I got absolutely whacked by the wake from a Dash-8 the other day.
On finals I was asked to orbit to let him pass, which I did, I carefully maintained altitude and came back behind and above him, reasonably close but not worried as he was going 2x my speed and was by then well below me. Knowing that wake descends at something like 300' per minute (correct ?) I was quite sure that I could not meet the wake from an aircraft below me. Pretty dumb really, of course he was descending faster than his wake! So I learned that it is perfectly possible for an aircraft's wake to be above it, if it is descending!! I don't recall the books mentioning that. Lesson learned, bigger orbit next time.