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This paper examines the characteristics of the urban heat island (UHI) in a large Australian city (Melbourne) using local area weather data and the US National Centers for Environmental Prediction (NCEP) reanalyses data of mean sea level pressure (MSLP). Conventional methods to determine the UHI from climatological data often involve comparing minimum temperature data between urban and rural environments. The findings presented in this paper show that comparing simultaneous measurements of temperature at one urban and three nearby airport sites at 0600 (Australian eastern standard time (EST)) provided a better estimate of the UHI magnitude. Analyses of these data between 1973–1991 were grouped according to the daily UHI magnitude. For each UHI group of the daily data, mean anomalous synoptic conditions from the 19‐year mean monthly MSLP were examined for the influence of the different MSLP conditions on varying UHI magnitudes. These synoptic conditions included the entire range of weather conditions over the study period. Over the 19‐year period, daily analyses of the regional climatological 0600 EST temperature data revealed a UHI between −3.16° and 6.0°C. The reanalyses of the NCEP MSLP data, in association with the local area climatological data, suggest that statistically significant anomalous anticyclonic conditions were associated with the warmest 17% and coolest 1% of UHI events. The position of the centre of the anticyclone was critical to UHI genesis and development. Statistically significant mean low pressure anomalies were associated with UHI values between 0° and 1°C. These occurred on 40% of the days between 1973–1991. Melbourne's urban area, independent of the topography, was found to inhibit early morning advection events of warm continent air and result in an urban cool island (UCI). Copyright © 2000 Royal Meteorological Society

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