expanses of water even if only seasonal. o Open water can be excluded if representative of the region such as coastal or large scale inland water/swamp/muskeg (ie. 25% of 100km radius, approximately equal in most directions). A ‘significant’ source of heat (or expanse of water) is defined to have an impact if it comprises "more than 10% of the horizontal surface within a 10m radius surrounding the sensor or comprises more than 5% of the horizontal surface within 5m radius."

It passes Class 4

T4) Sensor located at more than 10m from significant heat sources or reflective surfaces (buildings, roads, concrete surfaces, car parks, unnatural exposed ground areas, etc.). This also includes expanses of water even if only seasonal. o Open water can be excluded if representative of the region such as coastal or large scale inland water/swamp/muskeg (ie. 25% of 100km radius, approximately equal in most directions). A ‘significant’ source of heat (or expanse of water) is defined to have an impact if it comprises more than 50% of the horizontal surface within a 10m radius surrounding the sensor or comprises more than 30% of the horizontal surface within 5m radius.

 

I'm not sure how the thicker black rails would count but I still think set up in this case looks ill-advised when compared to other site perimeters. I'd be surprised if they did not have a slight influence.

 

I'm not sure about the record being homogenous. Maybe, but it's still a class 4 site with some other minor variables to take in that might well have varied over time.

 

Going from memory, your more detailed Canadian source looks to probably make Northolt a Class 3, down from a Class 2 when the road was expanded. And Heathrow looks worse that St James - probably still Class 4 rather than 5 - but having lots of changes that have made it worse over time.  St James looks reasonably consistent over time but Northolt and Heathrow don't.

I stand by my previous thoughts that these three stations do not meet the Met Office's own siting guidelines, although they are vague. The latter seem to tie in with Class 2 guidelines from WMO and Meteo France and USCRN. I don't think any of these stations would meet Class 2. I haven't even looked at measurement sensors other than temperature, which looks like they would throw some fails at Class 2 or 3.

 

 

Fair enough, still not sure which of our judgements is correct without seeing all the metadata but its made for an interesting discussion regardless. Good stuff 🙂

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On 20/07/2019 at 16:47, DaveL said:

There is nothing but short grass (and a low enclosure fence) within 30 metres of the Kew Gardens site (map here) - (photo here) and beyond that, the trees and shrubs are reasonably spread out until you get to 40 metres (on part of one side only), with any buildings or large concrete etc. areas other than paths over 90 metres distant and only on the NW side.

I haven't been through the official site classification regulations but if Kew doesn't meet class 1 on temperature, I would be surprised if anywhere else in London does, unless I have missed something. That's why I did the Kew - Heathrow comparison - perhaps the best versus the worst, but we shall see.

I will be doing  more work on this, mean monthly max and min comparisons for example, and hopefully eventually I can compare changes over the years but it's not straightforward, given the way the daily temperature data is presented by the CEDA website ("Met Office MIDAS Open").

I think Heathrow and Kew were good choices on your part and appreciate your ongoing efforts.  I doubt I have the experience, expertise or time to try do such analysis.

I'm not certain Kew makes Class 1 technically. It's maybe borderline but it does look to pass the common sense test in my mind. It depends on definition:

• T4) Sensor located at more than 100m from significant heat sources or reflective surfaces (buildings, roads, concrete surfaces, car parks, unnatural exposed ground areas, etc.). This also includes expanses of water even if only seasonal  -  There are a few heat sources between 50m and 100m but are they big enough to be significant in the scheme of things to fail Class 1? I'm tempted to say probably not.


• T10) Sensor radiation shield away from all significant shade when the Sun is higher than 5 deg. above local geographic horizon. - There are numerous big trees at 40-50m movingn east to south. They definitely obstruct well over 5 degrees - but do they offer significant shade? I think you'd have to guess at borderline without actually going there to check canopy.

 

I do think Kew, as maybe just about creeping into Class 1,  is good for the UK and great for London. It's clearly fine for measuring a green oasis in London, if that is what we want to measure. Class 3s and Class 4s might be also fine for measuring the temperature close to obstructions, roads and paths in London, if that is also what we want to measure. That IS the temperature in those spots and could be representative for broadly built up areas..

