The Climatology and Prediction of the Chicago Lake Breeze

Abstract

As part of an extensive mesoscale study of the Chicago lake breeze, ten summer months of data (1966?68) were used to compile a climatology of this phenomena and devise a simple forecasting technique. The lake breeze was found to occur at the Chicago shoreline on 36% of all days (out of a sample of 307). The time of onset ranged from before 0600 to 1&30 LST, with 0800?0900 LST being most frequent. The lake breeze front, defined as a narrow (1?2 km) band of convergence separating airflows with overland and overwater trajectories, was found to push inland anywhere from 1 block to over 40 km. Experience verified that light gradient winds, strong insolation, and land air temperatures warmer than the lake water surface were the favorable conditions for formation. However, in an attempt to arrive at more objective forecasting scheme, the work of Biggs and Graves was extended to Lake Michigan. A dimensionless number, σ = V2g/(Cp?T),is defined where Vg, is the geostrophic wind speed from the 1200 GMT surface chart, Cp the specific heat of dry air, and ?T the difference (°C) between the maximum air temperatures at an inland location and the mean water temperature of the southern basin of the lake. Used as a hindcasting technique, it was found that a critical value of 10.0 emerged, above which a lake breeze would not be expected to occur at the shoreline. A hindcast accuracy between 90 and 95% was obtained by applying several additional criteria regarding the amount of cloudiness and the direction of the gradient flow with respect to the shoreline.