Flying a single seat modern glider with a 15 metre wingspan, bumpy air below cloud base is usually indicating a thermal. Rising air may continue up into the cumulus cloud, and if it is interesting weather, the bumps may form cu nimb, or a thunderstorm, with tops up to 36,000 or so.
But the nastiest bump I ever met in my glider was in clear air over Scotland, near Aboyne, climbing in wave, at about 15,000 feet (using oxygen, of course). Evidently a system of weather traveling from the Atlantic, heading easterly, encountered a higher mass of clear weather moving from the Scandinavian area, heading west. I really thought the glider would break apart, it was so violent a wind shift. Changed my mind about going any higher that day.
I think any pilot meeting that wind shift would not have enjoyed the experience, whatever the size of his aircraft. Curlytips, have you ever met this kind of weather in a Boeing?