Dew Point Calculator (from Temperature & Humidity)

Because RH is relative to temperature, it cycles daily even when moisture doesn't: the same air mass can read 95% at dawn and 50% mid-afternoon. Dew point stays put — it tracks the water actually present. Human comfort follows the absolute moisture (it governs how fast sweat evaporates), so a 21°C dew point feels sticky whether the thermometer says 26 or 35. Forecasters quote dew point for exactly this stability.

The standard ladder (°C / °F): under 10 / 50 crisp; 10–15 / 50–59 comfortable; 16–18 / 60–65 humid edges; 18–21 / 65–70 sticky, AC working; 21–24 / 70–75 oppressive; above 24 / 75 tropical — Gulf-coast or monsoon air. The world's extreme dew points (Persian Gulf coast, ~35°C/95°F) produce heat indices beyond 60°C. One degree of dew point matters more to comfort than one degree of temperature in summer.

Temperature minus dew point. Small spread = air close to saturation: fog forms when it closes near the ground (a 2°C spread with clearing skies at dusk is a fog forecast), and cumulus cloud bases sit at roughly the altitude where rising air cools to its dew point — about 400 ft per °F of spread, the basis of our cloud-base estimator. METARs report both numbers side by side precisely so the spread is one subtraction away.

Any surface colder than the dew point sweats: single-pane windows in winter (inside air at 20°C/50% RH has a 9°C dew point — glass below that drips), cold-water pipes in humid basements, AC ducts in attics. The cure is either dropping the dew point (dehumidifier, ventilation) or warming the surface (insulation, double glazing). This calculator gives the threshold temperature; a cheap IR thermometer finds the surfaces below it.