one molecular layer would occur at
awater content of 0.03%.
Considering that multilayers can form
before amonolayer is established, it is not too surprising that at
a few multiples of 0.03%, the RH will approach 100%, which is
consistent with the high humidity water content of crystalline
solids in the 0.1%–0.2% range.
The important physical perspective
is that although the moisture level is small for crystalline
solids, water molecules occupy a significant fractional coverage
of a solid’s surface even at a moderate RH of 50%.