This is the time of year when water is evaporating off the surface of the Great Lakes. The highest rates typically occur in late summer/early fall when surface water temperatures are warmest but remain relatively cold compared to air temperatures. The timing of when the evaporation season starts depends on when the ice cover is gone from the lakes' surface.
Lenters, John D., John B. Anderton, Peter D. Blanken, Christopher Spence, and Andrew E. Suyker. "Assessing the Impacts of Climate Variability and Change on Great Lakes Evaporation." (2013).