Cooling rates, sky view factors and the development of intra-urban air temperature differences

Intra-urban cooling rates were monitored for a year in the centre of Goteborg on the Swedish west coast. Five sites with different building geometry - from dense canyon to open space - within a radius of 300 m were analysed. Results showed two modes of cooling during the night. In the first mode, the cooling was site-dependent the open space cooled the most and the dense canyon the least. In the second mode, which began about 3-4 hours after sunset, all sites cooled at the same rate. Our hypothesis is that in the early part of the evening both radiative divergence and sensible heat dominate the cooling. However, as the evening progresses, a spatially homogeneous inversion is established which controls the radiative cooling, making the cooling independent of both surface geometry and surface type. From April to November the sites cooled rapidly around sunset and the cooling slowly decreased during the rest of the night. However, between December and March, the cooling was less intense, with an almost constant rate during the entire night. It has been suggested that this might be the result of the draining of stored solar heat during the summer and a continuous flux of anthropogenic heating during the winter.