Relative Humidity

The amount of water vapor in the air at any given time is usually less than that required to saturate the air. The relative humidity is the percent of saturation humidity, generally calculated in relation to saturated vapor density.


The most common units for vapor density are gm/ . For example, if the actual vapor density is 10 gm/m3 at 20°C compared to the saturation vapor density at that temperature of 17.3 gm/m3 , then the relative humidity is

Calculation


Saturation vapor pressureDewpointRelative humidity calculation
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Dewpoint

If the air is gradually cooled while maintaining the moisture content constant, the relative humidity will rise until it reaches 100%. This temperature, at which the moisture content in the air will saturate the air, is called the dew point . If the air is cooled further, some of the moisture will condense.
Relative humiditySaturation vapor pressureCalculation of dewpoint
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Empirical fit of saturated vapor density versus Celsius Temperature


The behavior of water vapor density near the boiling point 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. This can then be used to estimate relative humidity for given conditions. The saturated vapor pressure reaches 760 mmHg at 100°C, the standard boiling point. The saturated vapor pressure roughly parallels the saturated vapor density; numerical values are included in the vapor density table.

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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 C = F.

Empirical fit of density dataRelative humidity
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