But for me the problem with Northolt and Heathrow is they have changed over time within 30m - and even within 10m in Heathrow's case. Does a new record say it's got hotter in the Heathrow area or just that it's got more encroached upon?  St James may have been more consistent over time or not. I don't know. Does the Met Office keep track of this stuff? If USCRN is trying to standardise to Class 1 for a rural network to measure untainted national trends and avoid the need for debated adjustments, what level of consistency should our Met Office be aiming for? Maybe we should have a specifically urban network and specifically rural one and be aiming at consistency (lack of further encroachment) over time. Would that be a reasonable aim or not? I don't know. It is only in researching siting for my own domestic station that I have realised what a can of worms national network siting seems to be - though it's interesting to try pin down what affects what.


 

 

Edited by Aleman

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20 hours ago, danm said:

Why is Heathrow so hot?

https://www.bbc.co.uk/news/uk-44980493

Yes, and I am sure that all of us know that. But the immediate site surroundings are part of the equation, and there are for good reasons official siting standards which are not always being met. That's what we are discussing here (I think!).

I suspect that there might be some focus on Cambridge University Botanical Gardens soon, for just those reasons. It may or may not have been well sited yesterday, but Google Maps definitely raises the question!

 

Edited by DaveL

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On 25/07/2019 at 18:44, danm said:

Why is Heathrow so hot?

https://www.bbc.co.uk/news/uk-44980493

We've already covered that this is about the Met Office not complying with its own station siting rules. Read back through the thread and see that the North runway and planes argument is a red herring. They're further away than the Class 1 limit of 100m so they're fine. The station fails on loads of other things within 30m and even within 10m. It looks like a Class 4 station.

Edited by Aleman

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Cambridge University Botanical gardens looks like it might even be a Class 5 on the current Google Streetview.  Large trees at 20m stand out but they are far from the biggest problem. Google's street view shows reflective plastic sheeting in some kind of large cloche formations only a few metres from the temperature sensor to the west, with cornfields immediately adjacent north (2 metres?) and south (6 metres?). Corn that will grow taller than the Stephenson Screen at about 2m distant! The station plot is only 5m x 5m and is crowded in with heavy distorting influences within 10m and 30m.  It's basically in an alloment right up to the Stephenson Screen and will vary with what they are growing  - and watering - each year. It's a joke to say the UK might have just had its hottest ever day of 38.7C on this.

 

Edited by Aleman

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Someone on another thead has loaded this as allegedly an image for Cambridge Botanical Garden today:

botanic.jpeg.f8ba0ab1949b0fd386465c8c3cc

It looks completely different. Whether or not the Met Office accept the new record, you surely have no continuity if it's a 7-foot cornfield and reflective plastics within a few metres one year and than a barren pile of dirt the next. What will they be growing next year - sunflowers? Is this the science UK meteorology is based on? Where's the standardisation to eliminate variables that science calls for?

Edited by Aleman

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If the Radcliffe Observatory in Oxford can happily downgrade to a Class 5 site every June (see here), then it shouldn't be surprising what goes on around lesser stations.

Edited by James102

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11 hours ago, Aleman said:

Someone on another thead has loaded this as allegedly an image for Cambridge Botanical Garden today:

botanic.jpeg.f8ba0ab1949b0fd386465c8c3cc

It looks completely different. Whether or not the Met Office accept the new record, you surely have no continuity if it's a 7-foot cornfield and reflective plastics within a few metres one year and than a barren pile of dirt the next. What will they be growing next year - sunflowers? Is this the science UK meteorology is based on? Where's the standardisation to eliminate variables that science calls for?

I would think not many  'garden' locations are very suitable as  gardens are  generally set up to be sheltered as is shown there.
Dry soil below it will heat up much like concrete or tarmac, but it's the lack of wind flow is most serious. It will be getting wafted by very warm air trapped in the area sheltered by the trees and buildings even if they are 100feet away.

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13 hours ago, Aleman said:

We've already covered that this is about the Met Office not complying with its own station siting rules. Read back through the thread and see that the North runway and planes argument is a red herring. They're further away than the Class 1 limit of 100m so they're fine. The station fails on loads of other things within 30m and even within 10m. It looks like a Class 4 station.

The whole area is basically a concrete jungle, comparing his to how it was when an 'airport' was a grassy field with a hangar at one end in the 1950s shows why a lot of these readings are now showing several degrees of UHI

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5 hours ago, 4wd said:

I would think not many  'garden' locations are very suitable as  gardens are  generally set up to be sheltered as is shown there.
Dry soil below it will heat up much like concrete or tarmac, but it's the lack of wind flow is most serious. It will be getting wafted by very warm air trapped in the area sheltered by the trees and buildings even if they are 100feet away.

