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Variability of urban fog during the last 50 years in Anhui Province (China) and its possible underlying mechanisms are analyzed based on daily meteorological records from 78 stations. The effects of urban development and aerosols on fog formation are discussed through an analysis of trends in fog frequency and comparison with a number of meteorological parameters, coal consumption and vehicle usage as indicators of economic development and aerosol production, and visibility as an indicator of aerosol load. The impact of urbanization on fog is different at various stages of urban development. The number of annual fog days in most cities studied has increased since the 1960s but decreased after the mid 1980s in large, old cities. New cities, on the other hand, are characterized by still increasing fog frequency. During the last thirty years, fog dissipation time occurred later, average fog duration increased and visibility decreased at most urban stations. The annual total coal combustion of Anhui Province correlates negatively with the average visibility within fog and average annual fog frequency in old cities. The difference in minimum temperature between cities and towns is negatively (positively) correlated with fog frequency in old (new) cities. Our study therefore supports previous findings that the number of fog days in cities is influenced by increased (i) temperatures associated with urban development and (ii) aerosol concentration due to urban development and industrial activity. [Copyright &y& Elsevier]