the water content of the atmosphere, at saturation, increases rapidly with increasing temperature.
at any temperature the absolute amount of water in the air, expressed either as mol percent or lb water per lb air, increases with increasing RH.
the atmosphere has a perfectly well-defined -- but quite uneven -- lower boundary, the surface of the land and the oceans.
its upper boundary is not as well-defined; the atmosphere simply becomes thinner and with increasing height until it is as thin as outer space.