A number of factors contribute to the temperature difference. Urban spaces are more built-up and support less vegetation, so the air-cooling effects of water evaporation and transpiration by plants are reduced. Instead, rainwater is quickly funnelled off, frequently underground, and is therefore lost as a cooling agent. In addition, people in a city cook, heat or air-condition their homes and produce warmth in many other ways, which further increase urban surface temperature. Finally, rural areas tend to be more reflective, and therefore return the Sun's energy back to space more efficiently than cities. As a result, more heat is absorbed by urban artificial building materials, such as asphalt and concrete, than by rural natural vegetation. Again, temperatures in cities increase, compared with countryside in the same broad region.
Lei Zhao and colleagues (Nature http://dx.doi.org/10.1038/nature13462; 2014) now suggest that yet another factor dominates the urban heat island effect, at least in humid climates and during the day. In these regions, heat is released to the lower atmosphere through convection more efficiently over the countryside than over cities. This effect can contribute temperature gradients of around 3 °C. The researchers come to this conclusion by analysing climate model simulations and satellite data for 65 cities across North America and determining the contributions of the various factors, separately for day- and night-time.
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