For air near room temperature and atmospheric pressure, the water vapour enhancement factor affects the result by approximately 0.5 % of value. The accuracy of these calculations depends slightly on the pressure and temperature of the gas concerned. The uncertainties associated with these equations are: (Formulae due to Sonntag, 1990, updated from formulae given by Wexler, 19.) This is a more accurate but complex alternative formula for vapour pressure (in pascals) from dew point (in kelvin) for water: Ln e i(t) = ln 611.2 + (22.46 t)/(272.62+ t) (Equation 3)įor the range -65 ☌ to +0.01 ☌, values given by this equation have an uncertainty of < ☑.0 % of value, at the 95% confidence level. Ln e w(t) = ln 611.2 + (17.62 t)/(243.12+ t) (Equation 2)įor information, 100 Pa = 1 millibar (mbar)įor the range -45 ☌ to +60 ☌, values given by this equation have an uncertainty of < ☐.6 % of value, at the 95% confidence level. Determining the three values starts with some simple temperature measurements. Vapour pressure can be calculated using the Magnus formula. This states that at a temperature t (in ☌), the saturation vapour pressure e w(t), in pascals (Pa), over liquid water, is Temperature is the measure of the energy in the air, relative humidity is the measure of water vapor in the air, and the dew point is the temperature at which the water vapor in the air will begin to condense into liquid water (reference 1).
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