Find dew point from temperature and relative humidity, plus comfort level.
The dew point is the temperature to which air must be cooled (at constant pressure and humidity) for water vapour to condense into liquid water. It is a direct measure of the absolute moisture content of the air — unlike relative humidity, which changes with temperature even when the actual moisture content stays the same. The Magnus-Tetens formula gives a fast and accurate approximation using only temperature and relative humidity. Dew points above 18°C feel muggy to most people; above 21°C is oppressive; above 24°C is dangerous. A dew point near the air temperature (small temperature–dew point spread) signals near-saturation and fog risk.
Magnus-Tetens dew point
γ = ln(RH/100) + (a × T) / (b + T); dew point = b × γ / (a − γ)
Relative humidity depends on temperature: the same amount of water vapour gives 100% RH at the dew point but only 40% RH at a much warmer temperature. On a hot day, 50% RH can feel very muggy, while 50% RH on a cold day is comfortable. Dew point is invariant to temperature and directly represents how much moisture is in the air, making it a more stable comfort metric.
Dew points below 10°C feel dry and comfortable. From 13–16°C is noticeably humid. Above 18°C most people begin to feel oppressed; above 21°C is genuinely uncomfortable; above 24°C is considered dangerously tropical. The exact threshold varies with acclimatisation.
A spread less than 2.5°C (4.5°F) means the air is very close to saturation. This is the condition for fog formation, heavy morning dew, and condensation on cool surfaces. A spread of 0 means the air is exactly at saturation (relative humidity = 100%).
No — by definition the dew point cannot be higher than the air temperature. If you observe the dew point ≥ temperature in a weather report, it means relative humidity ≈ 100% and the sensor has a small calibration error.