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So yesterday I came back home with a buoyant helium balloon (no comment, and before you ask, it's blue) and I left it in the house. Obviously, it rose to the ceiling.

This morning I woke up to a rainy weather and found in awe that my balloon is lying on the floor.

This is puzzling me because yesterday was a bright sunny day with a high atmospheric pressure. Today the atmospheric pressure dropped so I'd have expected the volume of the balloon to expand, and thus to become more buoyant.

However on second thought, the balloon was filled at an outside temperature of 30°C which now decreased to around 22°C which would account for shrinking the balloon from the ideal-gas law.

Ruling out the other (obvious) possibility that the balloon is not airtight, could one conclude that the drop in temperature offsets the drop in pressure and resulted in a decreased volume which made the balloon lose its buoyancy?

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  • $\begingroup$ "Ruling out the other (obvious) possibility that the balloon is not airtight," <-- this is the reason, are you serious??? $\endgroup$
    – Jared
    May 15, 2015 at 6:59
  • $\begingroup$ This would honestly surprise me. Those balloons are released by kids at fairs and some have been reported to drift for well over 36h. And I doubled the tying knot on this one. $\endgroup$
    – VH-NZZ
    May 15, 2015 at 7:10
  • $\begingroup$ That temperature drop means that the product $PV$ must reduce by a factor of roughly 295/303, or -2.6%. At most, pressure OR volume would drop by 2.6%. Considering air pressure can vary by at most 10% (say 950 hpa to 1050 hpa), if the balloon isn't airtight then even putting it under a hot light would make it rise again no? $\endgroup$
    – Xeren Narcy
    May 15, 2015 at 7:15
  • $\begingroup$ The facts of helium being able to diffuse through latex are somewhat interesting and do answer your question, but a far more interesting phenomena is what happens to a helium balloon in an accelerating vehicle. Given the balloon is centered in say a car, and the car accelerates, which direction does the balloon go relative to the reference frame of the car? $\endgroup$
    – docscience
    May 15, 2015 at 14:18

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The Helium atoms are so small they diffuse rapidly through the rubber walls of the balloon, and as soon as the volume has decreased to the point were the bouyant force is less than the weight of the balloon the balloon sinks to the floor.

If you're interested, some Googling found this school project investigating the helium diffusion through balloons. They report the volume (and therefore the bouyant force) roughly halves in eight hours.

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