Calculate dew point temperature and relative humidity from air temperature — using the Magnus formula with full comfort level analysis.
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The dew point is the temperature to which air must be cooled (at constant pressure and humidity) to become saturated with water vapour — causing dew, fog or condensation to form. When the dew point equals the air temperature, relative humidity is 100%. A dew point above 21°C (70°F) feels oppressively humid; below 10°C (50°F) feels comfortably dry.
Relative humidity (RH) is the ratio of the current water vapour pressure to the maximum possible (saturation) vapour pressure at that temperature, expressed as a percentage. 100% RH means the air is saturated and condensation will form. RH changes with temperature even if the actual moisture content stays constant — warmer air can hold more vapour.
The Magnus formula approximates dew point: γ = (a × T) / (b + T) + ln(RH/100), where a = 17.625, b = 243.04°C. Dew point Tₐ = b × γ / (a − γ). It is accurate to within 0.1°C for temperatures between −40°C and +60°C and is the standard formula used by meteorologists worldwide.
Dew point comfort scale: below 10°C (50°F) = very dry, can cause irritation. 10–16°C (50–61°F) = comfortable. 16–18°C (61–65°F) = becoming noticeable. 18–21°C (65–70°F) = uncomfortable, humid. 21–24°C (70–75°F) = oppressive. Above 24°C (75°F) = dangerously humid — heat illness risk increases significantly.
Dew point is critical in aviation. When the dew point spread (temperature minus dew point) falls below 2–3°C, fog or low cloud is imminent. Pilots use dew point to assess fog risk, icing conditions (carburetor icing occurs when dew point is within 3°C of temperature above 4°C), and to estimate cloud base height: cloud base (ft) ≈ (T − Tₐ) × 400.
Cold glasses cool the air immediately surrounding them below the local dew point, causing water vapour in that air to condense on the cold surface. This is the same mechanism that creates morning dew on grass — surfaces cool overnight through radiative heat loss until they reach the dew point temperature of the surrounding air.
Below 0°C, water can condense directly as ice (deposition) without passing through liquid. The frost point is the temperature at which ice would form — it is slightly higher than the dew point at the same vapour pressure because the saturation vapour pressure over ice is lower than over liquid water. Frost forms when surfaces cool below the frost point.
High dew points (above 18°C) severely impair the body\ ability to cool through sweat evaporation because the air is already nearly saturated. Running performance degrades measurably above 14°C dew point: a marathon runner might lose 1–2 minutes per mile above their normal pace when dew point exceeds 21°C. Elite races are typically cancelled or delayed when heat-dew point conditions exceed safety thresholds.
