This paper attempts to estimate over-water air temperatures and dew-point temperatures between about 1 and 15 m above Lake Ontario in winter. Multivariate statistical techniques are used to formulate regression models incorporating readily determined variables such as over-land temperatures, surface water temperatures, wind speed, and over-water fetch. Seventy-one to 92 percent of the variance can be accounted for by the resultant models with average standard errors of 1.1°C for air temperature and 1.4°C for dew-point temperature.

More than half of the modification in the lower atmosphere was found to occur in the first 10 min or over the first 1–2 mi of open water. Usually the change in air temperature from land to lake seldom exceeds 60 percent of the potential because it is limited by surface water temperature and barely reaches 40 percent with very cold air.

For selected winter weather situations, the spatial variations of temperature, humidity, and sensible and evaporative fluxes are examined. Bowen ratios of almost 2.0 were calculated for both warm air and cold air advection across Lake Ontario.