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?