oing 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 extreme； in winter； busy streets in cities can be 17℃ warmer than the side streets。 Areas near traffic lights can be similarly warmer than the areas between them because of the effect of cars standing in traffic instead of moving。 The maximum differences in temperature between neighboring urban and rural environments is called the heat…island intensity for that region。 In general； the larger the city； the greater its heat…island intensity。 The actual level of intensity depends on such factors as the physical layout； population density； and productive activities of a metropolis。
The surface…atmosphere relationships inside metropolitan areas produce a number of climatic peculiarities。 For one thing； the presence or absence of moisture is affected by the special qualities of the urban surface。 With much of the built…up landscape impenetrable by water； even gentle rain runs off almost immediately from rooftops； streets； and parking lots。 Thus； city surfaces； as well as the air above them； tend to be drier between episodes of rain； with little water available for the cooling process of evaporation； relative humidities are usually lower。 Wind movements are also modified in cities becau