E. S. Keene

The physical effect of atmospheric humidity has come to be recognized by all who deal in problems of house heating, sanitation and hygiene. The difference in effect of dry atmosphere, from that of air containing a desirable degree of moisture, is very noticeable in all buildings that are artificially heated. The effect of dry air is made apparent in the average home during the winter months by the shrinking of the woodwork and furniture. The absorption of the moisture from the building which is usually termed “drying out,” causes the joints in the floors and casements to open, doors to shrink until they fail to latch and drawers that have opened with difficulty during the summer then work freely.

Winter time is the season of prevalent colds, chaps and roughness of the skin, not so much on account of cold weather as because of dry air. The skin which is normally moist is kept dry by constant evaporation with the attending discomfort of an irritated surface and the results which follow.

The humidity of the air in which we live and on which we depend for life has much to do with the bodily comfort we derive in existence, and is suspected of being the cause of many physical ailments. Ventilation engineers not only recognize this condition but have found means of controlling it. It is possible to so control atmosphere temperature and humidity of buildings as to produce any desired condition.

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

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.

Relative Humidity.

—Suppose that on a damp day the outside temperature is 50° and that the atmosphere is 90 per cent. saturated. The air that comes into the house at this temperature and humidity is heated to 70°. The rise of temperature gives the air the property of absorbing additional moisture so that the relative humidity which was 90 per cent. is now much less. From the table relative humidity, will be seen that at 50° temperature and 90 per cent. saturation the air contains 3.67 grains of moisture. When the air is heated to 70°, it still contains the original amount of moisture but its relative humidity has decreased with the change of temperature. It is really the amount of moisture present—3.67 grains—divided by the amount necessary to saturate the air at 70°, which is 8 grains; this gives approximately a relative humidity 40 per cent. saturation.

As the temperature goes lower, less and less moisture is required to saturate the air. If saturated air at 0°F., which contains 0.48 grain of water, is raised to 70°F.—where 8 grains of water is required for saturation—the percentage of saturation would be ^0.48/₈ or 6 per cent.

The Hygrometer.

—The instrument most commonly employed for determining atmospheric humidity is the hygrometer. This appliance is composed of two thermometers mounted in a frame with a vessel for holding water. One of the thermometers is intended to register the temperature of the air and is called the dry-bulb thermometer. The bulb of the other—the wet-bulb thermometer—is covered with a piece of cloth or other porous material which is kept saturated with water, absorbed from the water holder. The dryness of the air is indicated in the wet-bulb thermometer by the decline of temperature due to evaporation.

The rate of evaporation from the wet-bulb covering will vary with the humidity and if the air is very dry the wet-bulb thermometer will register a temperature several degrees below that of the other thermometer. If the air is saturated with moisture, no evaporation will take place and the thermometers will read alike. The relative humidity of the air as indicated by the readings of the thermometers is taken directly from a humidity table. The table is made to suit any condition of atmospheric humidity and the determinations require no calculation.

Fig. 157 shows the U. S. Weather Bureau pattern hygrometer such as is used at the weather stations. The wet-bulb thermometer has a muslin or knitted silk covering which dips into a metal water cup as shown in the figure. It is important that the covering of the wet bulb be kept in good condition. The evaporation of the water from the covering leaves in the meshes particles of solid matter that were held in solution in the water. The accumulation of the solids ultimately prevent the water from thoroughly wetting the wick.

An observation consists in reading the two thermometers and from the difference between the wet-bulb reading and that of the dry-bulb, the relative humidity is taken directly from the table. To illustrate, suppose that the dry-bulb thermometer reads 60° and that the wet-bulb reads 56°. The difference between the two readings is 4°. In the table of relative humidity on page 202, 60° is found in the column headed, Air temp. t, and opposite that number in the column headed 4 is 78, which indicates that under the observed conditions the air is 78 per cent. saturated with moisture. This table is suited for air temperatures from 35°F. to 80°F. and depressions of the wet-bulb thermometer from 1°F. to 20°F. The table, therefore, has a range of variations which will admit humidity determinations for all ordinary conditions.

The Hygrodeik.

—In Fig. 158 is shown a form of hygrometer known as a hygrodeik, by means of which atmospheric humidity may be determined without the use of the tables. In the figure the wet-bulb and dry-bulb thermometers are easily recognized. A glass water bottle W is held to the base of the instrument by spring clips which permit its removal to be filled with water. Between the thermometers is a diagram chart from which the atmospheric humidity is taken. An index arm, carrying a movable pointer P, permits the instrument to be set for any observed thermometer readings.

