Temperatures in cities are often higher than in surrounding rural areas, an effect commonly thought to be associated with lower evaporative cooling from urban land. But on page 216 of this issue, Zhao et al. report that the phenomenon is caused by variations in heat convection to the lower atmosphere (L. Zhao et al. Nature 511, 216–219; 2014).
The authors analysed temperature data for 65 North American cities using computational models. They found that, in humid regions, convection dissipates heat less efficiently from cities than over rural land, because there is greater aerodynamic resistance to heat diffusion. The opposite is true in dry regions.
Furthermore, the temperature difference between cities and rural areas in humid regions is even greater in dry years, probably because of feedback between precipitation and temperature. The researchers suggest that reducing anthropogenic heat won't ease city-dwellers' discomfort, but increasing urban albedo — heat reflectivity — will.