Calculate the dew point from temperature and humidity — the temperature at which moisture starts to condense.
The dew point is the temperature at which the air can no longer hold all its moisture and water begins to condense. A dew point of 20 °C feels muggy; above 24 °C it is oppressive. Below 10 °C the air feels dry and comfortable.
The calculation uses the Magnus-Tetens approximation, which is accurate to within 0.4 °C for temperatures between -45 °C and 60 °C. The formula relates temperature, relative humidity and the saturation vapour pressure of water.
In construction and HVAC, the dew point determines where condensation will form inside a wall or on a window. If the surface temperature of a wall drops below the dew point of the indoor air, you get condensation and eventually mould.
γ = (17.27 × T) / (237.7 + T) + ln(RH/100). Dew point = 237.7 × γ / (17.27 − γ).
Below 10 °C feels dry, 10-16 °C is comfortable, 16-20 °C is humid, above 20 °C is oppressive.
No — the dew point is always ≤ the air temperature. When they are equal, relative humidity is 100%.
Because the glass surface is below the dew point of the indoor air. The water vapour condenses on contact.
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