Urban Heat Island | Causes and Effects of Urban Heat Island

An urban heat island (UHI) is a urban area which is significantly warmer than its surrounding rural areas. The temperature difference usually is larger at night than during the day, and is most apparent when winds are weak. Seasonally, UHI is seen during both summer and winter. By virtue of their high heat capacities, urban surfaces act as a giant reservoir of heat energy. For example, concrete can hold roughly 2,000 times as much heat as an equivalent volume of air. Latest studies shows that UHI is one among the main reasons for Global warming. The rising columns of warm air in urban areas are commonly called as "urban thermal plume". The temperature difference between urban areas and the surrounding rural areas can be as much as 5 degrees Celsius

Causes of Urban Heat Island
1. Deforestation : Cutting down the remaining greeneries in the Urban areas amplifies the amount of heat absorption. As population centers grow they tend to modify a greater and greater area of land and have a corresponding increase in average temperature.
2. Modification of the land surface by urban development which uses materials which effectively retain heat.
3. Waste heat generated by energy usage is a secondary contributor. Increase in the number of vehicles and industries top ups the effect. Waste heat from automobiles, air conditioning, industry, and other sources also contributes to the UHI. High levels of pollution in urban areas can also increase the UHI.
4. Materials commonly used in urban areas for pavement and roofs, such as concrete and asphalt, have significantly different thermal bulk properties (including heat capacity and thermal conductivity) and surface radiative properties (albedo and emissivity) than the surrounding rural areas. This causes a change in the energy balance of the urban area, often leading to higher temperatures than surrounding rural areas.
5. The tall buildings within many urban areas provide multiple surfaces for the reflection and absorption of sunlight, increasing the efficiency with which urban areas are heated. This is called the "urban canyon effect". Another effect of buildings is the blocking of wind, which also inhibits cooling by convection.  High levels of pollution in urban areas can also increase the UHI, as many forms of pollution change the radiative properties of the atmosphere.
6. Black surfaces absorb significantly more electromagnetic radiation, and causes the surfaces of asphalt roads and highways to heat. 

Effects of Urban Heat Island
1. First and foremost, the temperature increases when compared to rural areas.
2. UHIs can produce secondary effects on local meteorology, including the altering of local wind patterns, the development of clouds and fog.
3. The extra heat provided by the UHI leads to greater upward motion of wind, known as urban thermal plume, which can induce additional shower and thunderstorm activity. In addition, the UHI creates during the day a local low pressure area where relatively moist air from its rural surroundings converges, possibly leading to more favourable conditions for cloud formation and rainfall.
4. It increases the magnitude and duration of heat waves within cities.
5. It also affects the water quality in urban areas.

Latest News on Urban Heat Island
21 October, 2011 : Urban heat islands not major contributor to warming. Cities acting as “urban heat islands” do not significantly affect the overall warming of the planet, according to a study completed by environmental engineering professor Mark Z. Jacobson and graduate student John Ten Hoeve, who studies in the same department.
Cities do produce more heat than surrounding areas, however, according to the study, this only contributes two to four percent of the total warming since the Industrial Revolution, compared to 18 percent from black carbon and 79 percent from greenhouse gases. Another interesting result the study found was the ineffectiveness of white-roofs. Though painting roofs white has been touted as a way to reduce global warming, Jacobson found that it in fact contributed to global warming by reflecting sunlight back into the atmosphere, reducing cloudiness and increasing the absorption of pollutants. One such method is installing photovoltaic panels on roofs, which absorb sunlight and convert it to energy rather than reflecting it.