Moist air is a mixture — dry air with a little water vapour stirred through it — and describing its state takes more than one number. Psychrometrics is the bookkeeping of that mixture, and the whole subject opens with learning to tell four readings apart.

The dry-bulb temperature is what an ordinary thermometer reads. It knows nothing at all about the water in the air, which is precisely why it needs help. The wet-bulb temperature is what a thermometer reads with a damp sock on it and air moving past: evaporation steals heat from the bulb, so the wet bulb reads LOWER, and the drier the air the further it falls. That gap is the wet-bulb depression, and it is the whole measurement. The dew point is the temperature at which this air would start to condense — cool a mirror until it fogs and you have measured it directly.

The ordering is fixed and worth carrying: dew point ≤ wet bulb ≤ dry bulb, always, with all three meeting only in saturated air. If your three numbers come out in any other order, something upstream is wrong and no amount of further arithmetic will fix it.

The fourth reading is the relative humidity, a percentage — and a percentage of something, which is where beginners lose it. We come to that next.