Urban Heat Islands (UHI)

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As urban areas develop, changes occur in their landscape. Buildings, roads, and other infrastructure replace open land and vegetation. Surfaces that were once permeable and moist become impermeable and dry. These changes cause urban regions to become warmer than their rural surroundings, forming an "island" of higher temperatures in the landscape.

Heat islands occur on the surface and in the atmosphere. On a hot, sunny summer day, the sun can heat dry, exposed urban surfaces, such as roofs and pavement, to temperatures 50° - 90°F (27° - 50°C) hotter than the air, while shaded or moist surfaces - often in more rural surroundings - remain close to air temperatures. Surface urban heat islands are typically present day and night, but tend to be strongest during the day when the sun is shining.

In contrast, atmospheric urban heat islands are often weak during the late morning and throughout the day and become more pronounced after sunset due to the slow release of heat from urban infrastructure. The annual mean air temperature of a city with 1 million people or more can be 1.8° - 5.4°F (1° - 3°C) warmer than its surroundings. On a clear, calm night, however, the temperature difference can be as much as 22°F (12°C).

It has been reported that Malaysia's Federal administrative capital of Putrajaya, with its many grand buildings languishing, is an "urban heat island" with temperatures 5°C hotter than any other Malaysian cities, rising to 40°C.

Why Do We Care About Heat Islands?
Elevated temperature from urban heat islands, particularly during the summer, can affect a community's environment and quality of life. While some heat island impacts seem positive, such as lengthening the plant-growing season, most impacts are negative and include:

    •    Increased energy consumption: Higher temperatures increase energy
         demand for cooling and add pressure to the electricity grid during peak
         periods of demand. One study estimates that the heat island effect is
         responsible for 5 - 10% of peak electricity demand for cooling buildings in

    •    Elevated emissions of air pollutants and greenhouse gases: Increasing
         energy demand generally results in greater emissions of air pollutants 
         and greenhouse gas emissions from power plants. Higher air 
         temperatures also promote the formation of ground-level ozone.

    •    Compromised human health and comfort: Warmer days and nights, 
         along with higher air pollution levels, can contribute to general
         discomfort, respiratory difficulties, heat cramps and exhaustion, non-
         fatal heat stroke, and heat-related mortality. 

    •    Impaired water quality: Hot pavement and rooftop surfaces transfer their
         excess heat to stormwater, which then drains into storm sewers and
         raises water temperatures as it is released into streams, rivers, ponds,
         and lakes. Rapid temperature changes can be stressful to aquatic

What Can Be Done?
Communities can take a number of steps to reduce the heat island effect, using four main strategies:

    •    installing cool reflective roofs;

    •    creating green roofs (also called "rooftop gardens" or "eco-roofs");

    •    increasing tree and vegetative cover; and

    •    using cool pavements

Typically heat island mitigation is part of a community's energy, air quality, water, or sustainability effort. Activities to reduce heat islands range from voluntary initiatives, such as cool pavement demonstration projects, to policy actions, such as requiring cool roofs via building codes. Most mitigation activities have multiple benefits, including cleaner air, improved human health and comfort, reduced energy costs, and lower greenhouse gas emissions.