Humidity of the Air.

—The amount of water vapor in the air is called the humidity of the air. It may vary from a fraction of a grain per cubic foot in extremely cold weather, to 20 grains per cubic foot during the occasional hot weather of summer.

Since the amounts of moisture that air will hold depends on its temperature, and as the air is ordinarily only partly saturated, the varying amount of moisture are expressed either as relative humidity and stated in per cent. saturation or in the actual weight of water in grains per cubic foot and known as absolute humidity.

The relative humidity of the atmosphere is the amount of moisture contained in a given space as compared with the amount the same air could possibly hold at that temperature. Warm air will hold more moisture than the same air when cold. Air absorbs water like a sponge to a point of saturation. When the air is filled with moisture, any change which takes place to reduce the temperature also reduces its capacity to hold the water vapor and the excess is deposited as dew. This supersaturation ordinarily takes place near things which lose their heat faster than the surrounding air and the nearest colder surface acts as a condenser to receive the drops of dew. Grass being in convenient position is the commonest receptacle for dew formation. If the dew forms in the air it falls as rain, but if the temperature of the dew-point is below freezing, the dew immediately freezes and snow is the result.

In the consideration of problems that involve atmospheric moisture, both relative and absolute humidity are factors of common use, that are capable of exact determination. The relative humidity of the air is most readily determined and as it expresses the state of the atmosphere in which plants and animals live and thrive, as opposed to other conditions of humidity in which they sometimes sicken and die, it is one of the indicators of the quality of atmospheric air.

In the subject of ventilation, which is undertaken later, it will be found that a definite knowledge of atmospheric humidity has much to do with the successful operation of ventilation apparatus. Most people recognize the “balmy air of June” without realizing just why at the same temperature other seasons are not so delightful. In reality it is the condition of atmospheric humidity combined with an agreeable temperature that gives the kind of air in which we find the greatest degree of comfort.

The effect of moderately warm humid air is that of higher temperature than the thermometer indicates. When the atmosphere is near the point of saturation, the evaporation which usually goes on, from the surface of the body, practically ceases. In summer time a temperature of 85°F. with relative humidity of 90 per cent. saturation seems warmer than a temperature of 100° at 40 per cent. saturation, because of the cooling effect produced by the increased evaporation due to the drier air.

In winter, when most of the time is spent indoors, in an atmosphere that is very dry, the sensation of discomfort produced by the lack of humidity oftentimes leads to physical derangements that would never appear under more desirable conditions. The cause of many ailments of the nose, throat and lungs during the winter months is attributed by physiologists to breathing almost constantly the dry vitiated indoor air. The cause of dry air in buildings is not difficult to explain; it is a great deal more difficult to realize that the lack of water breeds so much discomfort.

In order to express the condition of humidity that may exist in the average dwelling, office or school-room during the winter, it is most convenient to refer to the results of varying atmospheric conditions that are given in Table 1—Properties of Air—which appears below. In the second column of the table, under the heading “Weight of vapor per cubic foot of saturated air,” will be found the amount of moisture in grains per cubic foot that will be required to humidify air at different temperatures. It will be seen that at 10° the air will contain, when fully saturated, only 1.11 grains of water, while at 70° temperature the same air would hold 8 grains of water. These amounts will be found in the column opposite the temperature readings. It is at once evident that when saturated air at 10° is raised to normal temperature 70°, the original amount of moisture is contained in an atmosphere capable of holding 8 grains of water. Its relative humidity will therefore be ^1.11⁄₈, practically 14 per cent. saturated. Unless moisture is received by the air from some other source this condition will produce a very dry atmosphere.

The normal atmospheric temperature of 70°F. with a relative humidity of 50 to 60 per cent. saturation produces a condition that is one of agreeable warmth to the average person in health and is recognized as the atmosphere most desirable. To some, this state of temperature and humidity is that of too much warmth and a temperature of 68°, with the same humidity, is most agreeable. At the same temperature, a reduction of the humidity to 20 per cent. saturation will produce a feeling of discomfort and the sensation will be that of a lack of heat. The cause for this latter feeling is due to excessive evaporation of moisture from the body.

