The Wet-and-Dry Bulb Psychrometer

Humidity is the hardest common quantity to measure and the easiest to measure badly. Hair hygrometers drift, electronic sensors need calibrating against something, and neither tells you what you usually want to know — will it condense, and will anything dry. The psychrometer sidesteps all of it. Two identical thermometers hang side by side; one has a wet muslin sleeve. Water evaporating from that sleeve takes heat away, so the wet bulb reads LOWER, and how much lower depends only on how readily the air will accept more water. Dry air cools it a lot. Saturated air cools it not at all. The instrument measures nothing but two temperatures, and from them you get relative humidity, dew point, and the drying power of the air — the numbers behind the food dehydrator, the timber store and every question about mould.