Drying and site conditions
Dew point from air temperature and relative humidity (Magnus formula)
- where
- intermediate Magnus term (dimensionless)
- relative humidity (%)
- air temperature (°C)
- dew point temperature (°C)
- dew point spread (°C)
Explain this equation
Works out the dew point: the temperature at which the moisture already in the air will start to condense. Use it on cold or humid days before plastering, rendering, priming or decorating.
T is the air temperature and RH the relative humidity from a hygrometer. γ is an intermediate value that combines the two. Td is the dew point; any surface colder than Td will get wet. D is how far the air is above its dew point.
This is the Magnus approximation. Saturation vapour pressure rises roughly exponentially with temperature, and 17.625 and 243.04 °C are fitted constants (Alduchov and Eskridge) that make the formula accurate to about 0.1 °C over normal site temperatures. The logarithm term turns the humidity into how far below saturation the air is.
Sense check: 20 °C at 50% RH gives a dew point of about 9.3 °C. At 100% RH the dew point equals the air temperature. Damp air (80–90% RH) has a dew point only a few degrees below the air.
Works out the temperature at which moisture in the room air will condense on a surface, from a hygrometer reading of air temperature and relative humidity.
Enter your values to see the result.
Good to know
Uses the Magnus form with the Alduchov–Eskridge constants (17.625 and 243.04 °C), which Lawrence (2005) shows is accurate to within about 0.1 °C between −40 °C and 50 °C. Any surface colder than the dew point will get wet: walls, ceilings and render backgrounds on cold mornings are the usual culprits. At 20 °C and 50% RH the dew point is about 9.3 °C.
Common mistake
Reading the hygrometer in a warm van or doorway and assuming the same air sits against a cold external wall. Measure in the room or on the scaffold where you are working.
Limits
Valid for −40 °C to 50 °C over liquid water. Below 0 °C the result is the dew point over water, not the frost point (which is slightly higher). Accuracy is limited by the hygrometer, typically ±2–3% RH.
Sources
- M. G. Lawrence, The relationship between relative humidity and the dewpoint temperature in moist air, Bulletin of the American Meteorological Society 86(2), 2005, pp. 225–233 (Magnus form with Alduchov–Eskridge constants 17.625 and 243.04 °C)
- O. A. Alduchov, R. E. Eskridge, Improved Magnus form approximation of saturation vapor pressure, Journal of Applied Meteorology 35(4), 1996, pp. 601–609
Results are calculations from the published methods and manufacturer figures shown, and depend on the data you enter. Always follow the manufacturers’ system and product data, BS 8212, BS EN 13914 and Building Regulations. Coverage rates vary by product, background and thickness. Check prices, VAT and CIS rules with your own records and HMRC.