Chapter X: Atmospheric Humidity (2)

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Air | Vapor |Depression of wet-bulb thermometer (_t_-_t´_)
temp.| press.|-----+-----+-----+-----+-----+-----+-----+-----
_t_ | _e_ | 9.0 |10.0 |11.0 |12.0 |13.0 |14.0 |15.0 |16.0
------+-------+-----+-----+-----+-----+-----+-----+-----+-----
30 | 0.164 | -20 | | | | | | |
31 | 0.172 | -14 | -50 | | | | | |
32 | 0.180 | -9 | -29 | | | | | |
33 | 0.187 | -5 | -20 | | | | | |
34 | 0.195 | -2 | -14 | -50 | | | | |
| | | | | | | | |
35 | 0.203 | +1 | -8 | -28 | | | | |
36 | 0.211 | 4 | -4 | -19 | | | | |
37 | 0.219 | 7 | -1 | -12 | -44 | | | |
38 | 0.228 | 9 | +3 | -7 | -25 | | | |
39 | 0.237 | 12 | 6 | -3 | -16 | | | |
| | | | | | | | |
40 | 0.247 | 14 | 8 | +1 | -10 | -35 | | |
41 | 0.256 | 16 | 11 | 4 | -5 | -21 | | |
42 | 0.266 | 17 | 13 | 7 | -1 | -13 | -59 | |
43 | 0.277 | 19 | 15 | 10 | +3 | -7 | -28 | |
44 | 0.287 | 21 | 17 | 12 | 6 | -2 | -17 | |
| | | | | | | | |
45 | 0.298 | 22 | 19 | 14 | 8 | +2 | -9 | -37 |
46 | 0.310 | 24 | 20 | 16 | 11 | 5 | -4 | -20 |
47 | 0.322 | 25 | 22 | 18 | 13 | 8 | +0 | -12 |-53
48 | 0.334 | 27 | 23 | 20 | 15 | 10 | +4 | -6 |-25
49 | 0.347 | 28 | 25 | 21 | 17 | 13 | 7 | -2 |-15
| | | | | | | | |
50 | 0.360 | 29 | 27 | 23 | 19 | 15 | 9 | +2 | -8
51 | 0.373 | 31 | 28 | 25 | 21 | 17 | 12 | 6 | -3
52 | 0.387 | 32 | 29 | 26 | 23 | 19 | 14 | 9 | +1
53 | 0.402 | 34 | 31 | 28 | 24 | 21 | 16 | 11 | 5
54 | 0.417 | 35 | 32 | 29 | 26 | 23 | 19 | 14 | 8
55 | 0.432 | 36 | 34 | 31 | 28 | 24 | 21 | 16 | 11
56 | 0.448 | 38 | 35 | 32 | 29 | 26 | 23 | 19 | 14
57 | 0.465 | 39 | 36 | 34 | 31 | 28 | 24 | 21 | 16
58 | 0.482 | 40 | 38 | 35 | 32 | 29 | 26 | 22 | 18
59 | 0.499 | 42 | 39 | 37 | 34 | 31 | 28 | 24 | 20
| | | | | | | | |
60 | 0.517 | 43 | 41 | 38 | 35 | 32 | 29 | 26 | 22
61 | 0.536 | 44 | 42 | 39 | 37 | 34 | 31 | 28 | 24
62 | 0.555 | 46 | 43 | 41 | 38 | 35 | 32 | 30 | 26
63 | 0.575 | 47 | 45 | 42 | 40 | 37 | 34 | 31 | 28
64 | 0.595 | 48 | 46 | 44 | 41 | 38 | 36 | 33 | 30