Table I.—Properties of Air

Temperature of the air	Weight of vapor per cubic foot of saturated air	Weight of cubic foot of saturated air
Fahrenheit	Grains	Grains
10°	1.11	589.4
11	1.15	588.1
12	1.19	586.8
13	1.24	585.5
14	1.28	584.2
15	1.32	582.9
16	1.37	581.6
17	1.41	580.3
18	1.47	579.1
19	1.52	577.8
20	1.58	576.5
21	1.63	575.3
22	1.69	574.0
23	1.75	572.7
24	1.81	571.5
25	1.87	570.2
26	1.93	569.0
27	2.00	567.7
28	2.07	566.5
29	2.14	565.3
30	2.21	564.1
31	2.29	562.8
32	2.37	561.6
33	2.45	566.4
34	2.53	559.2
35	2.62	558.0
36	2.71	556.8
37	2.80	555.6
38	2.89	554.4
39	2.99	553.2
40	3.09	552.0
41	3.19	550.8
42	3.30	549.6
43	3.41	548.4
44	3.52	547.2
45	3.64	546.1
46	3.76	544.9
47	3.88	543.7
48	4.01	541.3
49	4.14	542.5
50	4.28	540.2
51	4.42	539.0
52	4.56	537.9
53	4.71	536.7
54	4.86	535.5
55	5.02	534.4
56	5.18	533.2
57	5.34	532.1
58	5.51	534.9
59	5.69	529.8
60	5.87	528.6
61	6.06	527.0
62	6.25	526.3
63	5.45	525.2
64	6.65	524.0
65	6.87	522.0
66	7.08	521.7
67	7.30	520.0
68	7.53	519.4
69	7.76	518.3
70	8.00	517.2

The evaporation of moisture is always accompanied with the loss of heat required to produce such change of condition. This is known as the heat of vaporization and represents a definite amount of heat that is used up whenever water is changed into vapor. No matter what its temperature may be—whether hot or cold—when water is vaporized, a definite amount of heat is required to change the water into vapor.

Water may be evaporated at any temperature; even ice evaporates. A common instance of the latter is that of wet clothes which “freeze dry” in winter weather when hung on the clothes line. The rate at which evaporation takes place depends on the dryness of the surrounding air and the rapidity of its motion. In dry windy weather evaporation is most rapid.

As before stated, whenever water evaporates—at no matter what temperature—a definite quantity of heat is necessary to change the water into vapor. The exact amount of heat required to produce this change varies somewhat with the temperature and atmospheric pressure but it always represents a large loss of heat. At the boiling point of water (212°F.) the heat of vaporization is 970 B.t.u. for each pound of water evaporated, but at a lower temperature it is greater than that amount. At the temperature of the body (98.6°) the heat necessary to evaporate a pound of moisture from its surface is 1045 B.t.u.

It is the absorption of heat due to evaporation that cools the air of a sprinkled street. The more rapid the evaporation the more pronounced is the decline of temperature in the immediate vicinity. The same effect is produced when moisture is evaporated from the surface of the body. The acceleration of evaporation caused by a breeze or the blast of air from an electric fan is that which produces the chilling sensation to the body. During winter weather the effect of the cold wind is rendered more severe by evaporation of moisture from the body. In health, the body being in a slightly moist condition, the evaporation which goes on from its surface is what keeps it cool in warm weather, but if on account of excessive dryness of the surrounding air the evaporation is very rapid, a sensation of cold is the result.

Not only does excessively dry air produce the sensation of chilliness but the loss of heat from the body due to sudden or long exposure effects the general health and is conducive to chills that are followed by fever. In health the temperature of the body is constant and normally 98.6°F.; any condition that reduces that temperature tends toward a lowering of vitality and the consequent inability to withstand the attack of disease. In a very dry atmosphere the skin, instead of being slightly moist, is kept dry, the result of which is the irritation that produces chaps and roughness of the surface.

Reports of the U. S. Weather Department show that the relative humidity of Death Valley, which is the driest and hottest known country, during the driest period of the year—between May and September—averages 15.5 per cent. saturation. In winter, many buildings, particularly offices and school buildings are not far from that atmospheric condition, constantly. Under the usual conditions of house heating, there is an almost absolute lack of means to give moisture to the air. Almost without exception steam-heating plants and hot-water heating plants in office buildings and dwellings are without any provision for changing the atmospheric humidity.

