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This paper studies the urban heat island in Quebec City and its suburbs on one summer night. The region under study covers an area of 20 km by 20 km which streches from lake Saint-Charles on the north to Levis on the south and from the western suburb of Cap-Rouge to Beauport on the east. The total population of the region is about 400 000.On August 22, 1979 from 22 : 00 to 23 : 30 EDT, 109 measurements of wet-and dry-bulb temperatures were taken at preselected points. On that day, a large high pressure system gave clear skies to central Quebec while Quebec City Airport recorded a maximum temperature of 23 "C. The average wind measured at Duberger meteorological tower between 6 and 121 m above ground was 1.8 m SC' at 22: CO EDT and 0.9 m s-i at 23:30 EDT. Additional data were obtained from three thermographs installed at strategic points and five regular climatological stations, which were used to estimate cooling rates.During the period of measurement, the cooling rate averaged over the eight reference points Was about 0.7 'C h-'. However, the maximum cooling rate, which occured earlier, ranged from 2.2 'C h-i at Duberger to 4.4 'C h-' at Courville-de-Poissy.No relation seems to exist, in our case, between the maximum cooling rate and the heat island spatial structure. Comparison of these results with the ones given by Oke et al. (1972) shows that the eight stations behaved more like rural stations than urban ones.The temperature gradient measured between the center of old Quebec City and the suburbs averaged 6 -C while it was 9 'C for the coldest spot. Clo units were used to characterize the microclimates prevailing on the region for that night; 1 clo would be sufficient for comfort downtown while 1.5 clo is needed in the coldest spot for a slowly walking person. Large open areas like parks or the Lava1 University campus had a definite effect on temperature.The humidity was calculated for each point using standard psychrometric tables; no significant difference could be found over the area expect at points near or above the St. Laurent river where the average dew point was 1.5 "C higher.Each point was classilied into 6 categories according to its predominant land-use as reported by the observers. A multiple regression between temperature on the one hand and altitude and land-use on the other hand was tried; it showed that temperature was significantly correlated with both. Such an equation could be used by town-planners to locate temperature gradients in future urban development over the area; land uses could be planned to enhance or reduce these gradients.This study confirms the idea that urban climate is a mosaic of interacting micro-climates. More measurements, including possibly airborne infra-red thermal imagery, will bc taken during winter in order to assess the winter urban climate of Quebec City.

This paper describes the urban heat island in Quebec City on a typical clear winter night. This is a follow-up to a study made on a summer night in August 1980; the reader will find a detailed description of the region in Leduc et al. (1980). On February 25, 1980 at 21: 30 EST, nearly 200 measurements of temperature were taken at preselected points by 17 mobile observers; a mini-sonde was launched near the Duberger meteorological tower while upper air data were obtained from a sonde launched at Valcartier CFB. Additional data were available from the five regular climatological stations in greater Quebec City and from 2 other thermographs in order to evaluate cooling rates.On the day of the 25th, a developing low pressure area over the eastern coast brought cold air from the northwest toward Quebec City. On that day, the maximum temperature was -1 "C while the minimum on the following night dropped to -19 "C. At the time of the experiment, the sky was clear and the winds were westerly at 1.5 m s-' at the surface and northwesterly at 6 m s-l at 122 m. A strong inversion was present in the lowest 200 m while upper inversions were noted on the upper air sounding.As noted in the summer experiment, cooling rates, according to the Oke et al. (1972) criteria, reflected a rural behaviour at all stations equipped with a thermograph. It is to be noted that no data on cooling rates are available for the highly urbanized sectors.One notes a relatively warm zone in the downtown area where temperature reached -8 "C; there is also a cold zone, at the same location as during the summer experiment. The mean temperature gradient between the urban and rural sectors was 5 "C, which is comparable with that observed in August 1979 (6 "C). Thus, the urban heat island does not seem to be more intense in winter than in summer. Relatively large areas of uniform temperature were detected at places where the summer experiment indicated sharp spatial contrasts; this could be attributed to the presence of snow on the ground. The great number of measurements taken in the more urbanized sector revealed a complex spatial arrangement of smaller heat islands which are separated by boulevards or open areas (covered with snow), For the city dwellers, the temperature differences resulted in relatively sharp dontrasts in clothing requirements for comfort.Urban morphology was analyzed and a multiple regression model was formulated between temperature on the one hand and altitude, land use and distance to the warmest point, on the other hand. It shows that, for both winter and summer the most important factor in explaining the heat island is distance to the warmest point while land use comes second; distance explains the form and land use explains the line structure of the heat island.