Late-October Lake Breezes in Southern Ontario

On October 30, 1998, lake breezes were detected by surface observations along the northern shores of Lakes Ontario and Erie (map locations shown on satellite image below). The Lake Ontario lake breeze was detected in the Toronto area by surface observations at Toronto Island. The surface station at Burlington, roughly 40 km to the southwest of Toronto on the Lake Ontario shore, also measured changes due to a lake breeze. The Lake Erie lake breeze affected the surface station at Erieau.

The graphs shown below compare basic meteorological parameters at Toronto's Pearson International Airport and Toronto Island. The lake breeze arrived at the Toronto Island station between 11 EST and 12 EST. The wind at this station began to back to southerly and the temperature decreased slightly at this time. By 18 EST, the lake breeze was no longer affecting the Toronto Island site. In addition to significant differences in wind direction and temperature, the graphs show large differences in wind speed and dew point temperature between 12 EST and 18 EST.

The following map shows the synoptic conditions at 20Z or 1500 EST on October 30.

A ridge of high pressure was extending southward into the Great Lakes region while a weak low pressure system was moving to the south of the region. Skies over the area were mainly clear near Toronto and partly cloudy near Erieau as indicated by the following GOES-8 satellite images valid at 2015 UTC or 1515 EST.

From this image, it appears that lake breeze fronts resulted in no cumulus clouds or at least clouds too small to be detected by satellite. Doppler radar data from the King City radar approximately 40 km north of Toronto (not shown) also failed to indicate a lake breeze front (fine line signature).

Due to the absence of a lake breeze signature in the remote sensing data, I decided at 15:30 EST to investigate the degree of inland penetration by vehicle. I travelled south from the King City radar facility toward Toronto on Highway 400. Upon reaching the city limits, there appeared a distant haze to the south. Flags were showing a wind direction near NNW with a speed of roughly 10 knots.

At roughly 7 km from the lakeshore, I could visibily detect a change in moisture content ahead. As I drove a few hundred metres farther south, a flag indicated that the wind had shifted to southerly and decreased somewhat in speed. No clouds were visible. However, a layer of 'dirty' air was apparent over the downtown Toronto area. Closer to my home near the lakeshore, I could see my breath like it was mid-winter. The home thermometer showed 6C at 1700 EST. At 17:50 EST, it had become dark, the wind had shifted to the NNW at about 10 knots, and the temperature had increased to 8C. I could no longer see my breath. It appears that the lake breeze either dissipated or was forced to retreat by the moderate NNW winds.

Each autumn, Great Lakes water temperatures remain relatively warm while daytime inland air temperatures begin to decline. Lake breezes are uncommon by late autumn since the positive land-lake air temperature difference required to drive the circulation is usually absent. Satellite infrared measurements of lake surface water temperature for this day are shown below. 

Lake Ontario surface temperatures near Toronto ranged from near 9C near the lakeshore to 13C nearer to the centre of the lake. Lake Erie surface temperatures near Erieau were around 15C. Buoy water temperature measurements in these areas (not shown) were within 1C of these values. The temperature at Pearson Airport when the lake breeze was first detected at Toronto Island was near 9C. Simlarly, inland temperatures north of Lake Erie (not shown) were significantly less than the lake surface temperature. Why, then, did lake breezes develop, especially considering the moderate strength of the northwesterly wind?

Buoy observations from the western basin of Lake Ontario at 1100 EST gave a lake surface temperature of 11.8C and an air temperature of 6.9C (the wind was out of the NE at 11 km/h indicating that the lake breeze may have already begun to affect this station). Clearly, the air temperature over the lake was much ( 4.9C ) less than the water temperature. If one were to calculate the land-lake temperature difference using the lake air temperature rather than the lake water temperature, then on this day the land air temperature was several degrees warmer than that over the lake and a lake breeze was possible. The traditional method of calculating the land-lake temperature difference using the lake surface water temperature would discount the possiblity of a lake breeze at 1100 EST.

This phenomenon in which the lake air temperature is significantly cooler than the lake surface temperature is occasionally observed with differences reaching up to 10C. I have evidence that it occurs during the summer as well as the fall. I am not aware of its cause or of any research investigating the differences between lake air and water temperatures. If anyone is aware of such research, please let me know. Clearly, this phenomenon is critical to an understanding of the occurrence of late-autumn lake breezes.