A computer model of windpower flux was developed for Lake Mendota, Madison, Wisconsin, based on 3‐h wind speeds and estimates of above‐lake bulk Richardson number, and then was applied to the period 1 April–31 October (1966–1975). Fifteen years (1962–1976) of hourly records were also analyzed for Simcoe, Ontario (Lake Erie Basin).Expected power varies diurnally during all seasons, in phase with, or lagging slightly behind the solar cycle. The diel amplitude is about 46% of the expected daily mean. At both Simcoe and Madison, the expected daily power above an ice‐free lake attains a broad maximum of 12–14 × 105 ergs·cm−2·d−1 in winter and early spring, drops sharply in late April, and shows a broad summer minimum of 3–4 × 105 ergs·cm−2·d−1 during July–August. The autocorrelation functions for both spring windpower and daily insolation show evidences of periodicity of about 7 days, probably related to synoptic‐scale cold front passages. Following the front, with its characteristic wind directional shift from SSW to NNW, a characteristic calm develops accompanied by intense solar radiation.The spring power histogram features a median value about half of the mean, because of the high probability of observing very low windpower coupled with a very long tail on the frequency distribution. Low power also shows a strong clustering tendency in spring, because of both the strong inversion conditions in that season and the period of about 7 days in the synoptic‐scale weather shifts.