11/28/2023 0 Comments Water dew point calculator![]() To convert from dew point or frost point to relative humidity Conversions between these two parameters must be carried out via the intermediate step of evaluating both the actual vapour pressure of water and the saturation vapour pressure at the prevailing temperature. Unfortunately, there is no simple, direct formula for converting between dew point and relative humidity. Relative humidity (in %) = e/ e s × 100 (Equation 1) The relative humidity is the ratio of the amount of water vapour, e, in the air to the amount of water vapour, e s, that would be in the air if saturated at the same temperature and pressure. Where the condensate is ice, this is known as the frost point. In turn, fog is (slightly) less likey to start condensing over ice than over water with the same temperature.The dew point, or dew-point temperature, is the temperature at which dew, or condensation, forms as you cool a gas. Therefore, fewer molecules try to enter a volume of air above the surface. The reason why the saturation pressure over ice is lower than over water is because the molecules in ice are less likely to be able to flee from their grid than those in water. One set for water temperatures 0°-100☌, one set for Water < 0° and one for ice (which is not the same). Since the measurement accuracy for many sensors (especially humidity) is not better than that, I guess it is not possible or useful to find more precise calculations.īy the way: My book uses even three different sets of constants to calculate the saturation pressure. It does seem that there is not the correct formula, and every scientist (or meteorological institute) uses slightly different calculations. When calculating with 100% RH, it's results are slightly better than the previous formula for temperatures above 0☌, but below the error is also around 0.2☌. The result of the formula is within 0.2 of the previous one for the test cases. At that point what happens to the air around the swimming pool that warrants using different values for calculating the dew point just because there is ice a few metres away?ĭouble pa = CalculateActualVaporPressure (airTemperature, relativeHumidity ). if you put a few centimeters of water and wait a bit it would freeze and become ice. Say you're in -20C by an empty swimming pool. ![]() Also, I don't quite understand the physics of it. Sea water doesn't freeze at 0°, in fact most parts of the sea newer freeze, regardless of temperature.Īh that's why they use the terminology 'over water' and 'over ice'? Maybe we should put in both sets of numbers then and allow the developer to choose whether it's over water or over ice, and default to 'over water'. to estimate the probability of fog) this would give clearly wrong values. Especially for calculating the dew point over sea water (i.e. Water doesn't freeze just because the air is <0°, and also the ground is not necessarily frozen then. No, I don't think that would be a good idea. Should it be calling CalculateSaturatedVaporPressureOverIce when temperature.DegreesCelsius <= 0? This may or may not be related to (2) above, but I'm not sure. Our function CalculateActualVaporPressure is calling CalculateSaturatedVaporPressureOverWater. Should it be calling CalculateSaturatedVaporPressureOverIce when temperature.DegreesCelsius <= 0? This may or may not be related to (2) above, but I'm not sure.Ĥ. If the temperature is <= 0, and we decide to go with the alternate numbers from Arden Buck for the saturated vapor pressure calculation, should we also use these a,b,c,d values for the rest of the dew point calculation?Įither I'm blind or I'm not understanding the math, but I can't understand why Ps,m(T) is being calculated when it's then not used in the two lines below it leading to the dew point temperature. First question: should we change the saturated vapor pressure calculations to use the Arden Buck full equation? That is using two sets of numbers for a,b,c and d? Our functions CalculateSaturatedVaporPressureOverWater and CalculateSaturatedVaporPressureOverIce are using calculations from 1982 but Buck (1996) suggests different numbers. ![]() The 3rd set of formulas on (the part that starts "For greater accuracy") and I understand how they got Psm(T), which is the Arden Buck ( ) equation but specifically for temperatures > 0C. Well I gave it a good try, but I'm still clueless following and
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