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Relative Humidity / Dew Point

The relative humidity of an air-water mixture is defined as the ratio of the partial pressure of water vapor in the mixture to the saturated vapor pressure of water at a prescribed temperature. Relative humidity is normally expressed as a percentage.

The dew point is the temperature to which a given parcel of air must be cooled, at constant barometric pressure, for water vapor to condense into water. The condensed water is called dew. The dew point is a saturation point.

When the dew point temperature falls below freezing it is often called the frost point, as the water vapor no longer creates dew but instead creates frost or hoarfrost by deposition.

The dew point is associated with relative humidity. A high relative humidity indicates that the dew point is closer to the current air temperature. Relative humidity of 100% indicates that the dew point is equal to the current temperature (and the air is maximally saturated with water). When the dew point stays constant and temperature increases, relative humidity will decrease.

At a given barometric pressure, independent of temperature, the dew point indicates the mole fraction of water vapor in the air, and therefore determines the specific humidity of the air. - Wikipedia

Relative humidity of the air for temperature t and dew point d.
(from Smithsonian Meteorological Tables)

Relative Humidity (%)
Depression of
Dew Point t-d
-10°C 0°C +10°C +20°C +30°C
0.0 100 100 100 100 100
0.2 98 99 99 99 99
0.4 97 97 97 98 98
0.6 95 96 96 96 97
0.8 94 94 95 95 96
1.0 92 93 94 94 94
1.2 91 92 92 93 93
1.4 90 90 91 92 92
1.6 88 89 90 91 91
1.8 87 88 89 90 90
2.0 86 87 88 88 89
2.2 84 85 86 87 88
2.4 83 84 85 86 87
2.6 82 83 84 85 86
2.8 80 82 83 84 85
3.0 79 81 82 83 84
3.2 78 80 81 82 83
3.4 77 79 80 81 82
3.6 76 77 79 80 82
3.8 75 76 78 79 81
4.0 73 75 77 78 80
4.2 72 74 76 77 79
4.2 72 74 76 77 79
4.4 71 73 75 77 78
4.6 70 72 74 76 77
4.8 69 71 73 75 76
5.0 68 70 72 74 75
5.2 67 69 71 73 75
5.4 66 68 70 72 74
5.6 65 67 69 71 73
5.8 64 66 69 70 72
6.0 63 66 68 70 71
6.2 62 65 67 69 71
6.4 61 64 66 68 70
6.6 60 63 65 67 69
6.8 60 62 64 66 68
7.0 59 61 63 66 68
7.2 58 60 63 65 67
7.4 57 60 62 64 66
7.6 56 59 61 63 65
7.8 55 58 60 63 65
8.0 54 57 60 62 64
8.2 54 56 59 61 63
8.4 53 56 58 60 63
8.6 52 55 57 60 62
8.8 51 54 57 59 61
9.0 51 53 56 58 61
9.2 50 53 55 58 60
9.4 49 52 55 57 59
9.6 48 51 54 56 59
9.8 48 51 53 56 58
10.0 47 50 53 55 57
10.5 45 48 51 54  
11.0 44 47 49 52  
11.5 42 45 48 51  
12.0 41 44 47 49  
12.5 39 42 45 48  
13.0 38 41 44 46  
13.5 37 40 43 45  
14.0 35 38 41 44  
14.5 34 37 40 43  
15.0 33 36 39 42  
15.5 32 35 38 40  
16.0 31 34 37 39  
16.5 30 33 36 38  
17.0 29 32 35 37  
17.5 28 31 34 36  
18.0 27 30 33 35  
18.5 26 29 32 34  
19.0 25 28 31 33  
19.5 24 27 30 33  
20.0 24 26 29 32  
21.0 22 25 27    
22.0 21 23 26    
23.0 19 22 24    
24.0 18 21 23    
25.0 17 19 22    
26.0 16 18 21    
27.0 15 17 20    
28.0 14 16 19    
29.0 13 15 18    
30.0 12 14 17    

 

Information from CRC Handbook of Chemistry and Physics, 7first Edition, 1990-1991, David R. Lide