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Direct observations of urban surface temperature in daytime are made using a thermal scanner mounted in a helicopter over three land-use areas in Vancouver, B.C. The results reveal strong directional variations in the observed apparent surface temperature. The variations arise due to the differential patterns of irradiated and shaded surfaces within the sensor field of view created by the three-dimensional urban surface structure and the position of the Sun. The directional variations may be considered to be a form of effective anisotropy due to the large scale roughness of the urban surface. Variations in excess of 9 C were measured over a downtown area. Urban residential and light industrial land-use areas also exhibited strong effective anisotropy. The directional temperature variations are of similar magnitude to atmospheric corrections applied to thermal imagery. This implies that effective anisotropy should receive serious consideration in the interpretation of thermal imagery obtained over urban areas. The scale dependence of the effective anisotropy is discussed.