What can cause a big city to be warmer than the surrounding countryside?

There are many people in the city and so much traffic that it pollutes the air and makes it warmer where the country dosent have much traffic so there for its cooler

In cities, a phenomenon known as the "urban heat island effect" causes air temperatures to be warmer than in nearby suburban or rural areas and

The smoothness of the landscape and the local climate-not the materials of the concrete jungle-govern the urban heat island effect, a new study finds