What is an urban heat island?

In a city, air temperatures are often as much as 3-4 °C higher than over open country. These higher temperatures are generated by the combustion of fuels in factory, heating, and transport systems, and, more importantly, the release at night of heat which has accumulated during the day in the fabric of the city, for the bricks and concrete of the buildings act as enormous storage heaters. This effect is compounded by air pollution, which reduces night-time terrestrial radiation, and by the low humidity which results from the lack of vegetation. A urban heat island is developed during calm conditions; winds disperse heat.