Urban Heat Island

Urban areas are warmer than rural areas and this is called the urban heat island. 
On average 1-2'c warmer per year than its surrounding rural areas. 
High pressure systems with clear nights and sunny days tend to increase the intensity. 
Heat islands are the result of five main factors:

1. Anthropogenic heat; heat released by human activity
2. Height and arrangement of buildings; urban areas have more vertical areas which causes insulation that can warm surfaces and cause an albedo 
3. Nature of building materials; concrete will warm more rapidly than mud, I'm cities the surface area exposed is larger so it is able to store the heat and release it slowly over the night
4. The presence of water; if water is present then most of the surplus heat is used as sensible heat to warm the atmosphere 
5. Presence of pollutants; pollutants have both a cooling and a heating effect, photochemical smog can alter the amount of energy flowing in and out of the urban area, dusty cities can reduce the amount of sunshine that reaches the city surface, however the pollutant dome can absorb heat and prevent some of it escaping