What you describe is a sun-trap, but I don't think that will be as significant as the stevenson screen effect - Stevenson screens get heated by up to a degree in light winds and have been acknowledged by the WMO to heat the air inside by up to 2.5 deg C in extremis in a sunny flat calm. There was wind of 6 m/s at Cambridge airport on Thursday afternoon but that would be on open ground at 10 metres. Down at 1.5 metres behind the sheltering trees bushes and hedges, the wind would have been very light - I estimated maybe 1 m/s in the other thread, after reading up on those effects.

So, IMO, there was maybe half a degree of error in the true air temperature for the wider area the screen is supposed to represent (studies have shown up to 0.8 °C in light winds). That would put the true air temperature at 38.2°C, a July record as it's slightly above the maximum at Cambridge NIAB just outside the town and would make sense. However, the reported difference between those two stations of 0.6°C seems a tad high to my eyes and understanding and the trees etc. is the most likely reason.

But I expect the Met Office to ratify the 38.7 as they don't seem to be at all concerned about errors caused by the well known effects of shelter from the wind (as in the case of Brogdale too). That, to me, not to mention Burt/Eden in their comprehensive report and other respected meteorologists who support it, seems inexplicable - unless they are worried about opening a can of worms.

 

Edited by DaveL

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On 12/07/2019 at 09:29, Midlands Ice Age said:

Regarding B'ham...

I live on the outskirts of Bham, near a piece of 'Green' land, on the South East side of the conurbia. Bham Airport and Coleshill are my  nearest official stations.

There are several 'unofficial' temp recordings in this  area, 

In summer my daytime maxes are usually about right. (comparing with the more central areas); night time I do fall much lower.

In winter, I record much colder both night and day  (unless the wind is in the North or North west - when I get shielded from the colder weather by the 20 miles of suburbia to the West and North).  I find I am substantially colder when the wind is in the Easterly quadrant. I have also observed these affects many times with car temperature watching.

My impression is that wind direction can have a big affect on the UHI of a particular site.

To this point a paper was published about 6 months ago  that stated  that the UHI affect downwind had been detected up to 800 yards, and within 100 yards there was still a detectable amount of heating. Within 50 yards up to 1degree higher recordings were discovered at fairly regular interludes by the researchers.

My message is   - look at wind directions associated with any 'errant' temperatures. 

Re   the above

Knocker has just posted this link to a paper confirming the enhanced wind effect UHI, which is called UHA (Advection effect)

I saw a preliminary version  of this, and it seems as though its effect is quite substantial as I indicated above.

It seems as though this effect is still seen at 2 - 3Km and is thought to be 0.4C -0.6C at around 500 meters.

It seems clear that impacts caused are not yet being recognised.

It is in addition to the UHI adjustments

Bassett_et_al_Effects_Heat_Advection_Boundary_Layer_Meterology.pdf

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On 26/07/2019 at 20:49, Aleman said:

Someone on another thead has loaded this as allegedly an image for Cambridge Botanical Garden today:

botanic.jpeg.f8ba0ab1949b0fd386465c8c3cc

It looks completely different. Whether or not the Met Office accept the new record, you surely have no continuity if it's a 7-foot cornfield and reflective plastics within a few metres one year and than a barren pile of dirt the next. What will they be growing next year - sunflowers? Is this the science UK meteorology is based on? Where's the standardisation to eliminate variables that science calls for?

Jesus, wouldnt even site one of my amateur stations in that location. Or if i did I would be disregarding any readings on sunny, calm days. Shocking as you say to say this weather station holds our UK record.

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Anyone know why the max temp for Heathrow airport on 25th July shows 37.2c yet on the day the Met Office reported 37.9c at this site? Is there more than one station at Heathrow?

