The bigger a city, the hotter it gets. In cities with more than a million inhabitants the temperature can be up to 12 degrees celsius higher than in smaller urban areas.

The extra energy needed to cool buildings in large cities is tremendous - up to five to 10 per cent of all electricity consumed during the summer months.

Much of the sunlight that reaches New York hits the city's roofs, and the darker the roof the more heat it absorbs.

On a 90 degrees fahrenheit day, a black roof reaches a scorching 180 degrees fahrenheit, but a lick of specially designed white paint brings the heat down considerably, to 100 degrees fahrenheit.

By painting New York's roof tops white, the White Roof Project aims to cool the city and save energy, all with a simple paint job.