A very thorough post from Neru.
But, as others have pointed out, wrong answer. Any relation to the ground of either clouds or aeroplane is irrelevant; the aeroplane flies only in the air immediately surrounding it. That airmass may be doing 5kts or 500kts (relative to the earth) in any direction. But as far as the aeroplane is concerned, it is merely pushing (or being pulled by the prop!) through that airmass at the IAS resulting from the thrust/drag balance.
Consider standing on the ground as a 500kt airmass passed overhead with Elfatness sitting in his C150 flying in said airmass. He may look like (depending on which way the cesspit in pointing) he's flying at 600kts! But, on his ASI, it reads 100KIAS, because it is reading the relative windspeed past the pitot head. Doesn't matter which way he's actually pointing travelling (as long at he doesn't turn too tight
). It's the relative windspeed that determines the speed of the clouds whizzing by in that airmass. The speed, or direction, for that matter, over the ground, of the whole airmass, with C150 in it, is irrelevant.
Another example. Fill up one of those half seashell tubs with water. Now, build an island in the middle (not too big, you have to pick this thing up soon). Get a motorised boat and set it to motor around the island. Boat is observed passing island at a couple of knots. Boat speed relative to the ground the tub is on is also a couple of knots.
Now, pick up tub and walk around backyard. Tub is now doing 5 knots in various directions! But, relative to the island
in the tub, the boat is still only doing a couple of knots. Even though the water mass is moving relative to the earth, the boat still only does a couple of knots relative to the island. Sama Sama a moving airmass with aeroplane in it.