Edited by danm

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There is a Heathrow station besides the Heathrow Airport (3772) one but the figures you mention both appear in Heathrow Airport data. One is the highest reported hourly figure  - at 1pm - and one is the highest reached that day at any time, including figures between hours.

https://www.ogimet.com/cgi-bin/gsynres?ind=03772&lang=en&decoded=yes&ndays=2&ano=2019&mes=07&day=25&hora=21

 

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On 27/07/2019 at 13:52, DaveL said:

What you describe is a sun-trap, but I don't think that will be as significant as the stevenson screen effect - Stevenson screens get heated by up to a degree in light winds and have been acknowledged by the WMO to heat the air inside by up to 2.5 deg C in extremis in a sunny flat calm. There was wind of 6 m/s at Cambridge airport on Thursday afternoon but that would be on open ground at 10 metres. Down at 1.5 metres behind the sheltering trees bushes and hedges, the wind would have been very light - I estimated maybe 1 m/s in the other thread, after reading up on those effects.

So, IMO, there was maybe half a degree of error in the true air temperature for the wider area the screen is supposed to represent (studies have shown up to 0.8 °C in light winds). That would put the true air temperature at 38.2°C, a July record as it's slightly above the maximum at Cambridge NIAB just outside the town and would make sense. However, the reported difference between those two stations of 0.6°C seems a tad high to my eyes and understanding and the trees etc. is the most likely reason.

But I expect the Met Office to ratify the 38.7 as they don't seem to be at all concerned about errors caused by the well known effects of shelter from the wind (as in the case of Brogdale too). That, to me, not to mention Burt/Eden in their comprehensive report and other respected meteorologists who support it, seems inexplicable - unless they are worried about opening a can of worms.

 

On a slightly unrelated matter if winds are generally weaker at these sites due to surroundings would winter minima be too low as well?

As mentioned on the other page, this is a consequence of the very long running stations (this site began recording in 1904), their surroundings aren't optimal but its the anomalies which matter most with regards to long term temperature trend analysis. Provided changes are accounted for its fine.

Should we question temperature data from September 1911 or other such months with notable high maxima? It has always been an issue unfortunately. The anomalies at urban stations compared to rural shows identical trends so I don't agree with the comments with that 4wd made. There is more evidence to back this assumption from the comparisons I made between Heathrow and Benson a few pages ago.

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7 minutes ago, Aleman said:

There is a Heathrow station besides the Heathrow Airport (3772) one but the figures you mention both appear in Heathrow Airport data. One is the highest reported hourly figure  - at 1pm - and one is the highest reached that day at any time, including figures between hours.

https://www.ogimet.com/cgi-bin/gsynres?ind=03772&lang=en&decoded=yes&ndays=2&ano=2019&mes=07&day=25&hora=21

 

Got it. Cheers. 

Bit annoying some websites that list daily temperature data don’t take account of this. One I use regularly called tutiempo has updated its data for July and shows 37.2c as the max for Heathrow on the 25th. 

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10 minutes ago, Quicksilver1989 said:

On a slightly unrelated matter if winds are generally weaker at these sites due to surroundings would winter minima be too low as well?

As mentioned on the other page, this is a consequence of the very long running stations (this site began recording in 1904), their surroundings aren't optimal but its the anomalies which matter most with regards to long term temperature trend analysis. Provided changes are accounted for its fine.

Should we question temperature data from September 1911 or other such months with notable high maxima? It has always been an issue unfortunately. The anomalies at urban stations compared to rural shows identical trends so I don't agree with the comments with that 4wd made. There is more evidence to back this assumption from the comparisons I made between Heathrow and Benson a few pages ago.

It's probably a bit more complicated than that simple thought. Buildings blocking wind might generate enough heat to lift the temperature slightly in winter.  If the wind is blocked from the south, it might also block some winter sun out , but not if blocked from other directions. The windblock might also be decidous and not block so much in winter. So my casual thoughts are that you are thinking along the right lines but the answer is sometimes yes and sometimes no and each site would have to be considered separately.

 

 

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13 minutes ago, Aleman said:

It's probably a bit more complicated than that simple thought. Buildings blocking wind might generate enough heat to lift the temperature slightly in winter.  If the wind is blocked from the south, it might also block some winter sun out , but not if blocked from other directions. The windblock might also be decidous and not block so much in winter. So my casual thoughts are that you are thinking along the right lines but the answer is sometimes yes and sometimes no and each site would have to be considered separately.

 

 

Fair enough, yup the potential biases are complicated and difficult to model as the Bassett paper MIA linked shows. I'd be curious with regards to the WMO class 1 meteorological stations that are in the UK.

