The urban boundary layer in Montreal

Horizontal and vertical sampling of the atmosphere has provided new information on the form of Montreal's urban heat island. The horizontal pattern under clear skies with light winds shows a major heat island, with marked gradients at the periphery, and a multicellular inner core. Retarded urban cooling rates in the evening yield a maximum heat-island intensity around midnight. Combined horizontal and vertical temperature surveys show that under conditions of strong rural stability, the lowest layers of the urban atmosphere become progressively modified as air moves toward the centre of the city. The change in the form of the potential temperature profile is in good agreement with Summers' internal boundary-layer hypothesis. In Montreal differing heights of heat and SO₂ emission appear to produce more than one internal layer. SO₂ observations, and heat input calculations reveal two major emission sources in Montreal; one associated with an industrial complex, and the other with the downtown core.