a few weeks back i flew a sail plane toward one which was about as fat as that, but not as tall, .. it appeared in front of me when i was about 3k feet just losing altitude to enter the pattern.
since it was already in my path, i wanted to see what it felt like as i got closer but i thought i'd have to be nearly on top of it to really feel it since the top of it was so far down. i was wrong. it was about 2000 feet in front of me, and the top was maybe 1000 below me when the turbulence began. as i turned the plane 180 degrees i got a nice bump that would have probably put a hole in the canopy if i wasn't strapped in.
it's funny how no matter how much you read or are taught about stuff like that, it will never teach you like 1st hand experience will