The
cold air that passes over the lake is much colder than the lake itself.
As a result the lake water heats the air passing over it and causes it
to rise, making the lake effect snows (there are a couple other factors
involved, but I'll keep it simple). In summer the air is typically
warmer than the lake waters, thus the instability is not created since
there is little if any heating of the air passing over the water.
