This is due to density differences between the two air masses.

The density difference is a function of different temperatures and salinity.

Typically, the density difference is small enough for the Boussinesq approximation to be valid.

So the density differences are due to temperature gradients plus salinity.

However effects will be felt because of density differences between different body tissues, so an upper acceleration limit still exists.

So since the density difference between the air and the balloon is less at higher altitudes, it will not rise as high/fast.

Buoyancy is a force that exists because of a potential difference due to the density differences between at least two fluids.

Because France has a uniform quality of delivery, its postal density differences are much greater.

But I also think that temperature differences would make pretty small density differences.

This density difference allows the detection of schools of fish by using reflected sound.