The lake effect occurs when dry air mass moves over a large body of water. When there's a dry air mass, be it warm or cold, and it moves over some large lake, thus large body of water, it changes its properties. The dry air mass picks up a lot of moisture while passing above the large body of water. As it does that, the air mass becomes wet. As it continues to move, this air mass will eventually come upon a natural barrier which will make it accumulate and go up, at certain area, the water vapor to condensate, and lot of precipitation to occur. Because this process will be happening constantly, the area will be receiving solid amount of precipitation, though the side of the water body from which the dry air mass is coming will be relatively dry.
