Dewla

Meet Dewla, a drop of water that appears on a cold glass because the air there is too cold to hold her.

Air always contains some water vapour. The amount it can hold before vapour starts condensing depends strongly on temperature: warm air can hold far more than cold air. The dew point is the temperature to which air must be cooled, at constant pressure and moisture, for condensation to begin. When air touching a cold drink cools below its dew point, the excess vapour turns into droplets on the glass.

A worked number with the Magnus approximation: air at 20 °C and 50 percent relative humidity has a dew point of about 9 °C. So a glass colder than about 9 °C will mist up in that room, while a warmer one stays dry.

Weather forecasters use the dew point to predict fog, dew and frost, and it is a better guide to how muggy air feels than relative humidity alone.

The water on the glass came from the air, not through the glass, a common misconception. If the dew point is below 0 °C, the vapour can turn straight into frost instead of dew.