Apologies for a new thread, especially if it is on a topic where there has previously been a thread and/or much earlier discussion. However, I've been fretting over Petersen's 2003 paper headed "Assessment of Urban Versus Rural In Situ Surface Temperatures in the Contiguous United States: No Difference Found" since Phil Clarke provided the link: http://journals.ametsoc.org/doi/full/10.1175/1520-0442%282003%29016%3C2941%3AAOUVRI%3E2.0.CO%3B2 (sorry, I'm an IT amateur and don't know how to embed links).

When I said there was no point my analysing the paper, Phil responded with "You could submit a comment to the journal, you could 'publish' on the internet and ask for scrutiny, if your criticisms have real merit and you find something the reviewers and all subsequent readers - the paper has been cited 285 times - have missed, they will be impossible to ignore." Which is a fair point, I suppose. I'm not a scientist, and I haven't spent much time on it (I don't have the time, to be honest), but a few points jumped out at me:

1. Phil says the paper has been cited 285 times. I assume he's correct, as I have no reason to disbelieve him. That's a citation, on average, every 2-3 weeks in the last 13 years. Why? I assume they were all supportive and are citing it as evidence of the lack of UHI. I regard that as nothing more than the self-reinforcing consensus.

2. I regard his main conclusion as being stated at the end of the admirably short opening abstract:

"Contrary to generally accepted wisdom, no statistically significant impact of urbanization could be found in annual temperatures. It is postulated that this is due to micro- and local-scale impacts dominating over the mesoscale urban heat island. Industrial sections of towns may well be significantly warmer than rural sites, but urban meteorological observations are more likely to be made within park cool islands than industrial regions."

3. In coming to this conclusion, Petersen (it seems to me) is very much running "[c]ontrary to generally accepted wisdom". To arrive at his conclusion, he has to dismiss the findings of previous research into UHI (section 1c of the paper). At the risk of making this very long indeed, the following are the ones which need to be dismissed (and which he very fairly references, direct quotes from Petersen's paper below):

Cayan and Douglas (1984) found urban-affected heat island temperature increases of 1°–2°C common when comparing linear trends over three to five decades of urban stations with trends at nonurban sites

(Kukla et al. 1986). They looked at the difference in trends between rural and urban data over a 40-yr period for 34 station pairs and concluded that the urban contamination amounted to about 0.12°C decade−1.

Two different approaches were used by Karl et al. (1988) in an attempt to determine the effect of urbanization on the U.S. climate record... Average annual temperature was found to be 0.11°C warmer in cities of 10 000 people, 0.32°C warmer in a 100 000 population city, and 0.91°C warmer in a 1 000 000 population city. All of the warming in average temperature comes from minimum temperature as their annual assessment of daily maximum temperature indicates that urban sites tend to be cooler than rural during the warmest part of the day.

Using 42 pairs of urban–rural stations in China, Wang et al. (1990) found an average urban heat island of 0.23°C

Gallo et al. (1993) looked at clusters of stations and compared the relationship between the difference in rural and urban temperatures and a vegetation index. Most but not all of their rural–urban differences showed urban stations as warmer.

China's northern plains were the subject of a UHI analysis by Portman (1993). Using data from 1954 to 1983 and examining how the differences of residuals between each urban station and every rural station changed, the author determined that the mean annual urban warm bias increased 0.19°C during these 30 yr.

An analysis of the Barcelona heat island is presented in Moreno-Garcia (1994). In addition to transects, the author examined data from two stations. On an annually averaged basis, the urban site was 0.2°C cooler for daily maximum temperatures and 2.9°C warmer on minimum temperatures. It was noted that the two stations had the same instrumentation, similar elevations, and “similar” distances from the sea

The San Antonio, Texas, heat island was assessed by Boice et al. (1996) using 45 yr of data from one San Antonio station and three stations from surrounding small towns. The results indicate that San Antonio's minimum temperature increased at an average rate of 0.3°C relative to the other stations.

Todhunter (1996) determined that the Minneapolis–St. Paul mean urban heat island in 1989 was 2.1°C.

Using data from three different parts of the world, Camilloni and Barros (1997) determined that the urban–rural temperature difference decreases during periods when rural temperatures are increasing and increases when rural temperatures are decreasing.

