How They Distort Global Temperatures: The Urban Heat Island Effect

Posted: 03/01/2010 by Lynn Dartez in CFP

Now you see why the CRU and IPCC limited the number of stations they were using and restricted them to mostly urban stations to get the result they wanted

By Dr. Tim Ball  Monday, March 1, 2010

imageHow much do calculations of global temperatures represent the real temperature of the Earth? Every day new stories appear about temperature records with errors or deliberate omissions. An important part of the debate is something called the Urban Heat Island Effect (UHIE). A new article by Dr. Edward Long says, “The problem would seem to be the methodologies engendered in treatment for a mix of urban and rural locations; that the ‘adjustment’ protocol appears to accent to a warming effect rather than eliminate it.  This, if correct, leaves serious doubt for whether the rate of increase in temperature found from the adjusted data is due to natural warming trends or warming because of another reason, such as erroneous consideration of the effects of urban warming.”

In another paper we learn that, “The GISS adjustments to the USHCN data at Dale Enterprise follow a well-recognized pattern. GISS pulls the early part of the record down and mimics the most recent USHCN records, thus imposing an artificial warming bias.”

What are they talking about?

History Of The UHIE Problem.

German scientist A. Kratzer, working on the impact of pollution on trees in the Ruhr Valley in the 1930s, discovered urban temperatures were higher than the countryside. War interrupted the work, but shortly after T. J. Chandler studied the temperature of London, England. With a thermometer on his car he recorded temperatures along specific routes. When plotted they showed a distinctive concentric pattern with higher temperatures in the centre. In 1952 Chandler published “The Climate of London” and B.W.Atkinson later showed precipitation patterns were also affected.

Several cities were studied since and though each showed the concentric temperature patterns. The form is a distinctive dome of warm air with a centre height of about 1,000 feet over the hottest part of the city (Figure 1). A convection pattern inside the dome circulates the air as shown.

Figure 1: Schematic of the Urban Heat Island Effect

Temperature contrast between city and countryside is most extreme in cold climates and the dome is sometimes visible in northern cities on cold calm winter mornings. During the day, the dome rises like a hot air balloon and dissipates only to form the next night. With strong winds it is pushed downwind away from the city. Figure 2 is a map of England for August 10, 2003 with a distinctive UHIE over London but displaced to the north by a south wind. London was 36°C but Hastings just south of the city was 24°C.

Figure 2: Temperature lines (isotherms) for England 10 August 2003.
Source: UKMO

Alarmists linked it with the death rate and claimed both were examples of the impact of global warming. Actually, it was a UHIE and death rates were not higher than normal.

Physical Cause

Main reasons for the temperature differences are colour and structure of surface materials, which determine their ability to absorb and release heat. Concrete, asphalt, bricks and wood absorb heat quickly during the day and cool quickly at night while water grass and trees do the opposite. It‘s a major argument for the preservation of old parks and the creation of new ones in urban areas. Trees are more important in the city than outside.

Greatest surface changes are in the centre of the city, an area called the Central Business District (CBD), which is almost 100% solid surface and impervious to water. Urban drainage systems carry water away quickly while in the country it remains and evaporates slowly or is transpired by plants creating cooling.  When water evaporates it takes heat energy from the surrounding air or from the surface, which creates cooling just like when you sweat. Impervious surfaces exist in the suburbs but only 50 percent is covered. Roofs of houses, garages, driveways, roads and sidewalks all absorb heat in the day and radiate it back to the atmosphere at night. Storm sewers remove water very quickly.

Chandler type traverses along major routes provide longitudinal profiles of the temperature variation across the city. For example at 12:30 am on September 15, 1973, Oke and Hay, of the University of British Columbia, did a 26 km traverse across Vancouver, Canada. Results showed many interesting changes.

  • Temperatures ranged from about 7°C in the countryside to 15°C in the heart of the city (CBD).
  • The most dramatic drop from 14°C to 8°C occurs in about 3 km from the suburbs to the countryside.
  • Green spaces, even small ones, were distinctly cooler. Stanley Park with its tall trees was the coolest.
  • Highest temperatures were in the centre despite proximity to the harbour.
  • The parks, although cooler, were still warmer than the countryside.

Most weather stations are at airports and cities have expanded and enclosed the airport so the UHIE increased. When you separate urban weather stations from rural ones anywhere in the world a different trend of temperatures appears. Urban stations show distinctive warming while there is very little change in rural stations.

City size determines the amount of extra heat generated by industry, automobile and people. Studies at airports show jet aircraft and heat from darker surface runways cause different readings. Unless a station remains in an unchanged environment its readings will change over time. Adjusting for these is the challenge.

Anthony Watts recorded conditions and the extent of the changes at sites in the US.  Figure 3 shows the increasing temperature with additional artificial heat sources.

Figure 3: Maryville Weather Station

How agencies like the Goddard Institute for Space Studies (GISS) or the Climatic Research Unit (CRU) adjust for the UHIE is the crux of the problem.


Original raw data is recorded to one half a degree so reducing it tenths through statistics doesn’t mean much. Phil Jones claimed a 0.6°C increase since the end of the 19th century and said it is unnatural and clear evidence of warming due to human CO2 in the atmosphere. Jones still refuses to disclose which stations he used and how they were adjusted, especially for the UHIE.

The automobile was a major factor causing urban expansion through suburban development. Warwick Hughes has studied temperature data for years and was the person who received the following email from Jones when he asked for his data in February 2005, “We have 25 or so years invested in the work. Why should I make the data available to you, when your aim is to try and find something wrong with it.” He illustrated the UHIE with two classic plots.

Figure 4 shows the temperature curve for six Australian cities.

Figure 4: Temperature graphs for Six Australian cities
Source: Warwick Hughes, (1991)

Figure 5 shows the temperature curve for 27 rural sites in Australia for the same time period.

Figure 5: Temperature graphs for 27 rural Australian stations.
Source: Warwick Hughes (1991).

These graphs were widely distributed and buttressed arguments about what was wrong with the claim CO2 is causing global warming. Despite knowledge about the problem this was one of the first clear examples in the public arena. Hughes’ study ends in 1991 but Long brings it up to date and the urban/rural difference continues. (Figure 6)

Figure 6: Temperature differences rural/ urban

Figure 7: Comparison of adjusted rural/urban temperatures.

Now you see why the CRU and IPCC limited the number of stations they were using and restricted them to mostly urban stations to get the result they wanted. You also understand why Tom Wigley told Jones in a leaked email of November 6, 2009 that, “We probably need to say more about this (Difference between land and ocean temperatures). Land warming since 1980 has been twice the ocean warming and skeptics might claim that this proves that urban warming is real and important.” Exactly Tom!


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