urban heat island

Urban heat island effect on temperatures, a tale of two cities

The urban heat island effect (UHI) is the phenomenon that cities are usually much warmer than surrounding rural areas, especially at night. The main cause of UHI is that in cities, the land development, concrete and asphalt, absorbs heat during the day and gives it up during the night. Often, proponents of catastrophic anthropogenic global warming (CAGW) have claimed that the rising night-time temperature is proof of an enhanced greenhouse effect due to carbon dioxide emissions.

In this post, we will compare the temperature history of the Phoenix metropolitan area with that of Maricopa, AZ, a small rural town about 25 miles south of Phoenix (see map). The data come from the National Weather Service (NWS) in a report dealing with UHI and precipitation.

Map-maricopa-az-300x253NWS compares temperatures of Phoenix and Maricopa for the period 1961 to 2007. During that time the population of Phoenix grew from about 726,000 to over 4 million. Maricopa grew from less that 1,400 to 40,000.

NWS says “The mechanisms which lead to the creation of an urban heat island (UHI) have been studied for nearly a century and are well understood….The effects of the UHI are most pronounced during the summer (June-July-August) months.”

NWS first compares the average daily high temperatures in summer for Maricopa and Phoenix. They note that there is little difference in the two data sets.


NWS then compares the average summer low temperatures and we see a large divergence with the Phoenix temperatures much warmer than rural Maricopa.


NWS attributes this temperature difference to UHI. The steady rise in both data sets could reflect the increasing UHI in both places. If the difference were due to an enhanced global greenhouse effect, why is there a divergence, especially when high temperatures are similar?

 NWS notes that “no upward trend was found in the maximum temperatures, and in fact, the urban temperatures were consistently 0 to 4ºF cooler than the rural station. The hypothesized reason for this is the “oasis effect”, where solar energy is expended on evaporating water from an unnaturally moist surface in the urban areas due to irrigation activities.”

Many temperature stations are located in urban areas with the consequence that they report artificially higher temperatures. These high temperatures are averaged (or homogenized) into regional data sets that then get reported as more “global warming” than actually occurs. And that is one reason why 50% of warming claimed by IPCC is fake.

See also:

Warmer nights no proof of global warming

Most US maximum temperature records set in the 1930s

Global warming is a funny thing. Whenever we experience a heat wave the press proclaims it is the face of global warming, but when we have an unusual cold snap, it is merely natural variation.

The Arizona Daily Star had an interesting article today concerning the number of days in Tucson with maximum temperatures over 100° F. The story says the record was set in 1994 with 99 days over 100° F and speculates upon our chances of breaking that record this year. The “normal” number of days over 100° F is 62 according to the National Weather Service.

Also interesting is the statistic that July 4, 2012, had the coolest maximum temperature on record, 86°F.

Looking at a larger picture and a slightly different statistic, we see that the greatest number of maximum temperature records in the U.S. were set in the 1930s. Below is a graph compiled by Steven Goddard from U.S. Historical Climatology Network (USHCN) records:


Getting back to Tucson, we see from the graphics in the Star article that we have had more years, with more days over 100° F than “normal” within the last 25 years or so. That could be a reflection of the urban heat island effect. Our asphalt and concrete absorbs more heat and reflects it back at night. I discuss that in my post Warmer nights no proof of global warming.

A graphic from that post demonstrates the problem by comparing the temperature trend in urban Tucson with that from rural Tombstone:

Tucson-Tombstone temp

You can see that Tucson temperatures have been rising but there is no trend in the rural Tombstone station. Carbon dioxide works in mysterious ways.

So, the big news from the Arizona Daily Star is: Summers in Tucson are hot.

See also:

Human induced warming in Tucson

NOAA temperature record “adjustments” could account for almost all “warming” since 1973

Dr. Roy Spencer, Principal Research Scientist at the University of Alabama in Huntsville, analyzed the records of weather stations in the U.S. Historical Climatology Network (USHCN) which NOAA touts as the official U.S. climate record. He found that almost all of the reported warming since 1973 can be accounted for merely by the adjustments to the record made by NOAA.

Spencer also analyzed the British CRUTem3 and International Surface Hourly (ISH) data. His main conclusions are:

1) The linear warming trend during 1973-2012 is greatest in USHCN (+0.245 C/decade), followed by CRUTem3 (+0.198 C/decade), then my ISH population density adjusted temperatures as a distant third (+0.013 C/decade)

2) Virtually all of the USHCN warming since 1973 appears to be the result of adjustments NOAA has made to the data, mainly in the 1995-97 time frame.

3) While there seems to be some residual Urban Heat Island (UHI) effect in the U.S. Midwest, and even some spurious cooling with population density in the Southwest, for all of the 1,200 USHCN stations together there is little correlation between station temperature trends and population density.

