It can come from the top, but in most cases, the serious updraft from the bottom is the main cause. If you put the big one back on, get up to about 55mph and hold your hand palm toward windshield, and move it around behind the windhield. You will see where the major sources of turbulence is coming from. In my case, it was coming from right at the bottom corners and shooting up both sides of my body up to my helmet.