A heat island is the hotter area of the city surrounded by cooler urban fringe and rural areas
The higher urban temperature causes an urban heat island to develop over the city.
This heat island reaches its greatest intensity above the city center and diminishes towards the suburbs.
Within this heat island, the air is less dense and begins to rise. Cooler air from the suburbs moves in to replace this.
Urban heat islands exist throughout the year, but their effects are more pronounced at night and in the winter when temperatures are lower. This is also the situation when there is anticyclonic subsidence of air.
The following are the effects of urban heat island
- increase human discomfort especially in summer
- reduce ice and snow in winter, which is positive
- increase biological activities, for example, growing season length, which is positive
- cause heat stress and death during heat waves
- increase conditions suitable for the spread of diseases caused by insects such as mosquitoes and fleas
- increase smog
- reduce visibility due to smog, particulary in winter
- increase air conditioner use leading to release of more heat and green house gases, which degrade local air quality
Features of urban heat island during the day and night
During the day
- heat result in increased turbulance, thus a relative decrease in the stability of the lower atmosphere
- heat island grow vertically and therefore is less concentrated
- pollution os dispersed over a great area
During the night
- as it is cooler during the night the height decreases and is more concentrated
- the ‘heat bubble’ is now denser and shallower and takes on the dome shae due to cooling of air
- pollution is concentrated in the dome
How can the heat island effect be reduced
the reduction of urban heat islands depends on a number of factors. some of them are within and some outside the community control.
climate conditions and the relief can not be changed. decision-makers can select energy-saving strategies that will create a healthier and more sustainable environment.
creating urban green spaces like parks and roof gardens, appropriate building design, and encouraging the use of public transport can contribute to reducing the effects of the urban heat islands.