Can a cloud ever be stationary with respect to the ground surface? For some reason (experience, logic?) it seems to me such case is not possible. I read that stationary fronts are defined as two air masses such that
neither air mass is strong enough to advance into the other at least 5 knots (about 6 miles per hour or about 9 kilometers per hour) at the ground surface
But this is not referring directly to clouds' speed, nor discussing the particular case of speed equal to 0.
There are the lenticular clouds, which are defined as stationary, but it is not clear to me whether such stationarity is strict (0 knots per hour) or rather a very low speed relative to "normal" clouds.
Anyone can provide insights on this?