4) Despite homogeneity adjustments in the USHCN record to increase agreement between neighboring stations, USHCN trends are actually noisier than what I get using 4x per day ISH temperatures and a simple UHI correction.

When “global warming” only shows up after the data are adjusted, one can understand why so many people are suspicious of the adjustments.

See his post on the matter here.

Spencer has also examined the surface temperature records according to the population density near the stations. He finds “clear evidence of an urban heat island effect on temperature trends in the U.S. surface station network” which produce a warming bias to station records.

See his posts here and here.

Among his findings:

1) Essentially all of the +0.20 deg. C/decade average warming trend over the U.S. in the last 40 years computed from the CRUTem3 dataset (which the IPCC relies upon for its official global warming pronouncements) evaporates after population adjustment (no claim is made for countries other than the U.S.)

2) Even without any adjustments, the ISH data have a 20% lower warming trend than the CRUTem3 data, a curious result since the CRUTem3 dataset is supposedly adjusted for urban heat island effects.

Comment: These analyses may explain, in part, why the official surface temperature record does not agree with the record from satellites. It also implies that recent warming is not nearly as dire as some would have us believe, nor does the real temperature reflect any major influence by carbon dioxide emissions.

NOAA is not the only agency “adjusting” the temperature record. NASA’s GISS has been doing that for U.S. and global temperatures also (for instance see here.)

All the adjustments tend to make older temperatures, for example, those in the 1930s, colder while making all more recent temperatures warmer. This is statistically improbable if the adjustments were to be justified by some physical conditions. This whole process is troubling because it suggests corruption within the government-funded scientific establishment whose results apparently have to be politically correct rather than scientifically correct.


See also:

A Perspective on Climate Change a tutorial

NOAA accused of fabricating temperature data

The State of our Surface Temperature Records



Warmer nights no proof of global warming

The diurnal temperature range (DTR) is the difference between the daily low temperature at night and the daily high temperature during each day. In some places this range is decreasing because the nights are getting warmer. Proponents of carbon dioxide induced warming (AGW) tout this as proof that carbon dioxide is indeed warming the planet.

In the following graph, compiled from the weather station at Phoenix Sky Harbor airport, we indeed see that the nightly minimums (red) are rising while the daily highs (blue) are fairly steady. This could imply that carbon dioxide is trapping heat at night and delaying the nightly cooling in absence of sunlight.


Apparently, however, carbon dioxide works only in the cities. The same kind of data collected near the airport in rural Childs, Arizona (near Ajo), show no such rising nighttime minimum temperatures.


Rising levels of atmospheric carbon dioxide fail to explain the different DTR responses between rural and urban stations. However, the urban heat island effect does provide an explanation. During the day, concrete and asphalt in cities absorb energy from the sun and radiate it back during the night, thereby making the nights warmer.

Many researchers have studied the urban heat island effect. For instance, Gallo et al., from the National Climatic Data Center found: “Those stations that were associated with predominantly rural land use land cover usually displayed the greatest observed DTR, whereas those associated with urban related land use or land cover displayed the least observed DTR.” That means the night temperatures in urban areas were warmer than those in rural areas.

The warming effect of urbanization is shown by many examples. For instance, the next graph compares the stations in urban Tucson versus rural Tombstone:

Tucson-Tombstone temp

You can see that Tucson temperatures have been rising but there is no trend in the rural Tombstone station. Carbon dioxide works in mysterious ways. This difference between urban and rural stations is not a local phenomenon, but a world-wide one. For instance, the next graph compares the temperature records of urban and rural stations in South Africa:


 This dichotomy of temperatures shows that land-based temperature data have an artificial warming bias since most stations are in or near urban areas. Of course, the keepers of the data say they can mathematically “correct” the data to account for the urban heat island effect. But too often, it seems that they are “correcting” away inconvenient records. For instance, read how NASA is “correcting” away the warming of the 1930s here.

Once again, the hypothesis of  carbon dioxide caused warming fails in the light of actual data.

Reference cited:

Gallo, Kevin P.; Easterling, David R.; and Peterson, Thomas C., “The Influence of Land Use/Land Cover on Climatological Values of the Diurnal Temperature Range” (1996). Papers in Natural Resources. Paper 191, Journal of Climate 9.

Human-Induced Warming in Tucson?

090605 Wry Heat
We’ve all seen headlines that say, in effect, it was the warmest year since….. it was cooler. Is it really getting warmer or is at least some of the temperature rise artificially induced because of the places we put our thermometers?
In the U.S. there are 1,221 stations in the U.S. Historical Climate Network (USHCN); these are the official stations measuring temperature.
One of those stations occurs on the University of Arizona campus. Its temperature record certainly shows warming, see figure below.