Find the temperature at which air becomes saturated and condensation begins.
Overview
Dew point describes moisture, not just heat. Two days at 30°C can feel different if relative humidity differs: higher humidity slows evaporative cooling and raises discomfort because perspiration cannot dry as quickly.
Relative humidity is the current water vapor pressure divided by saturation vapor pressure at that temperature. Dew point is the temperature where those become equal if air cools at constant moisture content. It is often a better comfort indicator than RH alone.
Magnus-type formulas approximate saturation vapor pressure with exponential curves in temperature. They are accurate enough for weather education and HVAC rough planning but not a substitute for professional psychrometric charts in critical designs.
Pilots, runners, and facility managers watch dew point for fog risk, condensation on windows, and mold conditions. Rising dew point with stable temperature means increasing absolute humidity.
Steps
Use °C or °F consistently.
Provide percent between 0 and 100.
Apply Magnus approximation.
Interpret dew point against comfort guidelines.
Recalculate as temperature or RH updates.
Remember approximation error at temperature extremes.
Math
Td ≈ (b × γ(T,RH)) / (a − γ(T,RH)), γ = (aT/(b+T)) + ln(RH/100)
Magnus constants a and b fit vapor pressure curves. Solving for temperature when actual vapor equals saturation vapor yields dew point.
Worked cases
Magnus a=17.27, b=237.7
Dew point ≈ 24°C.
Lower vapor pressure
Dew point much lower, drier feel.
Td close to air temperature
Fog/condensation risk elevated.
Why use this
Questions