The temperature differentials between urban and rural areas, occurring on the ground, at the surface, and at various heights in the air. The UHI may actually be described in multiple ways with various methodological approaches to investigate each type, and is divided into: the subsurface heat island (Allen et al. (2003) Geothermics 32 state that shallow aquifers existing below many northern European cities have groundwater temperatures 3–4 °C higher than in surrounding rural areas) and urban heat islands in the air. Each city’s unique land cover patterns will influence the spatial character of its UHI; see Roy et al. (2011) Phys. Geog. 32, 2, 114. Spatial and temporal patterns of CO₂ concentrations are useful in understanding urban climate; see Wentz et al. (2002) AAAG 92, 1.