Urban Heat Island UPSC

Urban Heat Island

A recent study from IIT Kharagpur called “Anthropogenic forcing exacerbating the urban heat islands in India” noted that mean daytime temperature of surface urban areas going up by around 2 degrees C if compared to neighbouring areas. 

The same study also said that the relatively warmer temperature in urban areas, compared to suburbs, may contain potential health hazards due to heat waves apart from pollution. 

So why are these urban areas hotter than surrounding suburbs? 

• This happens because of the materials used for pavements, roads and roofs, such as concrete, asphalt (tar) and bricks, have higher heat capacity and thermal conductivity than rural areas, which have more open space, trees and grass. 

• Trees and plants are characterised by their ‘evapotranspiration’— a combination of words wherein evaporation involves the movement of water to the surrounding air, and transpiration refers to the movement of water within a plant and the subsequent lot of water through the stomata (pores found on the leaf surface) in its leaves. Grass, plants and trees in the suburbs and rural areas do this. The lack of such evapotranspiration in the city leads to the city experiencing higher temperature than its surroundings. 

• The higher temperatures of urban heat islands can be attributed to human activity, particularly to changes in land surfaces. Urban development requires the use of significant amounts of cement and asphalt for roofing purposes and to pave sidewalks and roads. These materials have thermal bulk properties that absorb more solar radiation than the surfaces found in rural areas. 

Additionally, these materials have different surface radiative properties, which means they emit energy as thermal radiation or heat. 

What is the impact of urban heat islands? 

Impact of Urban Heat Island on local climate 

• In addition to increasing temperatures, urban heat islands also influence local weather conditions. Some of these changes to weather conditions include: precipitation levels, cloud coverage, fog presence, humidity levels, and wind patterns. 
• Increased precipitation and thunderstorm activity occurs because the hotter temperatures result in an upward movement of surrounding air. 
• This upward movement has its greatest impact between 20 and 40 miles downwind of the city center, where precipitation levels can be 28% greater than upwind of the city center. 
• During the day, urban heat islands experience low pressure, which allows moist air from nearby rural areas to collect. This moist air counteracts with the hot, urban air and creates the ideal condition for cloud coverage and increased humidity. 
• Another potential benefit of urban heat islands is that they work to increase plant growing seasons in nearby locations. For example, some studies have found that growing areas located around 6.2 miles outside of city limits are extended by an average of 15 days. 

Impact Of Urban Heat Islands on Animals 

• Migration of animals towards cities 
• Due to this higher temperature in urban areas, the UHI increases the colonisation of species that like warm temperatures, such as lizards and geckos. 
• Thermal pollution of neighbouring water bodies: 
• In other cases, urban heat islands negatively affect food and water supplies within city limits. For example, the surface temperature of city buildings is significantly hotter than the surrounding air. Rain then runs off of these much hotter buildings, warming up in the process. This warmer runoff makes its way into nearby streams, rivers, lakes and other bodies of water. When the heated runoff enters these bodies of water, it goes on to increase their temperature as well. This is referred to as thermal pollution and it can increase water temperatures by as much as 20° to 30° Fahrenheit. Thermal pollution has a negative impact on local fish populations, sending them into thermal shock.
• The Impact Of Urban Heat Islands On Human Health 
• Urban heat islands not only affect the health of the environment and local wildlife, but also the health of humans. 
• Increased city temperatures can be fatal during summer heatwaves, particularly for senior citizens.
• Urban inhabitants also suffer during heat waves because the urban heat island prevents night time temperatures from decreasing as they do in rural areas. Research has found that urban heat islands not only increase temperatures during heat waves but also prolong their duration.
• Extreme temperatures lead to heat cramps, heatstroke, and heat exhaustion. Heatstroke may cause respiratory distress syndrome, impaired mobility, or decreased awareness.