: Focused on temperature and humidity change
Urban Heat Island
"The relative warmth of a city compared with surrounding rural areas."
This relative warmth is caused by "heat trapping due to land use, the configuration and design of the built environment, including street layout and building size, the heat-absorbing properties of urban building materials, reduced ventilation, reduced greenery and water features, and domestic and industrial heat emissions generated directly from human activities.
Causes of the urban heat island
1.
Reduced Natural Landscapes in Urban Areas
Trees, vegetation, and water bodies tend to cool the air by providing shade, transpiring water from plant leaves, and evaporating surface water, respectively. Hard, dry surfaces in urban areas – such as roofs, sidewalks, roads, buildings, and parking lots – provide less shade and moisture than natural landscapes and therefore contribute to higher temperatures.
4.
Heat Generated
from Human Activities
Vehicles, air-conditioning units, buildings, and industrial facilities all emit heat into the urban environment. These sources of human-generated, or anthropogenic, waste heat can contribute to heat island effects.
2.
Urban Material Properties
Conventional human-made materials used in urban environments such as pavements or roofing tend to reflect less solar energy, and absorb and emit more of the sun’s heat compared to trees, vegetation, and other natural surfaces. Often, heat islands build throughout the day and become more pronounced after sunset due to the slow release of heat from urban materials.
5.
Weather and Geography
Calm and clear weather conditions result in more severe heat islands by maximizing the amount of solar energy reaching urban surfaces and minimizing the amount of heat that can be carried away. Conversely, strong winds and cloud cover suppress heat island formation. Geographic features can also impact the heat island effect. For example, nearby mountains can block wind from reaching a city, or create wind patterns that pass through a city.
3.
Urban Geometry
The dimensions and spacing of buildings within a city influence wind flow and urban materials’ ability to absorb and release solar energy. In heavily developed areas, surfaces and structures obstructed by neighboring buildings become large thermal masses that cannot release their heat readily. Cities with many narrow streets and tall buildings become urban canyons, which can block natural wind flow that would bring cooling effects.
The Result of Urban Heat Island (UHI) effects
The Urban Heat Island (UHI) effect directly influences temperature patterns within cities. Due to the urban heat island effect, the temperature rises as you move to the city center and the temperature decreases as you move to the outskirts. Urban areas absorb and store more heat during the day through surfaces like concrete and asphalt. This stored heat is released slowly at night, leading to higher nighttime temperatures compared to rural areas.
The UHI effect is more pronounced during summer due to increased solar radiation and reduced wind, which limits heat dissipation. In summer, UHI peaks as urban surfaces like asphalt absorb and slowly release heat, especially at night, with limited evaporative cooling due to reduced vegetation. This amplifies heatwaves and increases health risks.
Overall, UHI creates persistent warming throughout the year, requiring adaptive strategies like increasing urban greenery to mitigate seasonal heat.
UHI Index
UHI Intensity is defined as the temperature difference between an urban area and its surrounding rural areas at a specific time. It is typically measured by comparing the temperature of urban and rural regions, expressed in degrees Celsius (°C). This intensity varies depending on urban structure, green space ratio, population density, and season, and is typically more pronounced at night.
ΔTu−r=Tu−Tr
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ΔTu−r : UHI Index
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Tu : Temperature of Urban
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Tr : Average temperature at Rural