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| Fig. 1: Illustration of temperature profile of regions with differing levels of development. (Courtesy of NASA) |
Urban areas are generally hotter than less developed surrounding areas due to the urban heat island effect. This effect has been documented in over 400 major cities across the world. [1] Experimental data has shown that the average temperature difference may surpass 4-5°C, and may exceed 10°C in some cases. [1] Fig. 1 illustrates what the temperature profile of an urban area and its surrounding areas my look like. The more densely populated an area is, the greater the urban heat island effect. Many characteristics of urban areas contribute to the urban heat island effect, such as a lack of trees, dark surfaces that absorb more sunlight, and waste heat from energy usage, such as for air conditioning.
Urban heat islands are problematic for human health, as higher daytime temperatures can increase the risk of heat stroke, heat exhaustion, and cause respiratory difficulties. The elderly and young children are the most susceptible demographic to heat related illnesses. [2] Higher temperatures also lead to poorer air quality, since it causes increased levels of ground level ozone and other pollutants. [3] Ground level ozone forms when nitrogen oxides and volatile organic compounds react in the presence of sunlight and hot weather.
Additionally, the increased electricity demand from air conditioning leads to more waste heat being emitted into the surroundings, causing a feedback loop. An analysis of existing studies found that for each degree of temperature increase, the rise in peak electricity load varied between 0.45% and 4.6%. This translates to an additional electricity use of about 22 W per degree of temperature increase, per person. [4] In cities, the average increase of the peak electricity demand in cities is around 3.7% or 215 MW per degree of temperature increase. [1]
Albedo measures the proportion of solar radiation that is reflected after striking the surface of a material, on a scale from 0 to 1. A material with a high albedo reflects more solar radiation than a material with a lower albedo. Using materials with higher albedos in cities could mitigate the urban heat island effect by reducing the amount of solar radiation that is absorbed. Globally, roofs account for about 25% of the surface of most cities, and pavement accounts for about 35%. [5] Since they make up a majority of the surface area of cities and commonly have lower albedos, there has been much interest in increasing the albedo of these surfaces.
Asphalt pavements typically have an albedo from 0.04 to 0.16 and concrete pavements typically have an albedo from 0.18 to 0.35. [6] However, the albedo of concrete can be much higher. White portland cement concrete has an albedo of 0.7 to 0.8 when new, and 0.4 to 0.6 when weathered. [5] Pavement albedo changes with time, with the albedo of concrete pavements decreasing and those of asphalt pavements increasing as they age. [7] New asphalt pavements have low albedos of around 0.1, which have been found to cause extremely high surface temperatures of 70 to 80°C in summer months. [8]
Simulations of increasing the albedo of pavement by 0.1 and 0.4 have found that the decrease in temperatures are roughly linear. An increase in pavement albedo of 0.4 in California cities showed a reduction in annual average surface air temperature from 0.18°C to 0.86°C, depending on the baseline climate, city size, and layout of the city. [9] Another study of a real world project using cool asphaltic and concrete pavements in Athens, Greece found surface temperature reductions of up to 7.5°C and 6.1°C in the summer, and a peak reduction of 11.5°C. [10]
Higher albedo roofing materials have become more prevalent as research has shown their effectiveness in reducing temperatures and lowering cooling costs. A study of one project found that increasing the albedo of a commercial building roof from 0.24 to 0.60 using a reflective coating decreased peak surface temperatures from 80°C to 49°C. [11] Additionally, the electricity usage of the building was reduced by 18%. Similar results have been found for residential buildings as well, with energy savings of up to 63% in some cases. [11] The energy effectiveness of these roof treatments is inversely correlated with how well insulated the home is.
The relative lack of vegetation in urban areas is a significant contributor to higher temperatures. Vegetation cover accounts for from 5 to 20 percent of the surface area in cities, 15 to 50 percent in suburbs, and 75 percent or more in rural areas. [12] Areas covered by vegetation are shaded, reducing the amount of sunlight that heat absorbing materials absorb. Temperatures inside parks in urban areas can be 1 to 3°C cooler than outside, and their cooling effect can extend several hundred meters outside. [12] This can create cool islands, which can help reduce the urban heat island effect.
© William Zhang. The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
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