A model is presented to calculate daily totals of global solar radiation. First, cloudless sky radiation is evaluated from transmission due to absorption and scattering. Cloud effects are added using cloud layer transmission. A simple expression to account for additional radiation due to reflection between the ground surface and cloud bases is also included. Atmospheric transmissions are obtained from previous studies. Precipitable water and observations of cloud type and amount for different layers in the atmosphere are the only meteorological variables required. The model is evaluated and tested with data collected at stations in and around Lake Ontario: a lake station near Grimsby in 1969 and land stations at Burlington, Scarborough, Peterborough, Trenton and Kingston, Ontario in 1972 and 1973 during the International Field Year for the Great Lakes. Good agreement between calculated and measured radiation was obtained at all stations, particularly for 5- and 10-day means. Model performance was largely independent of both cloud amount and season.