Dew Point

Dew point temperature (T_dp) is the temperature at which air becomes saturated (100% relative humidity) and water vapor begins to condense, assuming constant pressure and moisture content. It is a direct measure of the absolute moisture content (water vapor partial pressure) in the air, independent of air temperature. Higher dew point indicates more moisture in the air. When air is cooled at constant pressure, its capacity to hold water vapor decreases. At the dew point, the actual vapor pressure p_v equals the saturation vapor pressure p_sat at that temperature. Further cooling causes condensation: dew on grass, fog, or water droplets on cold surfaces (windows, pipes, walls). Condensation occurs when a surface temperature falls below the dew point of the adjacent air. Relative humidity (RH) is related to dew point: RH = p_v / p_sat(T) × 100%, where p_sat(T) is the saturation pressure at the dry-bulb temperature T. For a given moisture content (fixed p_v), cooling the air increases RH until it reaches 100% at T = T_dp. Psychrometric charts show dew point as the temperature where a horizontal line (constant humidity ratio) from the state point intersects the saturation curve. The saturation vapor pressure can be estimated using the Magnus formula: p_sat(T) = 0.6108 × exp(17.27T / (T + 237.3)) kPa, where T is in °C. Given RH and T, the dew point can be approximated as: T_dp ≈ T − (100 − RH) / 5 for typical comfort conditions (rough approximation). More accurate: T_dp = (237.3 × α) / (17.27 − α), where α = ln(RH/100) + (17.27T)/(T + 237.3). In HVAC and building science, dew point is critical for: (1) Preventing condensation on cold surfaces (windows, exterior walls, chilled beams, cold pipes) by ensuring surface temperatures stay above T_dp. (2) Sizing dehumidification equipment to maintain indoor T_dp below levels that cause discomfort or mold growth. (3) Assessing comfort: dew points above ~18°C feel humid and uncomfortable; above ~24°C are oppressive. (4) Industrial processes requiring controlled humidity (pharmaceuticals, electronics manufacturing, food storage).