It is 10 a.m. and already getting hot. You are wondering why you agreed to do fi eld work for the summer in Houston. On the radio, the weatherperson declares, “It will be another scorcher today with the temperature approaching 95 (°F) and a (relative) humidity of 90%.” Maybe it is the heat, but, as you wipe the sweat from your face, you long for the comfort of sitting in thermodynamics class. Which gets you to thinking— how does the fugacity of water in my sweat (3.5% NaCl by weight in water) compare to the fugacity of water in the hot Houston air? Explain.