Dynamics and controls of the near-surface heat island of Vancouver, British Columbia

Temporal aspects of the urban heat island (UHI) of Vancouver, British Columbia, 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. The largest UHI occurs 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 Bolt 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, which itself is controlled largely by soil moisture status. 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. Surface wetting caused by recent rain, fog, or melting snow also is found to reduce UHI magnitude. While not quantified, marine advective effects appear to modify the UHI, especially the summer daytime cool island.