Review and Critical Analysis of International UHI Studies

2.1.1 Population Size/Density
Population is a good indicator to express the character and features of a city [116]. The United
Nation’s (UN) World Urbanization Prospects: 2007 Revision Population Database measured the
urban to rural population spread to be 30% in 1950, 49% in 2005 and predicts a rise to 55.6% in
2010 [151]. With more than half of the world’s population living in cities, the impact on the
environment will be significant.
The earth’s surface has changed significantly due to urbanization and industrialization which in
turn has created urban climates, Yamashita [116], due to the increase in population; vegetative
surfaces have been replaced with manmade impervious surfaces in the form of high density
residential and commercial areas. The modifications to the earth’s surface caused by an increase
in population indirectly affect the radiation, heat and water balance of an area. Population
density, has a direct effect on heat generation in that the more people there are, the more energy
they consume and that consumption leads to heat emittance [007].
A number of studies have considered the relationship between population and heat island
intensity and concluded that a direct correlation can be identified [114]. The studies look at a
route that covers 10 settlements of varying sizes, measured by population, and temperature
recordings were made along the route. Oke’s, study [114] concluded that there was a correlation
between the intensity of an urban heat island and the size of the city.
A study by Brian Stone [119] said that residential areas with low density housing with higher
spatial distribution contribute more to the production of an urban heat island effect.
Sailor [124], in his study of anthropogenic heat flux in six US cities found that results were
substantially lower for those cities with lower population density. Atlanta and Salt Lake City had
maxima winter values of less than 15 Wm ‐2 compared to Chicago, San Francisco and
Philadelphia’s maxima values of between 65‐75 Wm ‐2 .
Limitations; Oke’s model assumes similarities between settlements and does not take into
account geographical variations in latitude and unique settlements like airports that can have
their own unique UHI.
Intensive urbanization on small areas of land limits the progression of further urbanization
because there simply will not be any land left to build on, so the results, if population continued
to be monitored would become less significant [114]. Another factor influencing further
comparisons is that a convergent thermal breeze circulation is created in large UHIs which
creates a cooling effect.
Another theory is that because there are so many other variables involved in the increased heat
content of an area, population alone should not increase the heat contents of an area [007].
2.1.2 Land/Building Material Thermal & Radiative Properties
Another effect of urbanization is that land surface cover changes as buildings are constructed and
replace vegetation. This affects the surface energy balance of the land. Common building
materials affect UHI in the following manners, through their solar reflectance, thermal emissivity
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