K.E. Runnalls and T.R. Oke
Physical Geography, 21, 283-304
Abstract
Temporal aspects of the urban heat island (UHI) of Vancouver, B.C. are demonstrated using differences of screen-level air temperature observed at an urban (downtown) and rural (farmland) site for three years. On an annual basis the UHI is at its maximum near the middle of the night and its minimum is in mid-afternoon. Growth of the nocturnal UHI is driven by rural cooling rates in the early evening which are much greater than the almost constant rates in the city. Growth starts earlier in winter. Largest UHI occur in the fall, and the smallest in the spring. In daytime there is often a ‘cool island’, especially in summer. There is an approximately inverse square root control of the UHI by wind speed and the effect of cloud type and amount follows the Bolz relation. Combining the two gives a ‘weather factor’ that is linearly related to maximum UHI magnitude. Seasonal variation of the UHI is shown to be inversely related to the thermal admittance of the rural site. This is done by calculating a ‘potential’ UHI that is free of weather effects; a value that approximately conforms to that predicted by the SHIM numerical model. External surface wetness is found to reduce UHI magnitude. Whilst not quantified, marine advective effects also appear to modify the UHI , especially the summer daytime ‘cool island’.