In school buildings that are not kept under a more desirable condition of temperature and humidity, the general health is impaired and the behavior of the pupils very markedly influenced. The tension of a school-room full of fidgety nervous children can be very promptly and greatly reduced by the introduction of water vapor into the air to 50 per cent. saturation.

All modern school buildings, auditoriums, etc., are provided—aside from the heating plants—with means of ventilating in which the entering air is washed and humidified to the desired degree, before being sent into the rooms.

The popular conception of the hot-air furnace method of heating is that it produces particularly dry air, when in reality it is the only type of house-heating plant in which any provision is made for adding water to the air. These furnaces are usually furnished with a water reservoir by use of which the humidity may be raised to a desirable point.

Much of the water which enters the air of the average home, during winter weather, comes from the evaporation that goes on in the kitchen. Usually on wash days the humidity is raised to a marked degree and that day is commonly followed by a short period of agreeable atmospheric condition. The arrangement of many houses is such that a much-improved condition of humidity might be obtained from the kitchen by continuous evaporation of water from a tea-kettle.

Relative Humidity
Depression of wet-bulb thermometer (t-t')

Air temp. t	1.0	2.0	3.0	4.0	5.0	6.0	7.0	8.0	9.0	10.0
35	91	82	73	64	55	46	37	29	20	12
36	91	82	73	65	56	48	39	31	23	14
37	91	83	74	66	58	49	41	33	25	17
38	91	83	75	67	59	51	43	35	27	19
39	92	84	76	68	60	52	44	37	29	21

40	92	84	76	68	61	53	46	38	31	23
41	92	84	77	69	62	54	47	40	33	26
42	92	85	77	70	62	55	48	41	34	28
43	92	85	78	70	63	56	49	43	36	29
44	93	85	78	71	64	57	51	44	37	31

45	93	86	79	71	65	58	52	45	39	33
46	93	86	79	72	65	59	53	46	40	34
47	93	86	79	73	66	60	54	47	41	35
48	93	87	80	73	67	60	54	48	42	36
49	93	87	80	74	67	61	55	49	43	37

50	93	87	81	74	68	62	56	50	44	39
51	94	87	81	75	69	63	57	51	45	40
52	94	88	81	75	69	63	58	52	46	41
53	94	88	82	75	70	64	58	53	47	42
54	94	88	82	76	70	65	59	54	48	43

55	94	88	82	76	71	65	60	55	49	44
56	94	88	82	77	71	66	61	55	50	45
57	94	88	83	77	72	66	61	56	51	46
58	94	89	83	77	72	67	62	57	52	47
59	94	89	83	78	73	68	63	58	53	48

60	94	89	84	78	73	68	63	58	53	49
61	94	89	84	79	74	68	64	59	54	50
62	94	89	84	79	74	69	64	60	55	50
63	95	90	84	79	74	70	65	60	56	51
64	95	90	85	79	75	70	66	61	56	52

Air temp. t	11.0	12.0	13.0	14.0	15.0	16.0	17.0	18.0	19.0	20.0
35	4
36	6
37	9	1
38	12	4
39	14	7

40	16	9	2
41	18	11	5
42	21	14	7	0
43	23	16	9	3
44	24	18	12	5

45	26	20	14	8	2
46	28	22	16	10	4
47	29	23	17	12	6	1
48	31	25	19	14	8	3
49	32	26	21	15	10	5

50	33	28	22	17	12	7	2
51	35	29	24	19	14	9	4
52	36	30	25	20	15	10	6	0
53	37	32	27	22	17	12	7	3
54	38	33	28	23	18	14	9	5	0

55	39	34	29	25	20	15	11	6	2
56	40	35	31	26	21	17	12	8	4
57	41	36	32	27	23	18	14	10	5	1
58	42	38	33	28	24	20	15	11	7	3
59	43	39	34	30	25	21	17	13	9	5

60	44	40	35	31	27	22	18	14	10	6
61	45	40	36	32	28	24	20	16	12	8
62	46	41	37	33	29	25	21	17	13	9
63	47	42	38	34	30	26	22	18	14	11
64	48	43	39	35	31	27	23	20	16	12

Relative Humidity (Continued)
Depression of wet-bulb thermometer (t-t´)

