Andrew B. Collier, Ph.D. Physics
Hot in the city, hot in the city tonight, tonight
Hot in the city, hot in the city tonight, tonight
Billy Idol, Hot in the City (1982)
Is there a reason to believe that cities are warmer than the suburbs and nearby countryside? Indeed, there is! The concept of an Urban Heat Island (UHI) has been around since the beginning of the 19th century when Luke Howard made temperature measurements in and around the city of London . A UHI is an area centered on a metropolis, where the temperature is appreciably higher than the surrounding rural areas. There are two major contributors to this effect:
- differences in land surface materials (those in a city tend to store more heat), and
- waste heat and pollution associated with high population density, traffic and industry.
This temperature difference is accentuated in conditions of low wind and at night, when urban areas can be up to 12 degrees Celsius warmer than the countryside .
The materials used to build our concrete jungles have thermal properties quite different from those found in rural areas. Concrete and asphalt have large heat capacities, so they absorb and store the Sun’s energy efficiently during the day. This energy is then released at night, maintaining elevated temperatures. The shape of a city’s buildings is also a factor, with tall buildings forming “urban canyons,” which effectively increase the surface area for heat absorption and also inhibit the cooling effect of wind.
These are not just theoretical ideas. UHIs have been found over numerous cities (for example, Washington, D.C. , Oklahoma City , Atlanta  and Houston [6,7]), and their influence on the local meteorology has been studied. Specifically, it has been found that they can have a pronounced effect on the distribution of precipitation and thunderstorms.
There are, however, UHI skeptics. Thomas Peterson, using rigorously prepared temperature data from 289 urban and rural stations, concluded that “Contrary to generally accepted wisdom, no statistically significant impact of urbanization could be found in annual temperatures” . Furthermore, David E. Parker showed that the temperature trends in urban areas do not differ significantly from the global trend, inferring that urbanization does not play an appreciable role in climate change . Other researchers, like Thomas C. Peterson and collaborators, used data for rural areas to come to essentially the same conclusions .
As Peterson points out, an analysis of the UHI effect using in situ temperature measurements is fraught with difficulties, so we are going to look at this question from a slightly different perspective.
Modern computers allow detailed physical modelling of atmospheric phenomena and make it possible to perform experiments that would be impossible in nature. Jong-Jin Baik and his collaborators did extensive numerical experiments, simulating the effects of UHIs on convection . Their simulations showed that an updraft cell forms on the downwind side of an UHI, with an updraft intensity that increases with the temperature difference between urban and rural areas, but decreases at higher wind speeds. This updraft could lead to precipitation and the formation of thunderclouds, and hence lightning.
We are going to use data from the World Wide Lightning Location Network (WWLLN) to examine the incidence of lighting in the vicinity of UHIs.
The WWLLN (http://www.wwlln.net) is a lightning detection network consisting of around fifty Very Low Frequency (VLF) radio receivers distributed across the Earth. Each receiver records the time of arrival of the burst of VLF energy liberated by lightning. The figure below is a spectrogram generated from data recorded by a WWLLN receiver at SANAE base, Antarctica, which shows the bursts of energy produced by lightning as bright vertical lines. The times from all of the receivers are then clustered and analyzed using a sophisticated algorithm based on the Time of Group Arrival (TOGA)  to yield the location of the lightning discharges.
Conventional lightning detection networks identify almost all lightning activity over a limited area, typically operating on a regional or national basis. In contrast, the WWLLN is not particularly efficient; it only captures around 15 percent of all lightning activity. But it does so over the entire planet! As a tool for performing global lightning research, the WWLLN is difficult to beat.
Lightning and Urban Heat Islands
Dixon and Mote  showed that Atlanta has an UHI which influences local rainfall. Are the effects of Atlanta’s UHI apparent in the WWLLN data? Below on the left is a density map showing the incidence of lightning around Atlanta for the three year period from 2010 to 2012. The histogram on the right shows the density of lightning as a function of distance from the city center. There is no real evidence of enhanced lighting activity near the city in either the map or the histogram. This is a little disappointing, but Atlanta is in a region which generally experiences only a moderate level of lightning activity , so perhaps expecting a UHI effect is unwarranted.
According to the WWLLN data neither Washington, D.C. nor Oklahoma City attract more thunderstorms than the surrounding countryside, although both cities have an UHI. But, again, these cities are not located in regions with frequent thunderstorms to start with!
Houston, however, is in the middle of an area with frequent thunderstorms . Richard Orville et al. [6,7] found 45 percent more lightning over urban Houston and concluded that it was due in part to enhanced convection caused by the UHI. Our analysis of the WWLLN data for Houston and its surroundings are consistent with their findings. The map below clearly shows an increased incidence of lightning centered on the city and the histogram indicates that lightning activity systematically decreases as you get further away from Houston.
Not much is known about the existence of UHIs in Africa or their effects on lightning activity. Yet Africa is the continent with the most lightning on the planet . Evidence of a warming effect due urbanization has already been observed in South Africa . Let’s be the first to take a look at the connection between lightning and African urbanization. Below are the data for the region centered on Johannesburg, South Africa. Here the effects of the UHI are remarkably clear: there is an area of roughly 20 km radius around the city of Johannesburg in which lightning is much more prevalent than the surroundings. The accompanying histogram shows that the density of lightning in this area is elevated by around 30 percent. Evidently Johannesburg’s UHI has a very strong influence on local lightning activity.
Why should we care about UHIs? Well, they could have an important impact on the health of urban residents. A UHI will exacerbate the effects of a heat wave. Heat waves kill many people every year, especially the elderly, with a mortality rate directly linked to peak temperatures . Increased temperatures also lead to poor air quality due to the formation of ozone at low altitudes.
Evidently, we should be making some efforts to mitigate urban heating. These effects can be reduced by using more light-colored and reflective surfaces in urban areas, which would result in a larger proportion of sunlight being reflected (rather than absorbed). Simply changing the colors of roofs and using concrete rather than asphalt would have an enormous effect.
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About Andrew B. Collier
Andrew B. Collier, Ph.D. Physics
Andrew lives in Durban, South Africa, with his wife and an extensive collection of used running shoes. He has a Ph.D. in Physics from the Royal Institute of Technology, Stockholm, but is currently masquerading as a Mathematician. He is interested in data analysis, automated FOREX trading, photography, cooking and running.
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