Relative Humidity of Air

The relative humidity of air can be expressed by partial vapor and air pressure - density of the vapor and air - or by the actual mass of the vapor and air

Humidity is the quantity of water vapor present in air. It can be expressed as an absolute, specific or a relative value.

Relative humidity can be expressed by partial vapor and air pressure, density of the vapor and air, or by the actual mass of the vapor and air.

Relative humidity is usually expressed in per cent and abbreviated by φ or RH.

Relative Humidity and Vapor Partial Pressure

Relative humidity can be expressed as the ratio of the vapor partial pressure of the air - to the saturation vapor partial pressure of the air at the actual dry bulb temperature.

Relative humidity by partial pressure

φ = pw / pws 100%         (1)

where

φ = relative humidity (%)

pw = vapor partial pressure (mbar)

pws = saturation vapor partial pressure at the actual dry bulb temperature (mbar)

The table below can be used to determine the vapor saturation pressure at different temperatures:

Temperature Saturation Vapor Pressure
(mbar, millibar, mb)
(oC) (oF)
-18 0 1.5
-15 5 1.9
-12 10 2.4
-9 15 3.0
-7 20 3.7
-4 25 4.6
-1 30 5.6
2 35 6.9
4 40 8.4
7 45 10.3
10 50 12.3
13 55 14.8
16 60 17.7
18 65 21.0
21 70 25.0
24 75 29.6
27 80 35.0
29 85 41.0
32 90 48.1
35 95 56.2
38 100 65.6
41 105 76.2
43 110 87.8
46 115 101.4
49 120 116.8
52 125 134.2
  • 1 bar = 1000 mbar = 105 Pa (N/m2) = 0.1 N/mm2 = 10,197 kp/m2 = 10.20 m H2O = 0.9869 atm = 14.50 psi (lbf/in2) = 106 dyn/cm2 = 750 mmHg

air water saturation vapor pressure diagram

If the water vapor pressure in the air is 10.3 mbar the vapor saturates on a surface with 45oF (7oC).

Note! The atmospheric pressure of air is 1013 mbar (101.325 kPa, 760 mmHg). As we can see the maximum water vapor pressure - the saturation pressure - is relatively small.

Example - Relative Humidity and Vapor Pressure

From the table above the saturation pressure at 70oF (21oC) is 25.0 mbar. If the vapor pressure in the actual air is 10.3 mbar, the relative humidity can be calculated as:

φ = (10.2 mbar) / (25.0 mbar) 100%

= 41%

Relative Humidity and Vapor Density

Relative humidity can also be expressed as the ratio of the vapor density of the air - to the saturation vapor density at the the actual dry bulb temperature.

Relative humidity by density:

φ = ρw / ρws 100%         (2b)

where

φ = relative humidity (%)

ρw = vapor density (kg/m3)

ρws = vapor density at saturation at actual dry bulb temperature (kg/m3)

A common unit for vapor density is g/m3. If the actual vapor density at 20oC (68oC) is 10 g/m3 and the the saturation vapor density at this temperature is 17.3 g/m3, the relative humidity can be calculated as

φ = (10 g/m3) / (17.3 g/m3) 100%

= 57.8%

Relative Humidity and Vapor Mass

Relative humidity can also be expressed as the ratio at actual mass of water vapor in a given air volume - to the mass of water vapor required to saturate at this volume.

Relative humidity can be expressed as:

φ = mw / mws 100%         (2c)

where

φ = relative humidity (%)

mw = mass of water vapor in the given air volume (kg)

mws = mass of water vapor required to saturate at this volume (kg)

Relative Humidity Chart - degrees Fahrenheit

Relative humidity from dry and wet bulb temperatures

Relative Humidity Chart - degrees Celcius

Relative humidity from dry and wet bulb temperatures degrees Celcius

Altitude and Correction Factors

Barometrical Altitude (mbar) Abs. Altitude Correction Factor for
φ
(m) (ft)
1013 0 0 1.000
1000 108 354 0.987
989 200 656 0.976
966 400 1312 0.953
943 600 1996 0.931
921 800 2624 0.909
899 1000 3281 0.887
842 1500 4922 0.731
795 2000 6562 0.785

Related Topics

  • Air Psychrometrics - The study of moist and humid air - air condition - psychrometric charts, Mollier diagrams, air temperature, absolute and relative humidity, moisture content and more

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