Air - Humidity Measurement from Dry and Wet Bulb Temperature

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

Air humidity can be estimated by measuring

dry bulb temperature, and
wet bulb temperature

Humidity measurement - dry wet bulbe temperature meeter

Dry Bulb Temperature - T_db - can be measured with a simple thermometer as shown above.

Wet Bulb Temperature - T_wb - can be measured with a standard thermometer with some wet clothing, cotton or similar, around the bulb. Note that continuously air flow around the thermometer is important to evaporate water from the wet clothing and achieve a correct wet bulb temperature.

Sufficient air movement can be achieved with a sling thermometer or similar.

Relative humidity can be estimated from the tables below or alternatively from a psyhrometric or a Mollier diagram.

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 vs. Dry and Wet Bulb Temperatures - SI Units
T_db- T_wb(^oC)	Relative Humidity - RH (%)
	Dry Bulb Temperature - T_db (^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 vs. Dry and Wet Bulb Temperatures - Imperial Units
T_db- T_wb(^oF)	Relative Humidity - RH (%)
	Dry Bulb Temperature - T_db (^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