Böhm (1998) used data from three urban, three suburban, and three rural stations to examine the Vienna, Austria, urban heat island. He found that the urban effect is strongly influenced by local surroundings and therefore could not be regarded for the city as a whole, with the magnitude varying from 0.2° to 1.6°C. The trend in urban warming varied as well, with two central city stations showing no increase in urban warming while the third had 0.6°C warming in 45 yr. In Vienna, the average urban heat island effect was found to be strongest in winter.

Data from two stations were used by Magee et al. (1999) to determine that the effect of the Fairbanks, Alaska, urban heat island grew by 0.4°C over a 49-yr period, with winter months experiencing a more significant increase of 1.0°C.

An analysis of surface air temperature compared to 0.91-m-deep soil temperature indicated an urban heat island increase of 0.2°C over the period 1889–1952 for Urbana–Champaign, Illinois (Changnon 1999).

Gallo and Owen (1999) identified clusters of stations in the contiguous United States and compared the relationship between the difference in rural and urban temperatures and a vegetation index. They found seasonal changes in the urban–rural differences that tracked changes in the vegetation index. Most, but not all, of their rural–urban differences showed urban stations as warmer with urban stations averaging 0.38°C warmer than rural.

One city, Tucson, Arizona, was the subject of several different analyses by Comrie (2000), including transects by vehicle-mounted thermistors, spatial examination of in situ data, and comparison of rural and urban temperature time series. The results indicated that Tucson's urban heat island warming was ∼3°C over the last century and >2°C of this occurred in the last 30 yr.

Using 20 yr of data from one urban station and three rural airport stations, Morris et al. (2001) determined that Melbourne's nocturnal urban heat island was 1.13°C.

One rural and one urban station were used by Kim and Baik (2002) to determine that Seoul warmed 0.56°C relative to its rural neighbor during the 24-yr period 1973–96.

Kalnay and Cai (2003) compared data from 775 urban contiguous U.S. (CONUS) stations with 167 rural stations and found that the urban warmed 0.18°C more than the rural during the 1980s and 1990s.

In fairness, 2 or 3 reports cited did not conclude there was an UHI effect. Basically, however, it seems that the weight of scientific evidence, before Petersen 2003 was published, was firmly on the side of there being a substantial UHI. It was the result of numerous studies over lengthy time scales from sites all over the world. Not good news for global climate alarmism. No wonder Petersen has been cited so often since his report emerged!

4. Petersen's dismissal of the earlier studies relied on adjustments to iron out other factors which he claims contaminated the measurements, thereby distorting the temperatures as recorded, and in effect artificially introducing an otherwise non-existent UHI:

"Specifically, careful attention will be paid to adjusting the data to account for the natural effects due to differences in elevation and latitude as well as the artificial effects due to differences in time of observations, differences in instrumentation, and the effects of non-standard siting practices, namely, rooftop installations. Once the data are adjusted for these factors, it will be possible to accurately assess the impact of urbanization on the climate record." Fair enough, so far as it goes, but...

5. ...his data, whilst selected carefully to avoid missing data etc, was very limited:

"Quality-controlled mean monthly temperature data for U.S. in situ stations were obtained from the National Oceanic and Atmospheric Administration/National Environmental Satellite, Data, and Information Service/National Climatic Data Center (NOAA/NESDIS/NCDC) archives. The analysis period selected was the same one used by Gallo and Owen (1999), the three years 1989–91. Ending the period in December 1991 allowed the analysis to avoid the confounding influence of the Automated Surface Observing System (ASOS) deployment, which started in 1992. Three years is long enough to produce robust means. A longer period would increase the problem of missing data."

In other words, 3 years of data from the contiguous US only, provided by sources that not everyone would regard as unbiased or disinterested, was used to overturn decades of research from all over the world, from a variety of data providers. Hmm....

6. Also, the way in which rural, urban and suburban are defined might surprise a non-American:

"Satellite night-light data are the latest tool used for determining which stations are rural and which are urban. For example, while Hansen et al. (1999) use map-derived rural/urban metadata in their global temperature analyses, Hansen et al. (2001) moved up to satellite-derived night-lights rural/urban metadata. The rural/urban classification metadata used in the analysis presented here was developed by Owen et al. (1998) using night-light data from the Defense Meteorological Satellite Program-Operational Linescan System. Their methodology divided 1-km2 grid boxes throughout the United States into urban, suburban, and rural classifications. Figure 1 shows how their metadata compare with other approaches. Advantages of the Owen et al. metadata include that they are objective (while map based is often subjective) and that night-lights, in the United States at least, are good indicators of urbanization whether residential or industrial. Owen's et al.'s urban grid boxes had an 84.4% agreement with data from the U.S. Bureau of the Census (1997)."

