Karl et al 1993, "A New Perspective on Recent Global Warming: Asymmetric Trends of Daily Maximum and Minimum Temperature", Thomas R. Karl,  Philip D. Jones,  Richard W. Knight,  George Kukla,  Neil Plummer,  Vyacheslav Razuvayev,  Kevin P. Gallo,  Janette Lindseay,  Robert J. Charlson, and Thomas C. Peterson.
Free ownload as pdf at BAMS

Karl et al 1993 has a Fig 7 which has caught my eye for years because it is strong evidence against their basic premise that the global closure of  daily temperature range (DTR = max minus min, their term in Range RNG) is due to factors such as changing cloud cover etc. and not increasing urbanisation.   Karl et al make a big point that the PRC An range bars,  just under where I have inserted Karl Err, are not perfect evidence that a decreasing Range RNG  is related to degrees of urbanisation.  Completely skimming over the fact  that ALL the PRC stations are very urban, NONE are even faintly rural. and there are likely homogeniety problems in PRC data.  I think it is quite possible too that  PRC air quality that could be a factor in the "negative UHI" in Su (Summer).   This obviously flows through and impacts on the RNG number (= Max minus Min) .  Anyone who has analysed datasets from mixed population groups for DTR knows how sensitive the DTR number is to confounding factors.
Karl et al then in their conclusions ignore the copybook evidence of UHI contamination  shouted at them by the vast majority of the results portrayed in Fig 7, where I have added PE for Perfect Evidence.
This reminds me of Lord Nelson at the naval Battle of Copenhagen in 1801 who on being told that there was a flag signal ordering him to disengage, put his telescope to his blind eye, declared he could not see any signal and famously carried on the action.
 I wonder at the justification for  using widely  different population thresholds in Fig 7.
Karl et al High, Medium and Low population groups  equate to.
plus 1 million, between 1 million and 160,000 and under 160,000 for the PRC and,
plus 500,00, between 500,000 and 50,000 and under 50,000 for Japan.

Reviewers should have insisted that Karl et al extend their Fig 7 analysis by graded population groups to ALL their various global regions, which would have been a more scientific approach than just presenting data from PRC and Japan.
I know of results from two of the Karl et al regions which would have supported the case that increasing UHI effect increases the closure of DTR.

[1]   The 1996 paper by Warwick S. Hughes and Robert C. Balling, Jr. "Urban Influences on South African Temperature Trends." International Journal of Climatology, Vol. 16, No. 8, pp. 935-940. Online at http://www.john-daly.com/s-africa.htm
demonstrates clearly in Fig's 3 and 4  that DTR is closing at a greater rate in Large urban stations than in small towns and the Karl stations.

[2]    The mid 1990's Previously unpublished  paper by Hughes and Balling , "Eastern Australia temperature variations 1930-1992"
has  their page 19 Fig 4  which demonstrates with crystal clarity how the UHI effect increases Mean T and closes the DTR  with increasing population. The Plummer series of 49 stations in Fig 4 is very near to the East Australian series of 44 stations in Karl et al.

Note how in the Abstract below Karl et al can not bring themselves to mention the UHI effect.

Karl et al ABSTRACT

Monthly mean maximum and minimum temperatures for over 50% (10%) of the Northern (Southern) Hemisphere landmass, accounting for 37% of the global landmass, indicate that the rise of the minimum temperature has occurred at a rate three times that of the maximum temperature during the period 1951–90 (0.84°C versus 0.28°C). The decrease of the diurnal temperature range is approximately equal to the increase of mean temperature. The asymmetry is detectable in all seasons and in most of the regions studied.

The decrease in the daily temperature range is partially related to increases in cloud cover. Furthermore, a large number of atmospheric and surface boundary conditions are shown to differentially affect the maximum and minimum temperature. Linkages of the observed changes in the diurnal temperature range to large-scale climate forcings, such as anthropogenic increases in sulfate aerosols, greenhouse gases, or biomass burning (smoke), remain tentative. Nonetheless, the observed decrease of the diurnal temperature range is clearly important, both scientifically and practically.

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