And new research suggests that this heat build-up greatly depends on the layout and design of streets and buildings of each city.
While grid-like cities boast a "crystalline" texture, more chaotic cities have a "glass-like" texture.
The heat island effect has been known for decades.
As the earth continues to warm due to greenhouse gases, heat is expected to become more severe particularly for cities where urban building materials such as concrete and asphalt can absorb heat during the day and radiate it back at night.
The study found these differences in city patterns was the most important determinant of a city's heat island effect. The effect can be quite dramatic because it can add as much as 10 degrees Fahrenheit to night-time temperatures in places such as Phoenix, Arizona. They then applied the results to patterns of buildings determined from satellite images of 47 cities in the USA and other countries, which led to a straightforward formula to describe how a city's design would influence its overall heat build-up.
The research could help city planners design more energy efficient cities.
Scientists combined the structural and temperature data to determine the relationship between the two variables.
MIT researchers used mathematical models to analyze atomic structures in materials and located the positions of atoms within them. The cities varied from 0.5 to 0.9. In hot locations, cities could be created to minimize the extra heating, but in colder places the effect might actually be an advantage, and cities could be designed accordingly. "If you are planning a new section of Phoenix, you don?t want to build on a grid, since its already a very hot place".
The differences in the heating effect seem to result from the way buildings reradiate heat that can then be reabsorbed by other buildings that face them directly, the team determined. But as the latest research shows, the heat island phenomenon isn't uniform. "But somewhere in Canada, a mayor may say no, we'll choose to use the grid, to keep the city warmer", said one of the researchers Roland Pellenq from Massachusetts Institute of Technology (MIT).