The 'urban heat island' (UHI) effect is a phenomenon where urban areas experience significantly higher temperatures than their surrounding rural counterparts. This temperature difference is most pronounced at night and during summer months.

Causes of Urban Heat Island Effect:

1. Altered Surface Properties:
   • Dark Surfaces: Urban areas are dominated by dark-colored surfaces like asphalt roads and concrete buildings, which have low albedo (reflectivity). These surfaces absorb a large amount of solar radiation during the day, storing heat and releasing it slowly at night.
   • Reduced Vegetation: The replacement of natural landscapes (forests, grasslands) with impervious surfaces leads to a significant reduction in vegetation. Plants cool the environment through evapotranspiration, a process largely absent in built-up areas.
2. Anthropogenic Heat Generation:
   • Energy Consumption: Buildings, vehicles, industries, and air conditioning systems in urban areas release substantial amounts of heat into the atmosphere. This waste heat directly contributes to warming.
   • Industrial Activities: Factories and industrial processes generate considerable heat, further exacerbating the UHI effect.
3. Urban Geometry (Canyon Effect):
   • Tall Buildings and Narrow Streets: The arrangement of tall buildings along narrow streets creates "urban canyons." These canyons trap heat, reduce airflow, and limit the escape of longwave radiation from the surface, leading to higher temperatures. They also reduce sky view factor, limiting radiative cooling.
4. Reduced Evapotranspiration:
   • Lack of Green Spaces and Water Bodies: Urbanization replaces natural vegetation and permeable surfaces with concrete and pavement, reducing the amount of water available for evaporation and transpiration. This diminishes the natural cooling effect that occurs in rural areas.
5. Atmospheric Pollution:
   • Aerosols and Greenhouse Gases: Urban activities release pollutants like aerosols and greenhouse gases. While some aerosols can reflect sunlight, others can absorb it, and greenhouse gases trap heat, contributing to the warming of the urban atmosphere.
6. Heat Storage in Urban Materials:
   • High Thermal Mass: Concrete, bricks, and other building materials have high thermal mass, meaning they can absorb and store large quantities of heat during the day and release it slowly over several hours, particularly at night.

Effective Measures to Deal with UHI in India: Addressing the UHI effect in India requires a multi-pronged approach, integrating urban planning, architectural design, and policy interventions.

1. Promoting Green Infrastructure:
   • Urban Forestry and Parks: Increasing tree cover along streets, in parks, and open spaces provides shade and cools the air through evapotranspiration.
   • Green Roofs and Walls: Installing vegetation on rooftops and building facades reduces heat absorption, insulates buildings, and contributes to local cooling.
2. Using Cool Materials:
   • Cool Roofs and Pavements: Utilizing materials with high albedo (light-colored, reflective surfaces) for roofs and pavements reflects more sunlight and absorbs less heat. This can significantly reduce surface temperatures.
   • Permeable Pavements: Using permeable materials for sidewalks and parking lots allows water to infiltrate the ground, promoting evaporation and reducing runoff.
3. Optimizing Urban Design and Planning:
   • Strategic Building Orientation: Designing buildings to maximize natural ventilation and minimize sun exposure can reduce heat gain.
   • Wider Streets and Open Spaces: Planning for wider streets and creating more open spaces can improve air circulation and reduce the canyon effect.
   • Water Bodies: Integrating natural or artificial water bodies (ponds, lakes, fountains) into urban design can provide a localized cooling effect through evaporation.
4. Reducing Anthropogenic Heat:
   • Energy Efficiency: Promoting energy-efficient buildings, appliances, and industrial processes reduces waste heat generation.
   • Public Transportation and Non-Motorized Transport: Encouraging the use of public transport, cycling, and walking reduces vehicular emissions and heat.
   • Renewable Energy: Shifting to renewable energy sources for power generation reduces overall heat emissions from fossil fuel combustion.
5. Policy and Regulatory Frameworks:
   • Building Codes: Implementing building codes that mandate green building practices, cool roofs, and energy efficiency standards.
   • Urban Green Space Policies: Developing policies that protect existing green spaces and mandate the creation of new ones in urban development projects.
   • Awareness Campaigns: Educating the public about the UHI effect and the benefits of sustainable urban practices.

By implementing these measures, Indian cities can mitigate the adverse impacts of urban heat islands, leading to more comfortable, healthier, and sustainable urban environments.