Psychrometrics · the unseen field
Dewpoint — The moment air lets go of water.
Air holds only so much water, and the warmer it is the more it can carry. Cool it and nothing changes — until the state crosses the saturation curve. At that exact temperature, the dew point, the air can hold no more and the surplus condenses. Watch it happen below.
Cooling moves left.
Chilling the air doesn't add or remove moisture, so the marker slides straight left along a line of constant water content. Only its temperature falls.
Saturation is the ceiling.
The teal curve is the most water the air can hold at each temperature. It climbs steeply — warm air is a far larger reservoir than cold. Above it, everything is dry.
Dew forms at contact.
Where the constant-humidity line meets the curve is the dew point. Cool one degree past it and the air must shed water — it beads on the glass and runs down.