In a city, air temperatures are often as much as 3–4 °C higher than over open country. Causes include: the presence of large vertical faces; a reduced sky-view factor; surface materials of high heat capacities and conductivities; large areas of impermeable surfaces; anthropogenic heat fluxes; and atmospheric pollution (Grimmond (2007) Geog. Rev. 173, 1); for more detail, see Yow (2007) Geog. Compass 1, 6.