Imagine stepping out from the cool embrace of a lush green park onto a sun-drenched street, where asphalt shimmers underfoot and the concrete buildings rise on either side. It’s a sweltering summer afternoon, and the contrast is immediate and striking—a fast shift from the refreshing sanctuary of shade and moisture to the oppressive radiance of heat-retaining surfaces. A tangible wave of warmth engulfs you, with a harsh reminder of the Urban Heat Island (UHI) effect in action.
How City Design Intensifies Urban Heat Island Effect and Reduces Natural Cooling?
Urban heat island effect is one of the most palpable consequences of urban development, with its vast expanses of dark, impermeable surfaces, acting like sponges in absorbing and retaining solar radiation far more effectively than vegetated rural areas. The lack of evapotranspiration, which is a cooling process by which plants release water vapor, also exacerbates this phenomenon. Moreover, dense concentration of buildings creates an “urban canyon” effect, trapping heat and reducing airflow.
Understanding the Urban Heat Island Effect: Rising Temperatures and Their Hidden Dangers
Urban areas have become islands of higher temperatures, where the structures like buildings, roads, or other infrastructure absorb and re-emit the sun’s heat more than the capacity of what the natural landscapes, such as forests and water bodies, can afford. This higher temperature isn’t just an inconvenience; it drives up energy consumption for cooling, which leads to increased emissions of greenhouse gases and air pollutants, further posing a significant health risks.
How the Urban Heat Island Effect Endangers Vulnerable Communities in Large Cities?
The urban heat island effect is particularly high in daytime temperatures and reduced nighttime cooling in the humid regions and cities with larger and denser populations, than the outlying areas. Hot weather events lead to heat-related deaths and illnesses. Heat is of greatest concern for vulnerable populations, especially the older adults, young children, low-income groups, outdoor workers, and people with chronic health conditions, disabilities, mobility constraints, or taking certain medications.
Urban Heat Island Effect Causes:
Urban structures are highly concentrated with limited greenery. Therefore, they become “islands” of higher temperatures relative to outlying areas. These pockets of heat or heat islands, as referred to, can form under a variety of conditions, including during the day or night, in small or large cities, in suburban areas, in northern or southern climates, and in any season. The heat islands form as a result of several factors. Contributors to the urban heat island effect include:
1. Reduced Natural Landscapes: Urbanization reshapes landscapes at an unprecedented pace. Trees, vegetation, and water bodies tend to cool the air by providing shade and promoting evapotranspiration—the process by which plants release water vapor—and evaporation from surface water. In contrast, urban surfaces such as roofs, sidewalks, roads, buildings, and parking lots offer minimal shade and moisture. These hard, dry surfaces absorb and retain more heat, that significantly raising urban temperatures.
2. Urban Material Properties: The conventional materials that are used in urban infrastructure, such as asphalt and concrete, tend to reflect less solar energy, whereas, absorbing and emitting more solar radiation than natural surfaces. These materials reflect less sunlight and retain heat throughout the day, which is released slowly after sunset. This leads to elevated nighttime temperatures, and making urban heat island effect more pronounced during evening hours.
3. Urban Geometry: The dimensions and spacing of buildings within cities influence how the heat is absorbed and distributed. Dense clusters of tall buildings and narrow streets create “urban canyons” to trap heat and block natural airflow. These confined areas reduce the ability of surfaces to cool down, because large thermal masses absorb heat and release it slowly, especially when surrounded by other heat-retaining structures, which can block natural wind flow that would, otherwise, bring cooling effects.
4. Weather and Geography: Local weather patterns and geographic features further affect the severity of heat islands. Clear skies and calm conditions allow maximum solar radiation reaching urban surfaces while reducing the dispersion of heat. On the other hand, strong winds and cloud cover can help in mitigating heat buildup. Geography plays a role as well; for example, the surrounding mountains may block cooling winds from reaching cities or shape wind patterns in ways that influence how heat accumulates and disperses in urban areas.
5. Heat from Human Activities: Anthropogenic heat that is generated by vehicles, air conditioners, buildings, and industrial operations, adds to the overall urban heat load. These sources emit heat directly into the environment, and intensifying the heat island effect, particularly in the densely populated and industrialized areas.
Characteristics of Urban Heat Island Effect:
Usually, the urban heat island effect are measured by comparing temperatures in urban areas to those in the surrounding rural regions. Nonetheless, temperature variation can also occur within a city itself. Some urban areas are significantly warmer due to concentrations of heat-absorbing buildings and pavements, while others remain relatively cooler due to the presence of trees, parks, and water bodies. These internal differences are also known as intra-urban heat islands. There are two main types of urban heat island effect, based on where and how the heat is concentrated:
1. Surface Heat Island:
Surface heat island occurs when urban surfaces like roads, rooftops, and sidewalks absorb and emit more heat than natural surfaces. These heat islands are most intense during the day, particulary under direct sunlight. For instance, on a warm day, conventional roofing materials can become up to 66°F (19°C) warmer than the surrounding air.
2. Atmospheric Heat Island:
Atmospheric heat islands form when the air temperature over urban areas is higher than nearby rural areas. These are usually measured several meters above the ground and tend to vary less in intensity than surface heat islands. They can persist both day and night, though they are often more pronounced during the nighttime, when urban materials slowly release the stored heat.
Impacts of Urban Heat Island Effect:
Increased Energy Consumption:
The elevated temperatures caused by heat islands drive up the need for air conditioning, increasing both electricity usage and the peak energy demand. For every 2°F rise in temperature, air conditioning use may increase by 1–9%, especially in the regions where air conditioning is widely used, such as the United States. This surge in the demand often peaks during hot afternoons when homes and businesses are running appliances at full capacity. In extreme cases, this can overload energy systems, and forcing utility companies to implement controlled brownouts or blackouts to prevent widespread outages.
Elevated Emissions of Air Pollutants and Greenhouse Gases:
To meet the increased energy demand of the people, utilities typically rely on fossil fuel power plants, which emit greenhouse gases like carbon dioxide, which exacerbate climate change. Moreover, these plants release air pollutants that contribute to complex environmental issues such as:
- Ground-level ozone (smog) – It is formed when nitrogen oxides and volatile organic compounds (VOCs) react under sunlight and heat.
- Fine particulate matter – It is harmful to respiratory health.
- Acid rain – It affectis soil, water bodies, and buildings.
When conditions are hotter and sunnier, the potential for ground-level ozone formation increases, even if the emissions levels remain constant.
Impaired Water Quality by Urban Heat Island Effect:
The urban surfaces such as rooftops and pavements can significantly heat stormwater runoff, which eventually flows into nearby rivers, lakes, and streams. This runoff can cause:
- Sudden temperature surges in the aquatic ecosystems.
- Stress or death in the aquatic species, which are sensitive to rapid or extreme temperature changes.
- Altered reproduction and metabolism in fish and other organisms.
Studies have shown that urban streams experience more frequent and intense temperature spikes—up to 18°F higher—compared to the forested streams. This is because of the combined effects of heated runoff, larger discharge volumes, and higher baseline temperatures in urban waterways.
Mitigation Strategies of Urban Heat Island Effect:
- Green Infrastructure: Implementing green roofs, and urban forests, or other green spaces can help reduce the UHI effect by providing cooling through evapotranspiration and shading.
- Cool Pavements: Using light-colored or reflective pavements, which may reduce heat absorption.
- Building Design: In designing buildings, it may be kept in mind that they are equipped with energy-efficient materials and features, that can reduce heat generation within the building itself.
