Warm air can sustain a higher concentration of water vapor than cooler air without becoming saturated. (eso.org)
However, because atmospheric concentrations of water vapor tend to be at most only a few percent of the amount of air (and usually much lower), they are both often expressed in units of grams of water vapor per kilogram of (moist or dry) air. (eso.org)
As this sinking air mass weighs on the marine layer, the latter compresses into a thin layer with a relatively high concentration of water vapor. (eos.org)