Urban areas can get really hot compared to rural areas, creating what's called an Urban Heat Island effect. This effect is usually stronger at night or in winter. When you have mountains around, the heat from the city can either get trapped or escape more easily. In a valley, temperature changes are bigger than on flat ground, which can make the Urban Heat Island effect stronger or weaker. Scientists used computer simulations in Grenoble, France, to study pollution in February 2005. They found that the Urban Heat Island effect was about 5 degrees hotter at night and 3 degrees hotter during the day. They also used radon measurements to see how the heat from the city affects local pollution.