Urban Heat Island Effect

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“ An area of higher temperatures in an urban setting compared to the temperatures of the suburban and rural surroundings. It appears as an ‘island’ in the pattern of isotherms on a surface map”. -Glossary of Weather and Climate, Ira Geer, Ed.

History

The phenomenon “Urban Heat Island” was first investigated and described by Luke Howard in the 1810s, although he was not the one to name the phenomenon.

Heat Island (HI)

Heat islands develop when a large fraction of the natural land cover in an area is replaced by built surfaces that trap incoming solar radiation during the day and then re-radiate it at night (Quattrochi et al. 2000; Oke 1982). This slows the cooling process thereby keeping nighttime air temperatures high relative to temperatures in less urbanized areas. This increase in urban air temperatures as compared to surrounding suburban and rural temperatures is referred to as the heat island effect. Heat islands of varying extent and magnitude have been observed in most urbanized areas in the world.

Sketch of an urban heat island profile.

Background

The obeservational studies (observation of temperature data) have shown that the climate of a city or an urban area is significantly different from that of the surrounding rural areas. The differences are due to four factors. First, urbanization changes the thermal properties of the earth surface. For example, areas of concrete, asphalt and other building materials replace vegetation. The result of these changes is a higher surface temperature of the urbanized area. Second, buildings and other urban structures in the urban area are obstacles to the wind. The obstacles alter the general flow patterns and generate air turbulence. Third, urbanization changes the moisture conditions at the earth surface. The relatively moist vegetated surface of the original rural area is replaced by a dry surface. Very little evaporation occurs from the surface which results in a drier and warmer surface air over the city. Last, the urbanized area emits heatand pollutants to the atmosphere. As a consequence of the abovementioned factors, the urban area becomes warmer than the surrounding rural areas and the so-called urban heat island develops. The urban heat island is best developed when the prevailing winds are light and the skies are clear. During such conditions, the urban-rural temperature difference may reach values as much as 10°C. However, the average difference for all types of prevailing weather conditions is much smaller, with an average of about one to two degrees.

Urban Heat Island Complexities

a. Hourly and seasonal variability

b. Dependence on large scale weather conditions

c. Surface UHI verses air temperature UHI

d. Geographic/Topographic causation

e. Importance of other weather variables

Impacts of UHI

Impacts of UHI

Air Quality

Ozone concentrations are affected by emissions, mixing and dispersion chemical reactions in the atmosphere, which are, in turn, affected by the UHI.

a. Hydrocarbon emission from plants: Hydrocarbon (monoterpene and isoprene) emissions increase by 10% per°C.

b. Emission from human-related sources: Power plant emissions increase 5% per deg celcius (in summer). Motor vehicle running losses increase by 7-14% per°C. Fugitive emissions at fueling stations increase with temperature.

c.Pollutant mixing and dispersion: UHI raises mixing height. UHI creates an “urban plume”.

d.Atmospheric chemistry: Rate constants for many key reactions in the formation and destruction of ozone are temperature dependent. On hot summer days peak ozone concentration increases by 2 to 4% for each°C increase in air temperature.

Energy Consumption

a.Electric Utility Demand Depends on Air Temperature.

Human Health

a.Heat-related mortality: Mortality rate depend upon maximum or the minimum temperatures and humidity.

b.Heat related morbidity: Heat stroke, respiratory diseases.

Effects of urban heat island phenomenon

Effects in summer

Discomfort has been growing due to the increase of nights which minimum temperature is above 25℃ and higher temperature in daytime. It is reported that there is correlation between number of death and number of days which maximum temperature is above 30℃ and nights which minimum temperature above is 25℃.

The rise of temperature increases the demand for air conditioning and energy use. It is a vicious circle that increase of anthropogenic heat emission causes additional rise of temperature. It is also reported that urban heat island phenomena increase photochemical oxidants, and UHI is correlated to frequency of heavy rainstorm.

Effect in winter

It is reported that up-current of air by urban higher temperature is blocked by an inversion layer and then formulates a dust dome.

Causes of UHI

Causes of urban heat island phenomenon are:

  1. Anthropogenic heat emission by human activities of air-conditioning, electrical appliance, burning equipment, automobile and so on.

  2. Expansion of hard surface and reduce of green land and water and increase of buildings and road surface. Urban surfaces do not reflect much solar radiation (they gave low albedo) and so they heat up. Urban areas lack vegetaion and moisture, so they cannot cool themselves through evaporative cooling.

Albedo:

It is the diffuse reflectivity or reflecting power of a surface.It is defined as the ratio of reflected radiation from the surface to incident radiation upon it. Being a dimensionless fraction, it may also be expressed as a percentage, and is measured on a scale from zero for no reflecting power of a perfectly black surface, to 1 for perfect reflection of a white surface.

Albedo

Reflection of radiation

Global albedo Image

Mitigation Strategies of UHI

Urban heat island mitigation strategies will become essential for the future of urban development. It may become necessary to apply these methods to reduce the harmful impacts of increasing temperatures and the urban heat island.

Direct and Indirect Mitigation Strategies

Improvement of urban structure

In order to mitigate decrease of evapo-transpiration and the high temperature rise of the ground, which were caused by reduction of green space and water and expansion of impervious surface such as buildings and asphalt, the following measures for the improvement of urban surface, shall be carried out.

・ Facilitation of tree-planting in private houses, buildings and their sites provides evapotranspiration.

・ Promotion of tree-planting in public facilities such as government buildings

・ Construction of urban parks and promotion of tree-planting in public spaces such as ports, airports and sewage plants

・Green roof systems: Improve building energy performance. Cool rooftop surfaces. Have other environmental benefits.

・Porous pavement systems: Reduce runoff, Provide evaporative cooling benefits.

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Improvement of urban structure

In order to ensure wind flow through green spaces and water, following measures for the improvement of urban structure shall be carried out.

・ Development of network with green and water in the region across the prefectures by construction of large green spaces and making up the linkage between parks, rivers and roads

・ Feasibility study on the urban heat emission treatment system which carry heat from the very densely build-up area to rivers and sea through under-ground pipes with circulating water

・ Facilitation to utilize the city planning system to develop “less environmental burden cities”.