The scariest looking clouds for me are the really tall and dark ones I see when I look out the airplane window of a commercial turboprop.

I was returning home with my boss once in a Dash-8 and immediately noticed when the pilot made a turn on a short hop... then another. I was in a window seat and I could see fairly well what was coming. I gulped my drink and said to my boss, "Better drink up. We are flying into a storm." He pooh-poohed that and about thirty seconds later was wearing his drink.