Urban Heat Islands

The Urban heat islands (UHI), concrete buildings, and asphalt is making the heat waves around the world more intense. Studies are indicating that UHI is increasing with more people being part of such areas. What are Urban heat islands? How is it affecting the global climate? Read here to find out.

Surface temperatures play a critical role in how hot the surrounding area gets and also present a health risk during extreme heat events.

During heat waves, a substantial amount of the sun’s energy is absorbed and reflected by surfaces exposed to its rays, leading to their temperatures increasing significantly.

The process of urban development greatly changes the landscape. Natural and permeable surfaces are replaced by impermeable structures like buildings and roads. This creates what climatologists refer to as “urban heat islands”, areas within cities that experience significantly higher temperatures compared to nearby rural regions.

Urban Heat Islands

Urban heat islands (UHIs) are localized areas within urban environments that experience significantly higher temperatures compared to their surrounding rural areas.

UHIs occur due to human activities, modifications to the landscape, and the concentration of heat-absorbing materials in urban settings. They are a result of the urbanization process and can have various impacts on the environment, public health, and energy consumption.

The phenomenon, ‘Urban Heat Island’ was first investigated and described by Luke Howard in the 1810s.

Causes for the formation of heat islands

Heat islands form in urban and suburban areas because many common construction materials absorb and retain more of the sun’s heat than natural materials in less developed rural areas.

• Temperatures of dark, dry surfaces in direct sun can reach 88°C during the day, while vegetated surfaces with moist soil under the same conditions might reach only 18°C.
• Concrete, cement, and metal surfaces in urban areas tend to absorb heat energy rather than reflect it, contributing to higher urban temperatures.
• Cities have a low albedo, the reflecting power of a surface. The increased surface area of buildings results in more solar radiation absorption than reflection.

Heat Absorbing Materials: Urban areas are characterized by extensive use of heat-absorbing materials such as asphalt, concrete, and dark-colored buildings. These materials absorb and retain solar radiation, leading to elevated temperatures.

Reduced Vegetation: Urbanization often involves the removal of natural vegetation, such as trees and green spaces, which act as cooling agents through evapotranspiration and shading.

Human Activities: Heat generated by human activities, including industrial processes, vehicular traffic, and energy consumption, contributes to the overall temperature increase in urban areas.

Artificial Heat Sources: The presence of heat sources like air conditioners, industrial processes, and waste heat from buildings and vehicles further exacerbates this.

Modified Wind Patterns: Urban landscapes can alter local wind patterns, reducing natural ventilation and trapping warm air within the city.

Urban Form: The density, layout, and design of urban areas can impact the intensity and extent of UHIs. High-rise buildings, narrow streets, and compact urban layouts can intensify heat retention.

Read: Insolation and heat balance of earth

Impacts of urban heat islands

• UHIs lead to higher demand for cooling in buildings, increasing energy consumption, and associated greenhouse gas emissions.
• Elevated temperatures in urban areas can cause heat-related illnesses and exacerbate existing health conditions, particularly in vulnerable populations.
• UHIs can contribute to poor air quality, as higher temperatures can enhance the formation of ground-level ozone and other air pollutants.
• UHIs affect the water balance in urban areas, leading to increased runoff and reduced infiltration, affecting stormwater management.
• UHIs can disrupt local ecosystems, impacting flora and fauna and altering natural habitats.

Remedies

• Urban Planning: Incorporating green spaces, parks, and trees into urban planning can help mitigate UHIs by providing shading and promoting natural cooling.
• Cool Roofs and Pavements: Implementing cool roofs and pavements, which reflect sunlight rather than absorb it, can reduce heat absorption in urban areas.
• Green Roof and Walls: Installing green roofs and walls can help reduce surface temperatures and improve air quality.
• Energy-Efficient Buildings: Promoting energy-efficient building designs and cool building materials can reduce heat emissions from structures.
• Transportation Planning: Encouraging public transportation, biking, and walking can help reduce vehicular heat emissions.

By implementing these strategies, cities can effectively mitigate urban heat islands and create more comfortable and sustainable living environments for their residents.

Way forward

Cooler, shaded spaces in our parks, near buildings, in streetscapes, and alongside homes can reduce heat stress, particularly for those in our community who are most vulnerable to high temperatures.

Note: Cities in desert environments show a different trend known as the “urban oasis effect”. This effect is characterized by a cooler city center compared to the surrounding environments

Read: India Cooling Action Plan

UPSC Civil Services Mains Question

1. Bring out the causes for the formation of heat islands in the urban habitat of the world. (5 marks, 100 words).