How high would a half deflated ballon float outside of a third floor? this a question that has been haunting me for many years now.
If I have a half deflated helium baloon that floats one meter above the ground on a third floor and I take it outside, will it float one meter above the ground (0 floors) or will it float one meter over a third floor level?
I understand that there might be currents and other elements involved that will make this measure not exact but my guess is that if it floats at the third floor level it means that it is influenced by the pressure of the air above the ballon while if it floats one meter from the outside floor level, then the pressure from the air below the ballon takes a part as well (which is harder for me to understand)
I actually just tried to test it but it was quite windy so the balloon just stayed horizontal to the rope that was holding it.
 A: It will float in that height in which its density is the same as the outside density (that's the reason why we can swim on the water surface while stones sink to the ground). So if the balloon floats 1 m above 3rd floor level in your appartement, then you take it through the stairs outside (at ground level) and let it go, it will rise again to the third floor.
A: If no part of the balloon or its string is touching the ground then you seem to be describing neutral buoyancy. This means that the total weight of the balloon, its gas contents, and any attachments is equal to the weight of the air it displaces. Neutral buoyancy is an unstable condition as weight to displacement must be exactly equal. Minor changes in any of the initial conditions may cause minor changes in density inside or out that could change its buoyancy to slightly positive or slightly negative so that it will rise or sink. But yes ideally, with all conditions the same, a neutrally buoyant object would remain at the same height above ground level whether inside or outside the building. However in reality,  inside and outside of the building will rarely have identical conditions, as you found in your experiment.
A: The giveaway in your question is the mention of the rope. An untethered balloon will rise because it is less dense than the air it displaces, and will do so until it reaches a point at which its density equals that of the air it displaces. The chances of that being a constant 0.5m above the ground for an untethered balloon are effectively zero.
What is actually happening is that the balloon is rising to the point at which its buoyancy is cancelled by the weight of the rope hanging beneath it (and possibly with a contribution from your weight if the rope is not entirely slack). That is what is causing it to stay around 0.5m above the ground. I predict that were you able to perform the experiment by faithfully replicating the indoor conditions outside, it would still only rise 0.5m outside, because the dominant factor is the rope, not the air pressure difference.
If the balloon was not encumbered by the rope, then it should float at the same height whether inside or outside the building (assuming the pressure in and out were the same).
In reality, the balloon's behaviour outside would be influenced by turbulence, downdrafts etc, so the experiment would be hard to perform.
