According to the passage, what is the main reason cities are warmer?

Cities are often warmer than the countryside around them. Such a warm patch is called an urban heat island. By day, a city may be about 1 to 4 degrees Celsius warmer than the land around it, and at night about 1 to 3 degrees warmer.

The main reason is what cities are made of. Buildings, roads and other structures soak up the Sun's heat and give it off again far more than fields and forests do. On a warm day, an ordinary roof can be as much as 37 degrees Celsius hotter than the air. Trees and water cool the air with their shade and by giving off water, but a city has fewer of them. Tall buildings close together form "urban canyons" that block the wind and trap heat in the narrow streets between them. And cars, air-conditioners and factories add heat of their own.

The extra heat has a cost. People use more electricity for cooling: for a rise of about one degree Celsius, the electricity used for air-conditioning can grow by 1 to 9 per cent. Heat also speeds up the formation of ground-level ozone, a harmful pollutant, and heat islands add to illness and death in hot weather.

Cities can fight back by planting trees and other greenery, and by building green roofs, covered in plants, and cool roofs.