as by cars； is not the only source of this increased heat。 Two other factors contribute to the higher overall temperature in cities。 The first is the heat capacity of the materials that constitute the city； which is typically dominated by concrete and asphalt。 During the day； heat from the Sun can be conducted into these materials and stored — to be released at night。 But in the countryside materials have a significantly lower heat capacity because a vegetative blanket prevents heat from easily flowing into and out of the ground。 The second factor is that radiant heat ing into the city from the Sun is trapped in two ways： by a continuing series of reflections among the numerous vertical surfaces that buildings present and by the dust dome the cloudlike layer of polluted air that most cities produce。 Shortwave radiation from the Sun passes through the pollution dome more easily than outgoing longwave radiation does； the latter is absorbed by the gaseous pollutants of the dome and reradiated back to the urban surface。
Cities； then； are warmer than the surrounding rural areas； and together they produce a phenomenon known as the urban heat island。 Heat islands develop best under particular conditions associated with light winds； but they can form almost any time。 The precise configuration of a heat island depends on several factors。 For example； the wind can make a heat island stretch in the direction it blows。 When a heat island is well developed； variations can be extre