The index is really a graphical method of expressing the figures given in the table on pages 202-203. In the picture the wet-bulb thermometer reads 65°, the dry-bulb thermometer 77°. To determine the relative humidity under these conditions the movable arm is swung to the left and the pointer P placed on the left-hand scale at the line 65°. The arm is then swung to the right until the pointer touches the downward curving line beginning at 77°, the dry-bulb reading. The lower end of the arm H now points to the relative humidity, where 52 per cent. is indicated by the scale at the bottom of the index.

The same result is obtained from the table of Relative Humidity. The readings of the thermometers in the figure give a difference in temperature of 12°, the dry-bulb thermometer reads 77°. Referring this data to the humidity table, the column marked 12, for the depression of the wet-bulb thermometer and opposite 77° in the air temperature column, is found 53 which indicates the per cent. of saturation. The hygrodeik gives further the temperature of the dew-point, on the scale to the right; and the absolute humidity may be found by following the upward curving line nearest the pointer, at the end of which line is given the value in grains of moisture per cubic foot. The hygrodeik or other instrument of the kind is very largely used in places where relative humidity is regularly observed by those of limited experience, as in school-rooms, auditoriums, etc. Such records are not intended to be perfectly accurate and the readings of the hygrodeik are very well-suited for the purpose.

In using the hygrometer and the hygrodeik the instruments are stationary; they are usually hung on the wall in a convenient location for observation and are placed to avoid accidental drafts in order that the conditions surrounding the observation may be the same at all times. The evaporation which takes place from the wet bulb is due to natural convection and does not always reach the maximum amount. The evaporation is furthermore influenced by accidental variations and consequently the results cannot be considered exact.

Under conditions that demand more exact humidity records than are obtainable with hygrometer, the psychrometer furnishes means of making more accurate observation. The psychrometer shown in Fig. 159 is of the form used by the U. S. Weather Department. Like the hygrometer, it is composed of a wet-bulb and a dry-bulb thermometer but no water cup is attached to the instrument for moistening the wick of the wet bulb. When ready for use the wick is wet with water before each observation.

The greater accuracy to be attained by the use of this instrument is on account of the maximum evaporation which is obtained from the wet bulb for any atmospheric condition. The evaporation which takes place from the wet-bulb thermometer in quiet air is not so great as that which occurs if the same air is in motion. In moving air, however, there is a certain maximum rate beyond which no further evaporation will take place.

The motion of the air may be produced either by blowing on the bulb with a fan or air blast, or by whirling the thermometer. With the psychrometer the latter method is used. This instrument is provided with a handle which is pivoted to the frame and about which it is swung to produce a maximum evaporation from the wick. When a motion of the air is attained sufficient to produce a saturated atmosphere about the bulb, the temperature will remain constant.

A velocity of air or the motion of the wet-bulb thermometer 10 feet per second is that usually taken as the rate for observation and the swinging is kept up 3 or 4 minutes or until the temperature of the wet-bulb thermometer remains stationary.

Then the temperature of each thermometer is read and the humidity found in the table. Relative humidity determinations may be made at temperatures below the freezing point if sufficient precaution is taken in the observations. When the instrument is not in use, it is kept in the metallic case shown in the picture, to protect it from injury.

Dial Hygrometers.

—Various forms of hygrometers are in use, in which a pointer is intended to indicate on a dial the percentage of atmospheric humidity. That shown in Fig. 160 is one of the common forms. Instruments of this kind depend for their action on the absorptive property of catgut or other materials that are sensitive to the moisture changes of the air.

These instruments give fairly accurate readings in a small range for a limited time, but they are apt to go out of adjustment from causes that cannot be controlled. Unless they are occasionally compared with a standard humidity determination, their readings cannot be relied upon for definite amounts of atmospheric moisture.

The Swiss Cottage “Barometer.”

—Fig. 161 is one of the instruments of absorptive class that are sometimes used as weather indicators. The images which occupy the openings in the cottage are so arranged that with the approach of damp weather the man comes outside and at the same time the woman moves back into the house. In fair weather the reverse movement takes place. The figures are mounted on the opposite ends of a light stick which is fastened to an upright pillar. The movement of the images is caused by the change in length of a piece of catgut which is secured to the pillar and also to the frame of the house. Any change in atmospheric humidity causes a contraction or elongation of the catgut which moves the pillar and with it the images.

Since stormy weather is accompanied by a high degree of humidity and fair weather is attended with dry atmosphere, the movement of the images indicates in some degree the weather changes; but the device is not in any way influenced by atmospheric pressure and hence is not a barometer.

