The centre of a large city constitutes a “heat island”. The temperatures are highest near the centre of a city and diminish slowly towards the suburbs. In the central area of a metropolitan city the temperatures are 1 to 2 degrees warmer in the winter than the surrounding countryside.

The heat is generated by the concentration of objects and activities in the city, such as factories, motor vehicles, and homes. The surface materials such as brick, concrete, and pucca houses and streets of the city retain heat better than do the grass and fields in the rural areas.

Warm air cools as it rises from an urban centre and begins to circulate. An urban dust dome is created by dust, soot, and smoke carried by the wind. It retards the outflow of heat from the city, further enhancing the heat island effect.

The particulates in the dust dome produce a level of rainfall about 10 percent higher in the city than in the surrounding countryside. Cities also have 50 percent more fog in winter and 30 percent more in summer.

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Remember that a “heat island” has a shape and size that depend on the lay out of the urban area, buildings and industries. It results from urban heat generation and storage. The difference in temperature is greater at night than during the day.

Normally the heat islands are larger and more intense over big cities, the relationship is not direct. The heat island development is influenced by the spacing of buildings, and the various kinds of economic activities.

Thus, the intensity of a heat island depends on the density of buildings as well as that of population. Temperatures are normally higher where buildings are closely built and where there are narrow lanes and streets.