The method used had an 84.4% agreement with data from the US Bureau of the Census - or to put it another way, disagreement of 15.6%. Am I alone in thinking that's quite a high level of disagreement?

Furthermore: "The stations consisted of 40 clusters of stations well distributed around the country with a total of 289 stations (see Fig. 2). The Owen et al. (1998) methodology classified 85 of these stations as rural, 191 as urban, and 13 as suburban." Citing himself, Petersen defines rural sites as including many that I would regard as urban or suburban (I live in a town of maybe 6,000 inhabitants, and regard myself as urban - I used to live in a hamlet with a scattering of houses - THAT was rural):

"For example, the Global Historical Climatology Network (GHCN; Peterson and Vose 1997) consists of over 7500 temperature stations around the world that were identified as rural, urban, or an in-between class of small town using information on operational navigation charts and a variety of different atlases. A rural station was any station not associated with a town of over 10 000 population."

Personally, I think this contaminates his ultimate findings without more ado. It's not scientific, but living in the country (some 3 miles from where I now live) I have little doubt that it was distinctly colder than where I live now, in a small town. I can't prove it, but it feels as though it's the case.

7. The basic conclusion is that once adjustments are made for inhomogeneities (clumsy word - sorry), the UHI effectively disappears from the record. In fact, in some respects Petersen goes even further:

"Some of the largest cities in the United States were not represented in the 40 clusters. Could the large cities be showing urban warming while the smaller ones do not? To answer that question, the mean urban minus rural temperature difference was calculated for each cluster. An assessment of five of the largest cities—Boston, Massachusetts; Dallas, Texas; Detroit, Michigan; Salt Lake City, Utah; and Seattle, Washington—found that one (Detroit) did not have adequate rural and urban data to be analyzed while all of the rest had homogeneity-adjusted urban temperatures that were cooler than the homogeneity-adjusted temperatures of their rural neighbors."

Here he is actually saying that by the time the adjustments have been made, the countryside is proved to be warmer than cities! Does such a conclusion not ring alarm bells, and undermine the reasoning behind the adjustments? Am I alone in finding this to be a truly astonishing conclusion, apparently accepted by the "consensus" without demur?

I could go on, but I'm tired, and you're probably bored. Sorry it's been such a long post.

I use Met Office weather forecasts as a guide on this. On the TV, Rdio and Internet channels I frequent the usual thing in anti-cyclonic weather is that town will be a few degrees warmer in summer, and rural areas will be a few degrees colder in winter. The glens of Scotland will be even colder.

From this I deduce that the Met Office tacitly accepts a UHI value of between 2 and 5'C, and the further you get away from large conurbations the greater the difference.

2 to 5 degrees C is the difference I see every winter morning travelling 8 miles between my village home and urban workplace. The colder it is, the bigger the differential.

ISTR McIntyre did his usual stuff on a UHI paper, possibly this one. There is no doubt in my mind that UHI exists. The question is how much does it affect thermometer records and how much of the effect gets through to climate temp series. To me the correct thing to do would be to use pristine sites even if you have to chuck away a lot of data.

What's a homogeneity adjustment?

I knew pre-teens in the mid 1970s that London was always warmer. After ClimateGate I started to appreciate the lengths that Climate Science has gone to, to pretend UHI is not the cause of higher temperatures being recorded.

Googling "Climate Audit" "Jones 1990" and following the links, will lead to hours of reading, and possibly some familiar names! If you find any of Jones' missing data, do let him know.

Meanwhile Heathrow Airport claims another record, for being the worlds record breaking UHI.

Under Wx Observations, select "Station Markers". Zoom in.

http://eldoradocountyweather.com/current/misc/google-maps-radar/us-mosiac-radar.html

Forgot to mention, untick "radar mosaic". Temperature readings are unofficial, but according to present day climatology,1000 incorrect readings average out to wrong but accurate.

