URBAN HEAT ISLANDSby Aaditya Singh | 01-09-2017 07:52 |
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My apologies for a delayed report. I was travelling and did not have access to a network most of the time. URBAN HEAT ISLANDS (UHIs) What is Urban Heat Island? Some points: 1. An urban or built-up area significantly warmer than its surrounding open, natural or rural areas due to human activities. 2. Normally the temperature difference of higher than 10 degrees Celsius forms heat islands. 3. The temperature difference is usually larger at night and most apparent when winds are weak. 4. UHI effect is most noticeable during the summer and winter. 5. It occurs on the surface and in the atmosphere.
Causes of UHIs 1. Modification of land surfaces due to urbanisation and rapid urban developments 2. Waste heat generated by energy usage How urbanization creates UHIs? 1. Built surfaces are constructed with non-reflective and water-resistant materials that act as heat sinks, not only absorbing the radiated heat from the sun but also storing it for long and then releasing it in the form of heat. 2. Dark surfaces absorb significantly more solar radiation, causing roads and buildings to heat more than suburban and rural areas during the day. 3. Material Properties- Concrete and asphalt, commonly used for pavement and roofs, have significantly different thermal bulk properties and surface radiative properties. This leads to higher temperatures than surrounding rural areas. 4. Lack of Vegetation- Urban areas with high population end up having large built up areas and less green areas. Lack of evapo-transpiration due to lack of vegetation contributes to UHI effect. 5. Due to decreased vegetation, cities also lose the shade and cooling effect of trees, as also removal of carbon dioxide that occurs due to photosynthesis. 6. Urban canyon effect - Skyscrapers provide a canyon like environment and multiple geometric surfaces for reflection and absorption of sunlight and heat. 7. Blocking of wind by buildings inhibits cooling by convection and also prevents pollution from dissipating. 8. Waste heat from automobiles, air conditioning, industry, and other sources also contributes to UHIs. 9. High levels of pollution can also increase the UHIs, as many forms of pollution change the radiative properties of the atmosphere. 10. As UHIs raise the temperature of cities, they also increase the concentration of ozone, a greenhouse gas whose production accelerates with an increase in temperature. This further adds to heat retention. Impacts of UHIs Negative impact on all aspects of sustainability- environment, people and economy as indicated below: 1. Changes in Rain patterns- Monthly rainfall is greater downwind of cities, partially due to the UHI effect. 2. Changes in weather conditions- Higher temperatures lead to increased length of growing seasons, and decrease the occurrence of weak tornadoes. 3. Decreased air quality due to increased production of pollutants such as ground level ozone. 4. Decreased water quality as warmer waters flow into area streams and put stress on their ecosystems. 5. Increased energy consumption as increase in temperature leads to increase in demand for cooling, putting pressure on electricity supply. 6. Demand on water - As the surface and air get hotter, people consume more water for indoor and outdoor usage putting pressure on water supply. 7. Depletion of stratospheric ozone layer due to CFCs in the air-conditioning system. 8. Decreased Quality of life -Respiratory diseases, discomfort, heat stress and decreased productivity, besides heat related moralities. 9. Relation of UHI effect with Climate Change- According to research, UHI effect contributes to climate warming by about 30%. Vice versa many studies reveal that the severity of UHI effect increases due to climate change. Mitigation Methods 1. Reduction in building footprint and maximized open green spaces 2. Minimize hard surfaces –Use of permeable materials or surfaces such as vegetated roofs, porous pavement and grid pavers. Use open grid pavement system, which is at least 50% pervious and locating the parking space under the building will help reducing the urban heat island effect. 3. Use of reflective materials – Use high reflective materials with high solar reflective index (SRI) values for roofs and non-roof exterior surfaces. The SRI value is the combined value of reflectivity and emmitance. 4. Shading- Tree canopies and similar structures can provide shade. If coved by solar panels they can also contribute to renewable energy. 5. Smart construction- High albedo cool roofs with controlled SRI and vegetated green roofs. UHI Effect in the UAE UHI is quite common in cities in the temperate zone. UAE has a tropical desert climate. Out of seven emirates, Dubai, Abu Dhabi and Sharjah have seen rapid urbanization in recent years. Thus, areas with UHI effect are expected. However, according to findings of some studies, it is interesting to note that microclimate trends are reversed in UAE cities, where the suburbs are often warmer than the city centre by about 4 to 6 degrees C. This is due to the following factors: 1. Greenery in city areas- Moisture in vegetation reduces surface air temperature. 2. Wind-conscious layout of buildings can create "urban canyons" that channel the wind and reduce the "feels-like" temperature. Tall buildings also increase the total shaded areas from direct solar radiation, reducing downtown temperatures. 3. Local building materials, like marble and stone, trap less heat being more reflective than others like red brick and asphalt. 4. Suburban areas are warmer on the other hand as they are are barren sandy deserts- with little or no vegetation and few low-rise buildings if any. They often have wider roads, with larger heat-trapping asphalt area. 5. Presence of mangrove forests also helps reduce UHI effect. However there are pockets of densely populated areas with less vegetation that are obviously warmer. In such areas 'green landscaped roofs' ,'cool perforated pavements', urban vegetation, balcony gardens and covered parking spaces can keep UHI effect at bay. With ongoing construction, UAE government is committed to sustainable development with due focus on mitigation options and best practices to reduce UHI effect with participation of all stakeholders such as developers, building owners, utilities, industries and other concerned parties, Development of Dubai Sustainable City and Masdar City in Abu Dhabi are perfect examples of such development. Research and development in this direction is ongoing. Before concluding I would like to introduce an innovative product 'Jotashield Extreme', launched by Jotun Paints in UAE . Structures treated with this paint are cooler by 2 to 7 degrees C, as compared to those treated with normal paint, . A revolutionary pigment-binder technology in this paint offers twice the heat-reflective capabilities of ordinary exterior paint. It provides protection against ultraviolet and near infra-red heat, thus reducing indoor and surface temperatures, and consequently driving down energy consumption. Conclusion Greenery and vegetation is the most important safeguard towards reducing land surface temperature and hence the UHI effect. Thus proper management and integration of open and landscaped green spaces are elementary to mitigate the UHI effect in the urban cities of arid and semi arid regions like the UAE.
Sources and References
https://en.wikipedia.org/wiki/Urban_heat_island https://www.cleantechloops.com/urban-heat-island-effect/ http://www.ecomena.org/uhi-effect-sustainability/ http://gulfnews.com/news/uae/environment/thermal-mapping-hot-in-the-city-1.670035
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