Dew-point.

—Dew is formed whenever falling temperature of the air passes the point where saturation occurs. The reduction of the temperature of air raises the relative humidity because of the diminished capacity to contain moisture. As the temperature declines there will come a point at which the air is saturated and any further decrease of temperature will cause supersaturation. At this point the moisture will be deposited on the cooler surfaces in the form of drops. The temperature at which dew begins to form is known as the dew-point. The sweating of cold water pipes, the dew that forms on a water glass and other relatively cold surfaces is caused by a temperature below the dew-point of the air.

Dew-point Table
Dew-point in degrees Fahrenheit, barometer pressure 29 inches

The temperature at which dew forms will depend on the amount of moisture present in the air, but with a definite humidity and air pressure it will always occur at the same temperature. If the dew-point is above freezing, the dew will form as drops of water, but if it is at or slightly below the freezing point, the dew will appear as frost. White frost is formed when the dew-point is only a few degrees below the freezing point. A Black frost occurs when the atmospheric humidity is so low that dew does not form until the temperature is much below the freezing point.

To Determine the Dew-point.

—The dew-point may be found by a number of methods, usually described in works on physics but practical determinations are made with a hygrometer or psychrometer and a dew-point table. Accurate determinations must be made by the use of the psychrometer; those made by the hygrometer are approximate. Suppose the reading of the dry-bulb thermometer is 68 and that this is designated as t; at the time the wet-bulb temperature is 57 and is called t´. The depression of the wet bulb for these temperatures (t-t´) is 11°. In the dew-point table above is found in the dry-bulb column, opposite this number in the column headed 11—under depression of the wet-bulb thermometer—is 49, which is the dew-point for the observed conditions.

As another illustration, suppose the dry bulb of the psychrometer marks 65° and the wet bulb indicates 56°F.; then 65-56 equals 9° of the cold produced by evaporation. The dew-point is determined in exactly the same way as with the hygrometer. Opposite 65, in the dry-bulb column of the dew-point table, under the column of differences marked 9, is found the dew-point for the observed conditions. This is 49° at which temperature dew will begin to form.

Frost Prediction.

—The formation of dew is always attended with a liberation of heat—the heat of vaporization—which tends to check the further decline of temperature. The heat thus developed is usually sufficient to prevent the fall of temperature beyond a very few degrees, but at times when there is little moisture in the air the fall of several degrees of temperature is necessary before the heat liberated by the forming dew balances the heat lost by radiation and the temperature remains stationary.

This condition of things was pointed out many years ago by Tyndall, who in his book on “Heat” states: “The removal for a single summer’s night of the aqueous vapor which covers England would be attended by the destruction of every plant which a freezing temperature would kill.”

The frosts of late spring and early fall which occur at times of dry air and cloudless sky are often caused by local conditions that are not forecasted by the weather department and often may be successfully combated.

At the time of suspected frost, the temperature of the dew-point in relation to the freezing point determines the probability of a freezing temperature. If the dew-point occurs at 10° or more above the freezing point there will be little danger of a killing frost. As the difference in temperature between the dew-point and the frost point decreases, the danger of frost increases. If the dew-point falls at the freezing point, frost is a certainty.

In using the table on page 214, the open diagonal line may be considered the danger line and any dew-point falling below the temperature thus indicated will be considered dangerously near the frost point. This table differs from the other dew-point table only in the range of temperature. The dew-point is found in exactly the same way as before. In the use of the psychrometer and table as a means of frost prediction it is first necessary to make a reading of the wet-bulb and dry-bulb temperature described above. The dry-bulb reading is found in the left-hand column of the table; then follow the horizontal line opposite the figure, till the perpendicular column is reached indicating the difference in reading between the dry and wet bulb. The number at the meeting will be the temperature of the dew-point. For example, suppose the dry bulb stands at 65° and the wet bulb at 55°, the difference being 10° and dew-point under these conditions will be 47°.

If the dew-point is 10° or more above the freezing point there is no danger of a frost, but if the conditions are such as to give a temperature difference less than 10° above the freezing point there would be danger. If the dew-point falls below the open diagonal line of the table there is danger and that danger increases as the difference in degrees between the freezing point and the dew-point becomes less.

As another illustration, suppose that at sunset at the time of suspected frost the dry-bulb thermometer read 54 and the depression of the wet bulb showed 10°. Referring to the table it will be seen that for these conditions the dew-point falls at 33 which is only 1° above the freezing point. It is highly probable that frost would form.

Dew-point Table for Frost Prediction
Depression of the wet-bulb thermometer