Last updated: 2026-05-20

The Dew Point Explained

Enter the air temperature and relative humidity to get three complementary moisture metrics: the dew point, the absolute humidity, and the Temperature–Humidity Index (THI). Together they tell you far more about air quality and comfort than relative humidity alone.

Formulas

Dew point (Magnus approximation, accurate to ±0.4 °C±0.7 °F):

Absolute humidity (g/m³):

Temperature–Humidity Index (Thom 1959):

Understanding the Three Outputs

Dew point tells you the temperature at which condensation forms. Below 10 °C feels dry; 10–15 °C is comfortable; 16–20 °C is noticeable; above 24 °C is widely considered oppressive.Below 50 °F feels dry; 50–59 °F is comfortable; 61–68 °F is noticeable; above 75 °F is widely considered oppressive.

Absolute humidity tells you the actual mass of water vapour in each cubic metre of air. Unlike relative humidity, it doesn't change when you heat the room — useful for HVAC sizing, archiving sensitive materials, and incubation.

THI comfort bands (Thom 1959):

Practical Applications

• Condensation risk — If a window or wall surface is colder than the dew point, water will form on it. Typical single-glazed glass in winter can be 5–10 °C9–18 °F below room air temperature.
• Agricultural frost warning — Frost forms when the dew point drops below 0 °C32 °F on a still, clear night. Growers monitor dew point, not RH.
• Storage and archiving — Museums and archives typically target 45–55% RH at 18–20 °C64–68 °F. Absolute humidity keeps those limits meaningful regardless of temperature swings.
• HVAC design — Engineers size cooling coils to remove enough moisture to hit a target dew point, not a target RH.