Conservation of momentum doesn’t really come into it for this scenario. A balloon is a huge, low-density object so it’s basically always going to go at the same speed of the wind at whatever height it’s at.
The only way conservation of momentum could matter is if the balloon could get out of the atmosphere so it wasn’t being pushed around by the wind. But, of course, the way a balloon works is that it goes up as long as its density is lower than the density of the atmosphere it’s in. That means it’s always going to be stuck in fairly thick atmosphere, and always subject to the winds.
Conservation of momentum doesn’t really come into it for this scenario. A balloon is a huge, low-density object so it’s basically always going to go at the same speed of the wind at whatever height it’s at.
The only way conservation of momentum could matter is if the balloon could get out of the atmosphere so it wasn’t being pushed around by the wind. But, of course, the way a balloon works is that it goes up as long as its density is lower than the density of the atmosphere it’s in. That means it’s always going to be stuck in fairly thick atmosphere, and always subject to the winds.