URBAN HEAT ISLAND EFFECT
As urban areas develop, changes occur in their landscape. Buildings, roads, and other infrastructure replace open land and vegetated areas. 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 of 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 1–3°C warmer than its surroundings. On a clear, calm night, however, the temperature difference can be as much as 12°C. The term Surface Urban Heat Island (SUHI) is usually employed to make a distinction between UHI (when air temperature is considered) and SUHI when land surface temperature (LST) is considered.
Why are we concerned about Heat Islands?
Research on the trends of surface temperatures at rapidly growing urban sites in the U.S.A. during the last 30 to 50 years suggests that significant urban heat island effects have caused the temperatures at these sites to rise by 1 to 2°C. Urban heat islands have caused changes in urban precipitation and temperature that are at least similar to, if not greater than, those predicted to develop over the next 100 years by global change models. 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 in summer 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 cities.
• 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 ecosystems.
What Can Be Done?
Typically heat island mitigation is part of a local community's effort at managing its 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.
Communities can take a number of steps to reduce the heat island effect, using four main strategies:
• Increasing tree and vegetative cover;
• Creating green roofs (also called "rooftop gardens" or "eco-roofs");
• Installing cool—mainly reflective—roofs; and
• Using cool pavements.*
* Cool pavements include a range of established and emerging technologies that communities are exploring as part of their heat island reduction efforts. The term currently refers to paving materials that reflect more solar energy, enhance water evaporation, or have been otherwise modified to remain cooler than conventional pavements.
Remote Sensing in Mapping Urban Heat Islands:
UHIs have long been studied by ground-based observations taken from fixed thermometer networks or by traverses with thermometers mounted on vehicles. With the advent of thermal remote sensing technology, remote observation of UHIs became possible using satellite and aircraft platforms and has provided new avenues for the observation of UHIs and the study of their causation through a combination of thermal remote sensing and urban micrometeorology.
Satellite remote sensing, particularly NOAA AVHRR data, has been used in the study of urban heat islands. Because of the low spatial resolution (1-1 km at nadir) of the AVHRR data, these studies can only examine and map the phenomenon at the macro-level. The heterogeneity of the urban environment makes mapping of surface temperatures at this resolution very difficult, and the smoothing of signals within a pixel introduces bias in the radiant temperatures extracted. Successful utilization of Landsat Thematic Mapper (TM) thermal infrared data (with a spatial resolution of 120m) has been used to derive surface temperature data for some housing estates in Singapore. But the spatial resolution of the TM thermal infrared data is still inadequate to capture all the complex temperature changes of the urban environment.
Recent researches have utilized 5 m thermal infrared data acquired specially from an aircraft to characterize more accurately the thermal responses of different land cover types in the urban environment as input to urban heat island detection.
Thermal remote sensing has been used over urban areas to assess urban heat island effects, to perform land cover classifications, and as input for models of urban surface atmosphere exchange. The main surface parameter to be extracted from thermal remote sensing is Land Surface Temperature (LST) or simply surface temperature, which is of prime importance to the study of urban climatology.
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