| | | | | | | | |
65 | 0.616 | 49 | 47 | 45 | 43 | 40 | 37 | 34 | 31
66 | 0.638 | 51 | 48 | 46 | 44 | 42 | 39 | 36 | 33
67 | 0.661 | 52 | 50 | 48 | 45 | 43 | 40 | 38 | 35
68 | 0.684 | 53 | 51 | 49 | 47 | 44 | 42 | 39 | 36
69 | 0.707 | 54 | 52 | 50 | 84 | 46 | 43 | 41 | 38
| | | | | | | | |
70 | 0.732 | 55 | 53 | 51 | 49 | 47 | 45 | 42 | 40
71 | 0.757 | 57 | 55 | 53 | 51 | 49 | 46 | 44 | 41
72 | 0.783 | 58 | 56 | 54 | 52 | 50 | 48 | 45 | 43
73 | 0.810 | 59 | 57 | 55 | 53 | 51 | 49 | 47 | 44
74 | 0.838 | 60 | 58 | 56 | 54 | 53 | 50 | 48 | 46
| | | | | | | | |
75 | 0.866 | 61 | 60 | 58 | 56 | 54 | 52 | 50 | 47
76 | 0.896 | 62 | 61 | 59 | 57 | 55 | 53 | 51 | 49
77 | 0.926 | 64 | 62 | 60 | 58 | 56 | 54 | 52 | 50
78 | 0.957 | 65 | 63 | 61 | 59 | 58 | 56 | 54 | 52
79 | 0.989 | 66 | 64 | 62 | 61 | 59 | 57 | 55 | 53
| | | | | | | | |
80 | 1.022 | 67 | 65 | 64 | 62 | 60 | 58 | 56 | 54
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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
-----+----+----+----+----+----+----+----+----+----+----+----+----+----
Dry- | | | | | | | | | | | | |
bulb | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | 12 | 13
temp.| | | | | | | | | | | | |
-----+----+----+----+----+----+----+----+----+----+----+----+----+----
70 | 69 | 67 | 66 | 64 | 62 | 61 | 59 | 57 | 55 | 53 | 51 | 49 | 47
69 | 68 | 66 | 64 | 63 | 61 | 59 | 58 | 56 | 54 | 52 | 50 | 48 | 46
68 | 67 | 65 | 63 | 62 | 60 | 58 | 57 | 55 | 53 | 51 | 49 | 46 | 44
67 | 66 | 64 | 62 | 61 | 59 | 57 | 55 | 54 | 52 | 50 | 47 | 45 | 43
66 | 64 | 63 | 61 | 60 | 58 | 56 | 54 | 52 | 50 | 48 | 46 | 44 |
65 | 63 | 62 | 60 | 59 | 57 | 55 | 53 | 51 | 49 | 47 | 45 | 42 | 41
64 | 62 | 61 | 59 | 57 | 56 | 54 | 52 | 50 | 48 | 46 | 43 | | 40
63 | 61 | 60 | 58 | 56 | 55 | 53 | 51 | 49 | 47 | 44 | 42 | 41 | 38
62 | 60 | 59 | 57 | 55 | 53 | 52 | 50 | 48 | 45 | 43 | | 39 | 37
61 | 59 | 58 | 56 | 54 | 52 | 50 | 48 | 46 | 44 | 42 | 41 | 38 | 35
60 | 58 | 57 | 55 | 53 | 51 | 49 | 47 | 45 | 43 | | 39 | 36 | 33
