Measured Variables

Author: Hans-Jürgen Schwarz

English Translation by Sandra Leithäuser
Back to Air Humidity Measurement

Abstract

The moisture content in air can be measured by its dew point, saturation humidity, relative humidity (RH), absolute humidity and the moisture level.

Dew point

The dew point temperature is the temperature at constant pressure and constant water vapour content at which air is saturated with water vapor. If there is a higher supply of water vapor condensation will take place. This can also occur if the air cools down (as then it will be supersaturated with moisture). The dew point temperature is equal to the water vapor saturation temperature and cannot exceed 100ºC at standard pressure.

Saturation humidity

Saturation humidity is the amount of water vapor contained in one cubic meter of air at a given temperature.

Saturation Humitity (modified figure from the German Wikipedia, original upload 18.03.2005 by Markus Schweiß)

Relative humidity

Relative humidity is the ratio between the water vapor partial pressure (P_w)in the atmosphere and the maximum possible water vapor pressure, i.e., the water vapor saturation pressure (PS) at that same temperature .

Relative humidity (RH) is the ratio of the water vapor present in the air to the maximum amount of water vapor possible in the air at the same temperature.

Failed to parse (syntax error): {\displaystyle RH = 100\cdot\frac{P_W}{P_s(T)}[\text{\%}]}

The relative humidity, being a ratio, is a dimensionless quantity and is indicated by in %. Because the saturation pressure depends exclusively on the air temperature, the relative humidity is also temperature dependent. The relative humidity decreases as temperature increases and vice-versa.

Even at temperatures below the freezing of water (0°C/32°F) the air contains water vapor and ice can pass directly into the gaseous state by sublimation, as can be observed in winter when snow disappears although the temperature does not rise above its melting point. Similarly, at low temperatures water vapor can directly turn into the solid state by deposition (rime, graupel/ snow pellets).

Table 1: Effect of +/- 1°C temperature fluctuations at different temperature and humidity levels (the change in RH is not symmetrical)

RF\T	10°C	20°C	30°C	50°C	70°C
10.00%	± 0.7 %	± 0.6 %	± 0.6 %	± 0.5 %	± 0.5 %
50,00%	± 3.5 %	± 3.2 %	± 3.0 %	± 2.6 %	± 2,3 %
90.00%	 6.3 %	± 5.7 %	± 5.4 %	± 4.6 %	± 4.1 %

At temperatures below 0°C the water vapor pressure of liquid water (supercooled) is different to that of ice.

Table 2: RH and its interdependence

Temperature [°C]	at 960 hPa at relative humidity: [g/m³]			at 1013 hPa [g/m³]
	100% r.F.	80% r.F.	50% r.F.	100% r.F.
30	29	23.2	14.5	30.4
20	16.5	13.2	8.25	17.3
10	9.0	7.2	4.5	9.4
0	4.6	3.7	2.3	4.8

Mixing ratio

The mixing ratio X or moisture level in [g/kg] represents the ratio of the mass of water vapor mass to the mass of dry air

${\displaystyle$RH=100\cdot {\frac {\text{Mass of water vapor in the atmosphere}}{\text{Mass of dry air}}}[{\frac {g}{kgAir}}]}

Absolute humidity

The absolute humidity (AH) corresponds to the amount of water vapor that is contained in a given volume of air.

Absolute humidity (AH) is the amount of water vapor that can be contained in one cubic meter of air.

${\displaystyle$RH=100\cdot {\frac {\text{Mass of water vapor contained in the atmosphere}}{\text{Volume of humid air}}}[{\frac {g}{m^{3}}}]}

The unit for absolute humidity is g/m³. Measuring the absolute humidity has the advantage that the actual amount of water present in the air is obtained, regardless of the temperature.

The relationship between temperature, absolute humidity and relative humidity can be represented by the Mollier Diagram .

In order to understand the true condition of humidity in the atmosphere, the temperature must be stated as well as the relative humidity, because of their interdependence.

Example: Air at 25ºC can hold up to 23 g/m³ of water vapor maximum at saturation. If it only contains 11.5 g/m³, the relative humidity will be 50% RH.

The water vapor content of the air is not only dependent on air temperature, but also, to a lesser extent, dependent on the air pressure. The air pressure fluctuates with weather conditions, the height of the measuring point above sea level and the latitude of the measuring site.

relative humidity