Thank you all for your comments, and thank you, lapogus, for the links.

Rhoda, good question. The following quote from Petersen's abstract should answer it, I hope:

"All analyses of the impact of urban heat islands (UHIs) on in situ temperature observations suffer from inhomogeneities or biases in the data. These inhomogeneities make urban heat island analyses difficult and can lead to erroneous conclusions. To remove the biases caused by differences in elevation, latitude, time of observation, instrumentation, and nonstandard siting, a variety of adjustments were applied to the data. The resultant data were the most thoroughly homogenized and the homogeneity adjustments were the most rigorously evaluated and thoroughly documented of any large-scale UHI analysis to date. "

To remove the biases caused by differences in elevation, latitude, time of observation, instrumentation, and nonstandard siting, a variety of adjustments were applied to the data.

Would exactly the same adjustments be applied to neighbouring rural sites? Or is UHI measured entirely from models? The logical thing to do would be to compare readings from a variety of sites urban, semi-rural, rural and remote-rural to give a value. I assume this is how the Met Office gets its several/a few degrees/much colder in Scottish Glens used in its forecasts.

From what little I've read it would appear that the larger the urban area the greater the UHI effect. For long temperature series there will an increase in UHI as the city grows. There are not many major cities worldwide which have had a reducing population in the 20th century. I find it hard to believe that this doesn't have an effect on measurements.

SandyS

My thoughts exactly!

Is there an Urban Heat Inhomogeneitity effect? There must have been a big semi rural one over the Climate Research Unit when Jones was tucking into his Chinese data and fortune cookies.

RR. I sure I don't have to remind you of one of the most egregious cases of adjustment:

https://wattsupwiththat.com/2009/12/08/the-smoking-gun-at-darwin-zero/

If they can do this with a relatively small community (with minimal UHI?) just imagine what they can do when "adjusting" in an ever increasingly urbanized setting.

In fairness, RR, I do see the case for adjustments. If a reading (or series of readings) taken in a town 1,000' lower than an adjacent rural reading (or series of readings) are simply compared without adjustment to reflect the known effect that altitude has on temperature, then you're comparing apples and pairs. Ditto with similar readings taken, for example, one in the early morning, one in the mid-afternoon. Again, without adjusting to reflect the fact that the temperature is extremely likely to be warmer in mid-afternoon than in early morning, you're not comparing apples with apples. They're not like-for-like comparisons. The theory behind the adjustments is a reasonable one - it's how they pan out in practice that's the problem.

I perhaps should have given more credit to Petersen for the fact that his critique of the 18 or so historical comparisons between rural and non-rural locations is based very much on the fact that the earlier studies do not make it clear that appropriate adjustments were made, and that therefore the comparisons are not like for like. I think his work is admirable in its clarity and in its exposition of the way in which he arrived at the conclusions he did. The problem is that the adjustments are patently wrong, whatever the good theory behind the need to make them. It's the problem, yet again, of putting all your trust in models. A conclusion that big towns are cooler than the countryside - the adjustments and the models "prove" it - is so obviously bonkers that I am appalled (but sadly not surprised) that such a nonsense has apparently been cited 285 times, presumably with approval. Why has nobody from the bloated climate science industry spent more time and applied more expertise than that of which I am capable to challenge this? - there's plenty to challenge, that's for sure. Oh, that's right, you're not likely to get a grant for helping to prove that the UHI is real and significant, and that the annual temperature hikes that appear in the adjusted figures are not so real or so extreme as they would have us believe.

Ratty

I do take your point generally, and it's the same point EM made on the Zombie thread when we first started discussing this. His point (I may paraphrase slightly) was that UHI is irrelevant so long as urban temperature readings are going up and broadly speaking rural temperatures are going up by more or less the same amount. If that's the case, what does it matter?

But I think (and I think Petersen thinks, though we disagree on the outcome) that this is wrong, because if there IS an UHI effect, then it WILL distort the temperature record. For example, if a reading is taken at the same location, every day for 100 years, and that location was rural 100 years ago, but is now urban, if there is an UHI effect, its higher later readings cannot be trusted, because they might not reflect a true increase in temperature, but one that has resulted instead from the changing nature of the location.

