Credit: © ANNE-MARIE PALMER / ALAMY

More than half of the Earth's population are city dwellers. This concentration of humans in a relatively small space brings a variety of challenges, such as the supply of food and shelter and the disposal of sewage and waste. In a warming world, an accumulation of heat within urban areas has become problematic too: temperatures in cities often exceed those in the surrounding countryside by several °C, a phenomenon termed the urban heat island effect.

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.

It turns out that the relative importance of convection efficiency depends on the local background climate, that is, whether a city is located in a humid or arid environment. In a humid setting, rural areas will be characterized by lush vegetation, which is aerodynamically rough, whereas townscapes are comparatively smooth. In an arid climate, by contrast, sparser shrub- and grasslands are typically aerodynamically smoother than cities, so that the difference in convection efficiency actually reduces the urban heat island effect. Interestingly, the direct release of heat from human activities is vanishingly small in the simulations, but the researchers concede that this effect may not be represented well in their model.

Humans suffer more heat stress under high humidity. In the more humid regions of the globe, a retreat to the countryside would seem to be a good idea when a heatwave is rolling in.