59 | 57 | 56 | 54 | 52 | 50 | 48 | 46 | 44 | | 40 | 38 | 43 | 32
58 | 56 | 55 | 53 | 51 | 49 | 47 | 45 | 42 | 41 | 39 | 36 | 33 | 30
57 | 55 | 54 | 52 | 50 | 48 | 46 | 44 | | 40 | 37 | 35 | 31 | 28
56 | 54 | 53 | 51 | 49 | 47 | 44 | 42 | 41 | 39 | 36 | 33 | 30 | 26
55 | 53 | 52 | 50 | 48 | 46 | 43 | | 40 | 37 | 34 | 31 | 28 | 25
54 | 52 | 50 | 49 | 46 | 44 | 42 | 41 | 39 | 36 | 33 | 30 | 27 | 23
53 | 51 | 49 | 47 | 45 | 43 | | 40 | 37 | 34 | 31 | 28 | 25 | 20
52 | 50 | 48 | 46 | 44 | 42 | 41 | 38 | 36 | 33 | 30 | 27 | 23 | 18
51 | 49 | 47 | 45 | 43 | | 40 | 37 | 34 | 31 | 28 | 25 | 21 | 16
50 | 48 | 46 | 44 | 42 | 41 | 38 | 36 | 33 | 30 | 27 | 23 | 19 | 14
49 | 47 | 45 | 43 | | 40 | 37 | 34 | 31 | 28 | 25 | 21 | 17 | 11
48 | 46 | 44 | 42 | 41 | 38 | 36 | 33 | 30 | 27 | 23 | 19 | 14 | 9
47 | 45 | 43 | | 40 | 37 | 35 | 32 | 29 | 25 | 22 | 17 | 12 | 6
46 | 44 | 42 | 41 | 39 | 36 | 33 | 30 | 27 | 24 | 20 | 15 | 10 | 3
45 | 43 | | 40 | 37 | 35 | 32 | 29 | 26 | 22 | 18 | 13 | 7 | -1
44 | 42 | 41 | 39 | 36 | 33 | 30 | 27 | 24 | 20 | 16 | 11 | 4 | -5
| | 40 | 37 | 35 | 32 | 29 | 26 | 23 | 19 | 14 | 8 | 1 | -9
43 | 41 | 39 | 36 | 34 | 31 | 28 | 25 | 21 | 17 | 12 | 6 | -2 |-15
42 | 40 | 38 | 35 | 33 | 29 | 26 | 23 | 19 | 15 | 9 | 3 | -6 |-22
41 | 39 | 36 | 34 | 31 | 28 | 25 | 22 | 17 | 13 | 7 | 0 |-11 |-32
40 | 38 | 35 | 33 | 30 | 27 | 24 | 20 | 16 | 11 | 4 | -4 |-16 |-74
39 | 37 | 34 | 32 | 29 | 26 | 22 | 18 | 14 | 8 | 2 | -8 | 23 |
38 | 36 | 33 | 31 | 28 | 24 | 21 | 17 | 12 | -6 | -1 |-12 |-35 |
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=Prevention of Frost.=--From the discussion of frost formation it is evident that, the temperature of the dew-point being the determining factor in its probable occurrence, any expedient that may be used either to increase the humidity or to conserve the radiation of heat would prevent a dangerous decline of temperature. Frost prevention is practised in all fruit-growing regions and the method pursued depends on the kind of vegetation to be protected.

