Quantification of the influences of wind and cloud on the nocturnal urban heat island of a large city

Analyses taken over all observed weather conditions of daily 0600 EST clima te data from a network of monitoring stations in and around the large city of Melbourne, Australia, revealed a 20-yr mean urban heat island (UHI) valu e of 1.13 degreesC. The UHI varied seasonally between summer (1.29 degreesC ), spring (1.25 degreesC), autumn (1.02 degreesC), and winter (0.98 degrees C). Investigations undertaken with daily wind speed and cloud amount data e nabled a detailed investigation of the relative importance of factors such as the turbulent and radiative exchanges on Melbourne's UHI. Analysis of va riance and regression techniques were used to explore these processes and t o predict the behavior of the UHI in numerical terms for mean seasonal and annual periods between 1972 and 1991. Over the 20-yr period, analyses of th e association among Melbourne's UHI, wind, and cloud revealed that the UHI was inversely proportional to approximately the fourth root of both the win d speed and the cloud amount. This relationship explained more of the UHI v ariance during summer and the least variance during winter. Increases in th e amount of cloud cover and in the frequency of wind speeds in excess of 2. 0 m s(-1) resulted in a statistically significant (95% confidence level) re duction in UHI magnitude. The influence of wind in limiting Melbourne's UHI magnitude was greatest during clear to near-clear sky conditions. Similarl y increases in cloud were most restrictive to UHI development during calm t o low wind speeds. Unlike most previous studies, the linear regression anal ysis presented here revealed that cloud was more limiting than the wind spe ed to UHI development for all seasons except summer. Contour plots of the U HI are presented for the various associations between each category of clou d and wind. These plots enable a clear visual presentation of the most to l east favorable conditions for UHI intensity and development. The analyses i ndicate that low wind speeds and little or no cloud were typically associat ed with the largest UHI development. Eight octas of cloud and wind speeds i n excess of 5.0 m s(-1) were usually associated with modest (but still appa rent) UHI development.