This effect is more pronounced in bays and at the extremities of the lake, where the impelled water is concentrated in a small space by converging shores, especially if coupled with a gradually sloping inshore bottom which even further reduces the flow of the lower return currents. This condition is very pronounced at Green Bay Harbor. Northerly winds cause this on the S part of the lake and southerly winds have the same effect on the N part of the lake. They raise dangerous seas and generate hazardous currents at harbor entrances. Winds with southerly components are prevalent during the entire navigation season.
Northerlies are a little less frequent, but are common, particularly in spring.
The sea conditions are worst in October and November, when, lakewide, wave heights of 5 to 10 feet are encountered about 35 percent of the time. In October, S through SW winds are most often responsible, while by November W through N winds often generate rough seas. Seas of 10 feet or more are encountered 3 to 5 percent of the time from November through March. Extreme waves of 20 to 22 feet have been encountered. By August, the fall buildup begins.