Of course there are many such locations in both the UK and in the USA, which is why it is important to establish whether or not the UHI is real (given what a disproportionately large part UK and USA temperature records play in the whole thing). If it is, then rising temperatures at once-rural but now-urban sites can't be relied on as any sort of meaningful temperature record. But if the UHI isn't real, then higher temperatures recorded at those locations more recently than earlier, lower, temperatures, must mean that the temperature is rising.

Which is why Petersen's efforts at proving that the UHI is not real have met with such acclaim from the alarmists - they can claim that the rising temperatures at all those once-rural but now urban locations are real, not suspect. It's rubbish, of course.

Personally, I accept that temperatures are probably rising, as one would expect on leaving behind the horrors of the Little Ice Age. On the other hand, I doubt that they are rising as fast as some would have us believe; I believe that the UHI effect is real and shouldn't be ignored in such a cavalier fashion; I doubt that the warming we ARE enjoying is anything other than broadly beneficial; I accept that humankind may be playing a small part in it, but I suspect it's substantially natural, as it always has been in the past; and I abhor the expensive and damaging policy prescriptions which have resulted from all the ballyhoo. Petersen's paper, it seems likely, has helped to contribute to the current mess, which is why I dislike it so much.

Personally, I accept that temperatures are probably rising, as one would expect on leaving behind the horrors of the Little Ice Age. On the other hand, I doubt that they are rising as fast as some would have us believe; I believe that the UHI effect is real and shouldn't be ignored in such a cavalier fashion; I doubt that the warming we ARE enjoying is anything other than broadly beneficial; I accept that humankind may be playing a small part in it, but I suspect it's substantially natural, as it always has been in the past; and I abhor the expensive and damaging policy prescriptions which have resulted from all the ballyhoo.

You could have taken those words from my mouth on any conversation on this topic.

Mark Hodgson
Do you know if any of these papers take the weather conditions into account when calculating UHI? To my mind wind would have a major effect and night time cloud cover and day time fog also. When adjusting for UHI a smaller fudge factor is required on windy days than for still sunny ones. Doing the calculation taking weather conditions into account may give a different result that just averaging over a year.

SandyS

Thanks for your thoughts. All I can say for certain regarding Petersen's paper itself, is to take his own words from the abstract to his paper:

"To remove the biases caused by differences in elevation, latitude, time of observation, instrumentation, and nonstandard siting, a variety of adjustments were applied to the data." From that it's far from clear that he allowed for the factors you mention - which I agree would be relevant to the process.

Another (albeit understandable, given the size of the task) problem with Petersen's paper is that, as he says himself:

"As urban heat island literature is an extensive body of work, to limit the review, it starts in 1980." In fairness that means he analysed 21 (I think) papers over a period of 23 years, which is quite extensive (though certainly not comprehensive).

As for the papers he did cover, I haven't read any of them myself - no links were provided from the web version of his paper, and I have other things to do than spend dozens of hours hunting them down and many dozens more reading and analysing them. I'm not a scientist, and I'm not being paid to look at this stuff (unlike Petersen). Perhaps I should be more motivated and find the papers he didn't cover, and analyse them all - those he covered and those he didn't. But in reality, I'm just an interested bystander who likes hanging out with the others at Bishop Hill and pointing out things I see that are wrong in the world of climate science.

I'm slightly surprised that Phil Clarke hasn't been on here to comment on this thread, but maybe the points I've made have been a little close to the knuckle.

Let us address the question posed by this thread - does UHI exist? The answer can unequivocally be answered affirmatively.

In rural areas with a vegetation cover, much of the sun's energy is used up by evapotranspiration which has a cooling effect. In areas without vegetation most of the sun's energy strikes the surface and heats it. Deserts thus suffer higher temperatures. Urban surfaces resemble deserts with much less vegetation and so will heat up more than surrounding rural areas.

I was struck while watching Joanna Lumley's travelogue on Japan last night when she flew over Tokyo just how little greenery there was compared with London with all its parks and gardens. I would expect Tokyo to have a greater UHI effect than London.

It has just occurred to me that desert towns may exhibit a reverse UHI effect, since they may have more vegetation and exposed water surfaces than the surrounding desert. It would explain why gardens and water features are present in former arab towns and cities in southern Spain. They constitute cooler areas, not just because they are shaded.