Urban Heat Island

The Urban Heat Island (UHI) effect refers to the phenomenon in which urban areas experience significantly higher temperatures than their surrounding rural or suburban regions. This temperature difference results from human activities, dense construction, reduced vegetation, and the extensive use of heat-absorbing materials such as concrete and asphalt. The Urban Heat Island effect is one of the most prominent and well-documented examples of how urbanisation alters local climate and environmental conditions.

Definition

An Urban Heat Island is defined as a localized increase in air and surface temperature in urban environments compared to nearby non-urbanised or rural areas. The phenomenon can be observed both during the day and at night, although it is usually more pronounced after sunset, when urban surfaces release the heat absorbed during the day.
The temperature difference between urban and rural areas can range from 2°C to 6°C, and in extreme cases, may reach up to 10°C or more, depending on city size, structure, and climatic conditions.

Causes of the Urban Heat Island Effect

The Urban Heat Island effect arises from a combination of physical, structural, and anthropogenic factors associated with urbanisation.
1. Surface Characteristics:

• Urban materials such as asphalt, concrete, and metal have high heat absorption and low reflectivity (low albedo).
• These materials store solar energy during the day and release it slowly at night, raising ambient temperatures.

2. Lack of Vegetation:

• Vegetation provides cooling through shade and evapotranspiration (the release of water vapour from plants).
• Replacing green areas with buildings and roads reduces natural cooling and increases heat retention.

3. Building Geometry (Canyon Effect):

• Closely packed tall buildings trap heat between them, limiting airflow and reflecting radiation between surfaces.
• This creates a “canyon effect,” where heat is retained in narrow urban spaces.

4. Waste Heat Emissions:

• Energy consumption from vehicles, air conditioners, factories, and lighting adds anthropogenic heat to the atmosphere.
• The concentration of population and activities amplifies this effect, particularly in dense city centres.

5. Reduced Wind Flow:

• High-rise structures disrupt natural air circulation, decreasing ventilation and allowing heat to accumulate.

6. Air Pollution:

• Urban areas have high concentrations of greenhouse gases and particulates, which absorb and re-radiate heat.

Types of Urban Heat Islands

Urban Heat Islands can be classified into two main types based on where the heat concentration occurs:

1. Surface Urban Heat Island (SUHI):
   • Refers to the increase in land surface temperature due to heat-absorbing materials.
   • Detected using satellite thermal imaging or ground-based sensors.
2. Atmospheric Urban Heat Island (AUHI):
   • Refers to higher air temperatures within the urban boundary layer.
   • Divided into two sub-layers:
     • Canopy Layer: The layer of air near the ground where humans live and interact.
     • Boundary Layer: The layer above rooftops extending up to several hundred metres.

Diurnal and Seasonal Variation

• Daytime: During the day, solar radiation heats up both urban and rural surfaces, but rural areas cool faster due to vegetation and open soil.
• Night-time: The UHI effect is strongest at night when rural areas lose heat quickly while urban materials release stored heat slowly.
• Seasonal Influence: The effect tends to be more intense during summer and dry seasons, when clear skies and low humidity enhance heat absorption.

Measurement and Detection

The Urban Heat Island effect is measured using:

• Ground-based weather stations comparing urban and rural temperature data.
• Remote sensing and satellite imagery to map surface temperature differences.
• Thermal infrared cameras for localised urban temperature studies.

Prominent space-based sensors such as MODIS (Moderate Resolution Imaging Spectroradiometer) and Landsat are commonly used to monitor surface heat variations across cities.

Impacts of Urban Heat Islands

1. Environmental Impacts:

• Alters local climate patterns, leading to reduced rainfall or changed wind flows.
• Increases ground-level ozone and air pollution due to higher temperatures.
• Intensifies energy demand for air conditioning, contributing to greater greenhouse gas emissions.

2. Human Health:

• Elevated urban temperatures can cause heat stress, dehydration, and exacerbate cardiovascular and respiratory diseases.
• Increases mortality during heatwaves, particularly among vulnerable populations such as the elderly and outdoor workers.

3. Energy and Economic Effects:

• Increased demand for cooling leads to higher electricity consumption and economic costs.
• Power grids may face additional stress, resulting in blackouts during heat extremes.

4. Ecological Effects:

• Urban heat affects biodiversity by altering habitat conditions for birds, insects, and vegetation.
• Reduces soil moisture and impacts urban green cover.

5. Water and Air Quality Degradation:

• Higher temperatures accelerate evaporation from water bodies, reducing availability.
• Promotes the formation of smog and tropospheric ozone, worsening air quality.

Mitigation Strategies

Addressing the Urban Heat Island effect requires a combination of urban planning, green infrastructure, and technological interventions.
1. Increasing Vegetation and Green Spaces:

• Planting urban forests, green belts, and rooftop gardens enhances shading and evapotranspiration.
• Urban afforestation programmes improve cooling and air quality.

2. Cool and Reflective Materials:

• Use of cool roofs (high albedo materials) and cool pavements that reflect rather than absorb sunlight.
• Light-coloured building materials reduce heat storage.

3. Green Infrastructure:

• Development of green roofs and vertical gardens for cooling buildings and improving insulation.
• Permeable pavements reduce surface temperature by enhancing moisture retention.

4. Urban Planning and Design:

• Designing open layouts for improved airflow and ventilation.
• Reducing building density and promoting mixed-use zoning to decrease heat concentration.

5. Sustainable Transportation:

• Promoting public transport, cycling, and electric vehicles to reduce waste heat emissions.
• Encouraging compact, walkable neighbourhoods to limit vehicular use.

6. Water Features and Cooling Systems:

• Incorporating urban ponds, fountains, and water channels for evaporative cooling.
• Using energy-efficient air conditioning and waste heat recovery systems.

7. Policy and Public Awareness:

• Implementation of national urban climate adaptation plans.
• Educating citizens about heatwave preparedness and sustainable urban lifestyles.

Examples of Urban Heat Islands in India

Several major Indian cities exhibit strong Urban Heat Island effects due to rapid and unplanned urbanisation:

• Delhi: Experiences temperature differences of up to 6–8°C between urban and surrounding rural areas during summer.
• Mumbai: High density of concrete structures and vehicular emissions intensify night-time heat.
• Chennai and Hyderabad: Loss of wetlands and vegetation cover contributes to persistent UHI conditions.