Finds the temperature at which water vapour in the air starts to condense, using the Magnus formula. Any surface colder than this, such as a window or a wall, will run with water. It is why droplets form only on the cold spots at a given room temperature and humidity.
Air can hold only so much water vapour, and the limit falls as the air cools. The dew point is the temperature at which what the air already holds becomes the limit.
and are the Magnus coefficients, is the air temperature and the relative humidity.
Where a window or wall is colder than the dew point, the air touching it can no longer hold its vapour and the excess turns to droplets. The temperature and humidity may be the same throughout the room, yet only the cold spots run with water. In winter it is the windows that stream, because their surface sits far below the temperature of the walls.
At 20°C and 60% humidity the dew point is 12.0°C. A window surface at 10°C is two degrees below it and will condense. The saturation vapour pressure is 23.3 hPa, the actual vapour pressure 60% of that at 14.0 hPa, and the absolute humidity 10.3 g/m³.
At 100% humidity the dew point equals the air temperature. The air is holding all it can, and the slightest cooling turns vapour into water.
Raise the surface temperature by insulating better, lower the humidity by ventilating, or lower the room temperature. Insulation and ventilation dominate the remedies because those are the two variables in the formula.
The Magnus formula fits well from about −40°C to 50°C, and drifts outside that range.