URBAN HEAT ISLAND 

DEFINITION : An urban heat island occurs when a city experiences much warmer temperatures than nearby rural areas. The difference in temperature between urban and less-developed rural areas has to do with how well the surfaces in each environment absorb and hold heat.In most of the large cities, the temperature at the heart or the center of the city is noted to be higher than its surroundings or the  suburban  area.  The  phenomenon  is  called  Urban  Heat Island (UHI) effect

"Urban heat islands" occur when cities replace natural land cover with dense concentrations of pavement, buildings, and other surfaces that absorb and retain heat. This effect increases energy costs (e.g., for air conditioning), air pollution levels, and heat-related illness and mortality.

Why and How Does Urban Heat Island Happen?

Grass, trees and other vegetation have a natural ability to cool the air through moisture evaporation. This process is a byproduct of photosynthesis. Vegetation also provides shade to the ground, helps to reflect UV rays back at the sky and simply doesn’t retain heat. When building a city, trees, grass, plants and shrubs are naturally cleared away to build roads, buildings, parking lots and stores. 

The new materials that take the place of the plants retain heat instead of reflecting or dispersing it. Concrete alone can be up to 50 degrees warmer than the air temperature through this heat absorption. The more areas that are absorbing heat, the more heat that will be let off over time. Think about how rocks around a campfire continue to let off heat long into the night. Or how a stove element takes time to cool, even after you’ve cut off the heat source. Temperatures rise in those areas until you have the Urban Heat Island effect. 

Causes of Urban Heat Island and ItsEffects 

Use of construction materials : Asphalt and concrete, needed for the expansion of cities, absorb huge amounts of heat, increasing the mean surface temperatures of urban areas.

Decreasing Albedo: Many buildings found in urban areas have dark surfaces, thereby decreasing albedo and increased absorption of heat.

Air conditioning: Buildings with dark surfaces heat up more rapidly and require more cooling from air conditioning, which requires more energy from power plants, which causes more pollution. Also air conditioners exchange heat with atmospheric air, causing further local heating. Thus there is a cascade effect that contributes to the expansion of urban heat islands.

Urban Construction: Tall buildings, and often, accompanying narrow streets, hinder the circulation of air, reduce the wind speed, and thus reduce any natural cooling effects. This is called the Urban Canyon Effect.

transportation system: Transportation systems and the  use of fossil fuels also add warmth to urban areas.

Lack of Trees and green areas: which impedes evapotranspiration, shade and removal of carbon dioxide, all the processes that help to cool the surrounding air.

Waste Heat: Waste heat from vehicles, factories, and air conditioners may add warmth to their surroundings, further exacerbating the heat island effect

Urban haze: The haze of air pollution that hangs over many cities can act as a miniature greenhouse layer, preventing outgoing thermal radiation (heat) from escaping from urban areas.

Effects of Urban Heat Island 

Energy costs: Urban Heat Island effect increases energy costs (e.g., for air conditioning), air pollution levels, and heat–related illness and mortality. Increased consumption of air-conditioning for cooling can also contribute to global warming, which  contributes to climate change.

Poor air quality: UHIs often have lower air quality because there are more pollutants (waste products from vehicles, industry, and people) . These pollutants are blocked from scattering and becoming less toxic by the urban landscape: buildings, roads, sidewalks, and parking lots.

Poor water quality: Water quality also suffers. When warm water from the UHI ends up flowing into local streams, it stresses the native species that have adapted to life in a cooler aquatic environment.

Colonization by heat-loving species: Due to higher temperatures in urban areas, the UHI increases the colonization of species that like warm temperatures, such as lizards and geckos. Insects such as ants are more abundant here than in rural areas; these are referred to as ectotherms.

Heatwaves: Cities tend to experience heat waves that affect human and animal health, leading to heat cramps, sleep deprivation, and increased mortality rates.

MITIGATION

Green roofs: Using green roofs, which are roofs of buildings covered in plants, helps cool things down. we should paint rooftops green, and install solar panels there amidst a green background.

Planting trees: We should plant as many plants and trees as possible. They combat climate change; clean the surrounding air by absorbing pollutant gases  and trapping particulates on their leaves and bark; cool the city and the streets; conserve energy (cutting air-conditioning costs by 50%); save water and help prevent water pollution; help prevent soil erosion; protect people and children from UV light, etc.