Chicks Drowning In The Egg - The Physics.

Water exits from the liquid-egg into the air-sac and in the transfer process it transitions into water vapour (gas) and remains in gaseous form at incubating temperature. The water vapour molecules inside the air-cell are traveling at very high velocity.

A balance is reached when no more water vapour molecules are permitted into the air-cell because it is at saturation point. That saturation point = 100% RH.

Now each time that a water gas molecule exits via the shell to the ambient with the resultant tiny drop in pressure, a fresh molecule immediately enters the air-sac from the liquid-egg side.

As long as the RH outside the egg is below 100% all gas/pressure transfers from the air-cell are outwards to the ambient.

There is no mechanism by which water as gas can be forced into the air-cell. (Only immersion in water would raise the pressure gradient).

This is everyday physics for engineers.

Often what is thought to be water turns out to be albumen, egg-white, sticky, from too cold an incubation temperature. And once the embryo has died and the egg has cooled before opening, it would be condensate.