Climate Change and Urban Heat Island Effect (UHI) are the two meteorological phenomena of differing scales. The former at a global level and the latter at a local or regional level pose a significant threat to human populations with large-scale and potentially unpredictable impacts. As urban areas develop, changes occur in their landscape, Buildings, roads, and other infrastructure replace open land and vegetation. 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. Over the past decade UHI presence has been studied and documented in several cities around the world. UHI can be defined as “closed isotherms indicating an area of the surface that is relatively warm; most commonly associated with areas of human disturbance such as towns and cities.”

In metropolitan areas, the urban core shows a final “peak” in the urban heat island where the urban maximum temperature is found. The difference between this value and the background rural temperature defines the “urban heat island intensity” ( T ). The intensity
of the heat island is mainly determined by the thermal balance of the urban region and can result in a temperature difference of up to 10 degrees (Asimakopoulos et al., 2001).

Typical Cross Section of an Urban Heat Island

Characteristics of UHI
Some general characteristics of the UHI are as follows.

  • Average minimum temperatures increase more than maximum temperatures.
  • Average nighttime temperatures increase more than the average daytime temperatures.
  • Urban areas experience year-round increases in the minimum daily temperatures.
  • The UHI causes longer daily warm periods and shorter daily cool periods.
  • Compared to rural areas, the duration of the hot season is extended in urban areas.
  • Temperatures can substantially vary between urban neighborhoods—the amount of vegetation plays a significant role in damping the UHI.

Causes of UHI

The increase of the urban population has led to rapid changes in land use and land cover within cities. During recent years there are evidence of increased change in traffic congestion and pollution. These physical changes affect the quality of the environment, contributing to other phenomenon such as an increase in air temperature. Scientists have observed that air temperatures in densely built urban areas are higher than the temperatures of the surrounding rural country. Urban heat island (UHI) phenomenon has been observed all around the world in those cities which have seen rapid growth and urbanization. As more people move to cities “the economic engines of a country”, the land cover and land use will be subjected to change and due to which UHI phenomenon may increase and impact over society and environment. Various factors such as changing climate, urban sprawling, life style of people, geometry and geography of the cities play a vital role in increasing the urban heat island intensity.

 Causes of Urban Heat Island

Effects of UHI

  • The UHI causes people to experience uncomfortable, high temperatures for longer periods of the day; high temperatures arrive earlier and stay later in the day than they do in rural areas. This not only increases discomfort, but exposes people to higher risk of heat illnesses.
  • According to the world health organization (WHO) anthropogenic warming claims more than150,000 lives on an annual basis, First, direct heat related mortality and morbidity, and second a climate-mediated change on the incidence of infectious diseases.
  • Higher temperatures increase the use of air conditioning and evaporative coolers which increases energy costs and water use.
  • It also effect the elevation in the ground level ozone.
  • Urban warming will likely increase agricultural pests. This is a consequence of the increasing number of days when temperatures fall between 59 and 100 degrees F, temperatures suitable for insects.
  • The UHI has changed the sowing and harvest dates for crops.

Mitigation Strategies for Urban Heat Island Effects

Trees and vegetation are simplest way to reduce urban heat island effects. Urban temperatures can be reduced up to a substantial amount by planting the trees which help in increasing the albedo of the surfaces.

  • A green roof, or rooftop garden, is a vegetative layer grown on a rooftop can be cooler than air temperature while the conventional rooftops can be warmer up to 50°C. Green roof temperatures depend on the roof’s composition, moisture content of the growing medium, geographic location, solar exposure, and other site-specific factors.
  • Cool Roofs and Cool Pavements which using lighter-colored materials on buildings where light colors reflect more sunlight and trap less heat.
  • Passive ventilation is the most cost effective technology to keep the buildings cool in summer heat. Before airtight houses and mechanical ventilation systems, people used to keep their houses cool using natural methods like windows for getting cool air, walls with air-vents and large solid masses absorbing the heat.