Air temp. t	1.0	2.0	3.0	4.0	5.0	6.0	7.0	8.0	9.0	10.0	11.0
65	95	90	85	80	75	70	66	62	57	53	48
66	95	90	85	80	76	71	66	62	58	53	49
67	95	90	85	80	76	71	67	62	58	54	50
68	95	90	85	81	76	72	67	63	59	55	51
69	95	90	86	81	77	72	68	64	59	55	51

70	95	90	86	81	77	72	68	64	60	56	52
71	95	90	86	82	77	73	69	64	60	56	53
72	95	91	86	82	78	73	69	65	61	57	53
73	95	91	86	82	78	73	69	65	61	58	54
74	95	91	86	82	78	74	70	66	62	58	54

75	96	91	87	82	78	74	70	66	63	59	55
76	96	91	87	83	78	74	70	67	63	59	55
77	96	91	87	83	79	75	71	67	63	59	56
78	96	91	87	83	79	75	71	67	64	60	57
79	96	91	87	83	79	75	71	68	64	60	57

80	96	91	87	83	79	76	72	68	64	61	57
82	96	92	88	84	80	76	72	69	65	62	58
84	96	92	88	84	80	77	73	70	66	63	59
86	96	92	88	85	81	77	74	70	67	63	60
88	96	92	88	85	81	78	74	71	67	64	61

90	96	92	89	85	81	78	75	71	68	65	62
92	96	92	89	85	82	78	75	72	69	65	62
94	96	93	89	86	82	79	75	72	69	66	63
96	96	93	89	86	82	79	76	73	70	67	64
98	96	93	89	86	83	79	76	73	70	67	64

100	96	93	90	86	83	80	77	74	71	68	65
102	96	93	90	86	83	80	77	74	71	68	65
104	97	93	90	87	84	80	77	74	72	69	66
106	97	95	90	87	84	81	78	75	72	69	66
108	97	93	90	87	84	81	78	75	72	70	67

Air temp. t	12.0	13.0	14.0	15.0	16.0	17.0	18.0	19.0	20.0
65	44	40	36	32	28	25	21	17	13
66	45	41	37	33	29	26	22	18	15
67	46	42	38	34	30	27	23	20	16
68	47	43	39	35	31	28	24	21	17
69	47	44	40	36	32	29	25	22	19

70	48	44	40	37	33	30	26	23	20
71	49	45	41	38	34	31	27	24	21
72	49	46	42	39	35	32	28	25	22
73	50	46	43	40	36	33	29	26	23
74	51	47	44	40	37	34	30	27	24

75	51	48	44	41	38	34	31	28	25
76	52	48	45	42	38	35	32	29	26
77	52	49	46	42	39	36	33	30	27
78	53	50	46	43	40	37	34	31	28
79	54	50	47	44	41	37	34	31	29

80	54	51	47	44	41	38	35	32	29
82	55	52	49	46	43	40	37	34	31
84	56	53	50	47	44	41	38	35	32
86	57	54	51	48	45	42	39	37	34
88	58	55	52	49	46	43	41	38	35

90	59	56	53	50	47	44	42	39	37
92	59	57	54	51	48	45	43	40	38
94	60	57	54	52	49	46	44	41	39
96	61	58	55	53	50	47	45	42	40
98	61	59	56	53	51	48	46	43	41

100	62	59	57	54	52	49	47	44	42
102	63	60	57	55	52	50	47	45	43
104	63	61	58	56	53	51	48	46	14
106	64	61	59	56	54	51	49	47	45
108	64	62	59	57	54	52	50	47	45

The prevailing impression seems to exist that when air is heated, it loses its moisture. In reality, air that is heated only attains a condition in which its capacity for containing moisture is increased. If after being heated to a high degree—and is relatively very dry—the air is reduced to its original temperature, the amount of moisture will be the same as was originally contained. In heating houses with hot air, the seemingly dry condition is usually due to temperature alone. When a hot-air furnace is provided with the customary reservoir for moistening the discharged air, it may be made to produce excellent conditions of atmospheric humidity. The heated air readily absorbs the water evaporated in the furnace from the water reservoir and enters the rooms as relatively dry air but containing more moisture than the outside air; when it has been reduced in temperature by mixing with the cooler air of the house, its moisture content remains unaltered and at the lower temperature its relative humidity is increased.