Dew Point (approx)

This rule of thumb estimates the dew point, the temperature at which moist air would need to cool before water vapour starts condensing. It works because relative humidity tells you how close the air is to saturation: the higher the RH, the smaller the gap between the current air temperature and the dew point. The formula is only approximate, but it is useful for quick mental estimates in everyday weather conditions.

Use it when you want a fast estimate of dew point from ordinary weather data, such as in meteorology, outdoor planning, or checking when condensation might form on surfaces. It is especially helpful when you have temperature and relative humidity but not a full psychrometric calculation.

• Using °F values directly in the formula instead of °C
• Treating the result as exact even though it is only an approximation
• Confusing relative humidity with absolute humidity