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Dew point calculator

as a function of air temperature and relative humidity

and the frosting point if the air temperature is below 0°C.

Measure

  • air temperature using a thermometer;
  • relative humidity using a hygrometer.

The calculator determines the temperature at which dew or frost forms.

If air comes into contact with a surface whose temperature is below the dew point, condensation is deposited on the surface.

Online calculator

degrees Celsius.

%.

Method

Since the calculation is based on a table of water vapor saturation pressures, intermediate values must be managed graphically or by calculation.

By interpolating in a numerical data table, construct the saturation pressure function of water vapor psat(t) as a function of temperature t for temperatures t ≤ 65 °C.

For example, at t = 21 °C, the table shows psat(t) = 24.87 hPa

Calculate water vapor pressure pvap = Hr*psat(t).

For Hr = 45 %, we calculate pvap = 0.45*24.87 hPa = 11.19 hPa

Determine the tdew such that psat(tdew) = pvap ; this is the dew point we're looking for.

In the numerical table, we see that saturation pressure 11.19 hPa corresponds to a temperature between 8 °C et 9 °C. Interpolating, we obtain tdew = 8.6 °C

If the air temperature is negative, the frosting point is calculated using the following formula:

\[ \small \begin{align} T_{frosting} = & \ T_{dew} - T \\ & + \frac{2671.02}{\frac{2954.61}{T} + 2.193665 \ln(T) - 13.3448} \end{align} \]

where \(T_{frosting} \text{, } T_{dew} \text{ et } T \) are respectively the frosting point, dew point and air temperature, all expressed in kelvin.
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