Measuring Air Humidity

Measuring relative air humidity with dry and wet bulb temperatures

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Air humidity can be estimated by measuring

• the dry bulb temperature
• the wet bulb temperature

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 a continuously air flow 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 Mollier diagram.

Temperature in Fahrenheit

Relative Humidity - RH (%)
Difference Between Dry Bulb and Wet Bulb Temperatures  Tdb - Twb (oF)	Dry Bulb Temperature - Tdb (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

Temperature in Celsius

Relative Humidity - RH (%)
Difference Between Dry Bulb and Wet Bulb Temperatures  Tdb - Twb (oC)	Dry Bulb Temperature - Tdb (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