Urban building materials absorb heat during the day and radiate it at night, to a much greater extent than vegetation does. This, along with waste heat, creates a heat island effect that raises city temperatures above rural ones, particularly at night.

To come up with the result, the team looked at 47 city layouts from satellite images, using mathematical models designed to analyse atomic structures. Each city received a ranking for its local order parameter, which ranges from zero for total disorder to 1 for a perfect "crystalline" structure. The parameters for the cities varied from 0.5 to 0.9.

City structure appears to affect the heat island effect as buildings can reabsorb heat radiated out by buildings that are directly opposite them.

The findings could be useful for building new cities or expanding existing ones. "If you're planning a new section of Phoenix," said Roland Pellenq of MIT and France's National Center for Scientific Research, "you don't want to build on a grid, since it's already a very hot place. But somewhere in Canada, a mayor may say no, we'll choose to use the grid, to keep the city warmer."

In the state of Florida, the team estimated, urban heat island effects cause around $400 million in excess costs for air conditioning. Although a grid pattern makes it easier to plan utility lines, sewer and water pipes, and transport systems, the heat savings from a less organised city structure could be worth the extra complications in hot areas.

Research on construction materials may also help manage heat interaction between buildings in cities' historical downtown areas, according to the team.

The researchers published their results in Physical Review Letters.