It may represent understanding of the phenomenon, and has some common sense utility, but it may represent a complete misunderstanding of what is going on physically. Of all the statements about relative humidity that I have heard in everyday conversation, the above is probably the most common. When the air can't 'hold' all the moisture, then it condenses as dew. Relative humidity is the amount of moisture in the air compared to what the air can 'hold' at that temperature. How much moisture can the air 'hold'? Careful! There are dangers and possible misconceptions in these common statements about relative humidity. What will that do to the relative humidity? Empirical fit of density data Then presume you take that air into your house and heat it to 20☌ without changing the actual humidity. Pick a cold outside temperature and adjust the actual humidity so that the relative humidity is about 60%. Your degree of comfort depends upon the relative humidity. Heating your house tends to make the air excessively dry. The above calculation can be used to demonstrate a perennial winter problem in colder climates. Where the temperatures in the formula are in Celsius.Īnd the relative humidity is % then this calculation projects a dewpoint temperature of The empirical fit of the saturation humidity data is If you enter a value of actual humidity greater than the saturation vapor density, it will calculate a relative humidity greater than 100%, but that will not be a meaningful result - make sure the actual humidity is less than the saturation value.Īnother approximate calculation of the dewpoint temperature is given by Planetcalc in the form of a logarithmic fit of the humidity data. It seriously underestimates saturated vapor density near 100☌. Relative Humidity Calculation For an air temperature of C = F, the saturated vapor density is gm/m 3 (Calculated by an empirical fit to published data.) If the actual humidity in the air is gm/m 3, then the relative humidity is %.With this amount of humidity, the dewpoint is approximately ☌ = ☏.Ĭaution! The empirical fit used is only reliable up to 40☌. The saturated vapor pressure roughly parallels the saturated vapor density numerical values are included in the vapor density table. The saturated vapor pressure reaches 760 mmHg at 100☌, the standard boiling point.

This is the fit used in the calculation of relative humidity below, but it significantly underestimates the vapor density near the boiling point. If only the values up to 40☌ are used for the fit, a more precise fit of the data is obtained in the temperature region where relative humidity is of interest. The behavior of water vapor density is a non-linear function, but an approximate calculation of saturated vapor density can be made from an empirical fit of the vapor density curve But for the purposes of calculating relative humidity, the values near boiling are not important and are given too much emphasis in the empirical fit above. It is possible to produce what appears to be a good fit of the saturated vapor density of water all the way up to the boiling point. Relative humidityĮmpirical fit of saturated vapor density versus Celsius Temperature If the air is cooled further, some of the moisture will condense. This temperature, at which the moisture content in the air will saturate the air, is called the dew point. Saturation vapor pressure and density for waterĭewpointIf the air is gradually cooled while maintaining the moisture content constant, the relative humidity will rise until it reaches 100%. Careful! There are dangers and possible misconceptions in these common statements about relative humidity. The relative humidity can be equivalently defined in terms of the water vapor pressure in the air compared to its saturation vapor pressure. For example, if the actual vapor density is 10 g/m 3 at 20☌ compared to the saturation vapor density at that temperature of 17.3 g/m 3, then the relative humidity is The most common units for vapor density are gm/m 3. The relative humidity is the percent of saturation humidity, generally calculated in relation to saturated vapor density. The amount of water vapor in the air at any given time is usually less than that required to saturate the air.

Density of water in gm3