Why this window is fogging up

Air can carry invisible water vapour. When it touches glass colder than its dew point, that vapour has nowhere to stay gaseous.

T_d = bγ/(a − γ)

room airoutside

Droplets are qualitative — not a prediction of coverage.

Indoor air temperaturewarmer air can hold more vapour

15°C28°C

Relative humiditymore vapour → higher dew point

20%80%

Inside glass surfacea proxy for weather + insulation

−5°C25°C

Dew point

°C

Glass margin

°C

colder → condenseswarmer → dry

TRY

Dew point is the temperature at which air reaches 100% relative humidity when cooled without changing its water-vapour content. This page uses the commonly used Magnus approximation over liquid water: γ = ln(RH/100) + aT/(b+T), then T_d = bγ/(a−γ), with a = 17.62 and b = 243.12°C. The glass fogs in this model whenever its inside surface is at or below the calculated dew point. Real windows have surface-temperature variation, air movement and frames/edges that can be colder than the centre pane; these are omitted. See the US National Weather Service vapour-pressure reference for the underlying saturation-vapour-pressure relation.