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52 minutes ago, Aleman said:

There is a Heathrow station besides the Heathrow Airport (3772) one but the figures you mention both appear in Heathrow Airport data. One is the highest reported hourly figure  - at 1pm - and one is the highest reached that day at any time, including figures between hours.

https://www.ogimet.com/cgi-bin/gsynres?ind=03772&lang=en&decoded=yes&ndays=2&ano=2019&mes=07&day=25&hora=21

 

Why don’t websites that list this data show the highest temperature reached that day rather than just the highest hourly figure?

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12 hours ago, danm said:

Why don’t websites that list this data show the highest temperature reached that day rather than just the highest hourly figure?

Not all MetO stations have the actual max/min data freely available (only the hourly data is). So I assume some websites just use the hourly data for all stations as it's easier than using the actual max for some stations but not others.

It's a bit misleading though. If a website says "maximum" it should either pay to get all the data from MetO, or it should make clear it's only the max from the hourly readings.

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On 07/08/2019 at 23:49, Aleman said:

It's probably a bit more complicated than that simple thought. Buildings blocking wind might generate enough heat to lift the temperature slightly in winter.  If the wind is blocked from the south, it might also block some winter sun out , but not if blocked from other directions. The windblock might also be decidous and not block so much in winter. So my casual thoughts are that you are thinking along the right lines but the answer is sometimes yes and sometimes no and each site would have to be considered separately.

To add a bit to this issue raised by Quicksilver1969, yes it's not a simple answer but I recently came across an interesting report on the effect of tree shelterbelts on temperature. In summary it concludes that trees do lower windspeed for a considerable distance downwind, resulting in less mixing which tends to raise daytime maximum temperatures a little, and lower nighttime minima. This would not apply to all sites of course because if you get too close to trees on the south side, the sun is partly obscured and may counteract the otherwise positive anomaly, and at night the negative anomaly may be counteracted if a lot of open sky is obscured by the trees. They also say this:

"For dense shelterbelts with little through-flow, winds are reduced greatly near the windbreak, but wind speed fully recovers at about 15 times the height of the trees downwind."

Here is the article: Shelter Belts and their Impact on Wind Speed, Solar Radiation, and Air Temperature Inversions

Also relevant is the difference between the sites at say Kew Gardens (a good fairly open site IMO)  and the Cambridge Botanical Garden (a small and sheltered site with trees AND tall dense hedges i.e. which is far from open). I posted the following comparison image in the Guess the Temperature thread after someone made a not-too-serious remark about Kew Gardens. The image really belongs in here and shows very well the vast difference between the two sites - both are from Google Maps at the same scale. See how easily the Cambridge hedged/treed area (right image) fits onto the Kew open lawn, with a lot of room to spare (the position of the Kew screen is indicated by the small red rectangle) - and how cluttered the surroundings are at Cambridge in comparison. I think Cambridge BG is at least as bad as Brogdale and is probably worse.

885585367_kewgdsvscambridgebg.jpg.02d287

Edited by DaveL

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This recent study overlaps UHI and encroachment and is very interesting, in that it helps to understand the magnitude of the effect on temperature variability. It shows significant temperature effects from encroachment on one side.  It shows a difference of 0.31C at 30m on an overall average but over 0.8C at an average of peak times (evenings and nights after sunny summer days with low wind). This suggests temperature differences over 1C at an average of worst times at 30 m for encroachment on more sides, and also likely over 1C for encroachment on one side in the rarer worst conditions - like when records are set. Differences with distance tend to reduce to surprisingly low around midday before rising quickly into the evening (so be wary of studies that study encroachment/UHI but only analyse (late) morning measurements when encroachent effects might be almost insignificant).

There is some interesting stuff in it and it helps highlight why the WMO prefers no encroachment within 100m (Class 1) but sees no enroachment to 30m (Class 2) as being okay. It does highlight why stations that do not meet Class 2 can be seen as problematic. It raises significant questions about encroachment effects on readings at St James, Northolt and Heathrow Airport and maybe even Cambridge and it's new record.

https://www.dropbox.com/s/x12j51a7ws5riyd/Leeper impacts of small-scale urban encrochment.pdf?dl=0

Reading various articles online, Weather organisations around the globe seem to be moving towards attempting to standardise around Class 2 or better. Over to you, Met Office. (Perhaps start with publishing classification estimates for stations?)

Edited by Aleman

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