Relative Humidity of Air

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 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 = p_w / p_ws 100% (2)

where

φ = relative humidity (%)

p_w = vapor partial pressure (mbar)

p_ws = 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)	Saturation Vapor Pressure (mbar, millibar, mb)
-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

air water saturation vapor pressure diagram

If the water vapor pressure in the air is 10.3 mbar, the vapor will saturate when a surface reaches 45^oF.

Note! The atmospheric pressure of air is 1013 mbar (101.325 kPa, 760 mmHg). 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 70^oF 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 = ρ_w / ρ_ws 100% (2b)

where

φ = relative humidity (%)

ρ_w = vapor density (kg/m³)

ρ_ws = 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 at 20&sup0;C is 10 g/m³ and the the saturation vapor density at this temperature is 17.3 g/m³, the relative humidity is

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 = m_w / m_ws 100% (2c)

where

φ = relative humidity (%)

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

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