Air - Humidity Measurement from Dry and Wet Bulb Temperature

Relative humidity in moist air can estimated by measuring the dry and wet bulb temperature.

Example - The State of Air in the Mollier Diagram

If the dry bulb temperature of air is 19^oC and the web bulb temperature is 12^oC - then the state of air can be visualized in the Mollier diagram as indicated below.

the relative humidity is approximately 42%
the humidity ratio is approximately 0.0057 kg_h2o/kg_{dry_air}

Mollier diagram - state of air based on dry bulb and web bulb temperature

Relative Humidity - Temperature in Celsius

The table below can be used to estimate the relative humidity of air if dry and wet bulb temperatures are known.

Relative Humidity - RH (%)
T_db- T_wb(^oC)	Dry Bulb Temperature - T_db (^oC)
T_db- T_wb(^oC)	15	18	20	22	25	27	30	33
1	90	91	91	92	92	92	93	93
2	80	82	83	84	85	85	86	87
3	71	73	75	76	77	78	79	80
4	62	65	67	68	70	71	73	74
5	53	57	59	61	64	65	67	69
6	44	49	52	54	57	59	61	63
7	36	42	45	47	51	53	55	58
8	28	34	38	41	45	47	50	53
9	21	27	31	34	39	41	45	48
10	13	20	25	28	33	36	40	43

Relative moisture - dry and wet bulb temperature diagram - degrees Celsius Download and print Air - Relative Humidity vs. Dry and Wet Bulb Temperatures chart

Example - Relative Humidity

If the dry bulb temperature of air is 18^oC and the web bulb temperature is 12^oC - then the difference between them is 6^oC. By using the table above

the relative humidity is approximately 49%

Example - the State of Air in the Psychrometric Chart

The state of air with dry bulb temperature 75^oF and wet bulb temperature 62.5^oF can be visualized in the psychrometric chart as indicated below.

the relative humidity is approximately 50%
the humidity ratio is approximately 65 grains_h2o/lb_{dry_air}

Psychrometric chart - dry and wet bulb temperatures

Relative Humidity - Temperature in Fahrenheit

The table below can be used to estimate the relative humidity of air if dry and wet bulb temperatures are known.

Relative Humidity - RH (%)
T_db- T_wb(^oF)	Dry Bulb Temperature - T_db (^oF)
T_db- T_wb(^oF)	60	64	68	72	76	80	84	88
1	94	95	95	95	96	96	96	96
2	90	90	90	91	91	92	92	92
3	84	85	85	86	87	88	88	89
4	78	80	81	82	83	84	84	85
5	73	75	76	78	79	80	80	81
6	68	70	72	73	75	76	77	78
7	63	66	67	69	71	72	73	74
8	58	61	63	65	67	68	70	71
9	54	57	59	61	63	65	66	68
10	49	52	55	57	59	61	63	64