Urban growth effects on low-temperature fog in Edmonton

Abstract

Visibility observations at rural and urban airports in the Edmonton area during the severe winter of 1968–69 are compared with similar observations described in 1955 by G. W. Robertson for the winter of 1949–50. The new observations tend to confirm Robertson's finding that low-temperature fog in Edmonton is due mainly to saturation of the air by water vapour produced in burning natural gas. Visibilities in such fog are strongly temperature-dependent. The urban population more than doubled between 1950 and 1969, and new large sources of water vapour were added to the city. The effects of both changes are apparent in the visibility data. Computations of excess water content and temperature increases are made assuming that an isothermal layer develops as rural air passes over the city and is modified by the combustion of natural gas and motor vehicle fuel. The results are consistent with local measurements of surface temperature and vertical temperature gradients in the lowest 100 m over the city. Computations of excess water content are presented for fuel consumption rates corresponding to populations of 150000 to 800000. Measurements of the thickness of the fog layer over the city and of the frozen water content of ice fog for various visibilities are needed for additional verification of the calculations.