The Urban Heat Island (UHI) Effect is a phenomenon where urban areas experience significantly higher temperatures than their surrounding rural areas.
This temperature disparity is caused by human modifications of the landscape and altered energy use patterns within cities.
Key Characteristics and Causes of the UHI Effect
The sources identify several characteristics and underlying causes of why cities become "islands" of heat:
1. Material Properties and Heat Absorption
• Absorption and Retention: Cities are dominated by "grey" infrastructure—such as concrete, asphalt, and rooftops—which are impervious surfaces that absorb and retain heat. These dark surfaces absorb heat during the day and slowly release it after sunset, causing city temperatures to remain consistently higher.
• Efficiency of Absorption: Construction materials like asphalt or concrete can absorb as much as 95% of the sun's energy, which is then radiated back into the surrounding atmosphere.
• Heat Trapping: The concentration of buildings and human activity, coupled with the lack of vegetation, traps heat.
2. Lack of Natural Cooling Mechanisms
• Reduced Evaporation: Impervious surfaces prevent rainwater from infiltrating the ground and reduce natural cooling. Non-porous surfaces lead to less evaporation, a key cooling process.
• Diminished Vegetation: The density of buildings and the loss of natural habitats, along with fewer trees and lower water bodies, weaken the natural cooling systems that provide ecosystem services.
• Reduced Evapotranspiration: Natural processes like evapotranspiration, where vegetation cools the air by releasing moisture, are minimized, exacerbating the UHI effect.
3. Energy Consumption and Emissions
• Anthropogenic Heat Sources: The concentration of buildings and human activity creates urban heat islands. Heat is retained and emitted from the built environment.
• Increased Energy Consumption for Cooling: The intensified heat contributes to increased energy consumption for cooling. While air conditioning provides relief, it simultaneously contributes to energy consumption and greenhouse gas (GHG) emissions, further exacerbating the heat problem.
• High Vehicle Emissions: Factors contributing to urban heat can include high vehicle emissions from an increasing number of cars and the widespread use of air conditioning.
4. Characteristics of Urban Heat
• Spatial and Temporal Variation: The intensity of urban heat can vary depending on the time of day, season, and geographic location. Cities typically experience the highest temperature differences during nighttime and summer. Elevated nighttime temperatures are particularly important in urban heat island assessment, as they impact human health and the recovery from daytime heat exposure.
• Intra-urban Differences: The effect is not uniform across a city; spatial variations in temperature occur due to differences in land cover, building density, vegetation, and topography, creating patterns of heat exposure inequality. Central and densely built-up districts exhibit the highest temperatures. In Bangkok, central districts can be up to warmer than the average of the coolest peripheral districts.
• Intensification: The UHI effect intensifies hot extremes in cities. Warming associated with urban development is expected to be exacerbated in future years by temperature increases due to climate change.
Impacts of the UHI Effect
The Urban Heat Island effect poses significant threats to urban systems and residents, particularly when coupled with global warming:
• Human Health: Increased urban heat contributes to heat stress and exacerbates heat-related illnesses. Hot extremes have intensified in cities, causing adverse impacts on human health.
• Infrastructure and Services: The heat affects key infrastructure, including transportation, water, sanitation, and energy systems. Increased demand for cooling leads to power grid strain and can cause overloads and outages.
• Quality of Life: The UHI effect adversely impacts public health, economic productivity, energy consumption, and the overall quality of life for city residents.
• Vulnerability: Observed adverse impacts are concentrated among economically and socially marginalized urban residents, often suffering more from heat waves due to poor-quality housing with limited or non-existent cooling. The highest vulnerability is often concentrated in the dense, central business districts and historical centers of cities.
The UHI effect is a key heat contributor, alongside climate change and weather, that urban resilience strategies must address.
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