In the protection of orchards the use of smudge pots are probably the commonest means for preventing the loss of heat. The object is to create a cloud of smoke over and about the orchard so that it forms a protective covering which prevents the escape of the heat.

In the case of a light frost--that is, where the temperature falls only a few degrees below the frost point--the plants in small gardens and flower beds may be prevented from freezing by liberal sprinkling with water. This is done to raise the humidity of the atmosphere surrounding the vegetation. Most vegetation withstands the temperature at the freezing point without particular injury, and the freezing of part of the water liberates heat in sufficient quantity to prevent a further decline of temperature. This heat liberated on the freezing of water is described in physics as the heat of fusion and in changing part of the water into ice sufficient heat is liberated to check the further fall of temperature.

=Humidifying Apparatus.=--Opportunity for adding moisture, in the desired quantity, to the air of the average dwelling is limited to the evaporation of water in the heating plant, from vessels attached to the radiators or that which goes on in the kitchen. Household humidifying plants are within the range of possibility but there is not yet sufficient demand for their use to make attractive their manufacture.

In the hot-air furnace a water reservoir is usually a part of the chamber in which the air supply is heated. The water in the reservoir is heated to a greater or lesser degree, depending on the temperature of the furnace and vaporized both by heat and by the constantly changing air.

In the use of a steam plant or hot-water heating plant the opportunity of humidifying the air is very limited. One method is that of suspending water tanks to the back of the radiators from which water is vaporized. While this method is fairly efficient as a humidifier it is inconvenient and therefore apt to be neglected. In houses heated by stoves there are sometimes water urns attached to the top of the frame which are intended for the evaporation of water but as a rule they are not of sufficient size to be of appreciable value.

The quantity of water required to humidify the air of a house will depend _first_, on the temperature and humidity of the outside air; _second_, on the cubic contents of the building; _third_, on the rate of change of air in the building. If the ventilation is good the rate of atmospheric change is rapid and the amount of water in consequence must be correspondingly increased.

The data included in the following table showing the relative humidity and amount of water required were taken from a seven-room frame dwelling in Fargo, N. D., during particularly severe winter weather. The relative humidity determinations were made with a hygrodeik each day at noon. The house was heated by a hot-air furnace arranged to take its air supply from the outside.

The air supply is recorded under Cold-air intake. The furnace was provided with a water pan for humidifying the air supply. The amount of water evaporated each day is recorded in the column headed Evap. in 24 hours. The outside temperature ranged from -12°F. to -21°F. The weather was clear and calm except the last day, Jan. 12, which was windy. The higher humidity on that day was no doubt due to the greater amount of heat required from the furnace and the consequent evaporation of the water from the water pan.

The humidity determinations made by a hygrodeik, as before explained, are only approximately correct but sufficiently exact for practical purposes. The temperature is given in degrees Fahrenheit.

In the table it will be noticed that the outside air was used only a part of the time because of the severity of the weather. Attention is called to the quantity of water required to keep the humidity at the amount shown. This averages 27-1/2 quarts per day. At the time these observations were made the physics lecture-room at the North Dakota Agricultural College averaged 18 to 20 per cent. saturation during class hours, with observations made from a similar instrument. This is a steam-heated room with only accidental means of adding water to the air. The result was an atmosphere 3-1/2 per cent. above that of Death Valley.

HOT-AIR FURNACE

Readings taken at 12 o’clock noon each day

--------+-------+----+----+---------+-------------+------------------
| Temp. |Wet |Dry | Per | Evap. In |
Date |outside|bulb|bulb| cent. | 24 hours | Cold-air intake
| | | |saturated|quarts pints |
--------+-------+----+----+---------+-------------+------------------
Dec. 13 | -13 | 54°| 63°| 53 | | Closed 8 a.m.
Dec. 14 | -18 | 55 | 66 | 47 | | Open
Dec. 15 | -20 | 57 | 68 | 49 | 21 | Closed 7 a.m.
Dec. 16 | -18 | 57 | 67 | 51 | 20 1 | Closed 7 a.m.
Dec. 17 | -22 | 58 | 69 | 48 | 18 1 | Closed 7 a.m.
Dec. 18 | -16 | 55 | 65 | 51 | 17 1½ | Closed 6:30 a.m.
Dec. 19 | -10 | 57 | 68 | 47 | 20 1 | Closed 8 a.m.
Dec. 20 | 0 | 59 | 70 | 49 | 13 ¾ | Not open at night
Jan. 8 | -12 | 58 | 71 | 43 | 18 | Closed
Jan. 9 | -17 | 57 | 71 | 39 | 25 | Open 24 hours
Jan. 10 | -16 | 58 | 69 | 45 | 27 1 | Open 10 hours
Jan. 11 | -21 | 60 | 75 | 40 | 30 | Closed
Jan. 12 | -15 | 60 | 73 | 46 | 30 | Closed
--------+-------+----+----+---------+-------------+------------------

The amounts of water evaporated may seem large to those who are unaccustomed to quantitatively consider problems in ventilation but the small amount of water in the air at -21° must produce a very dry atmosphere when it is raised to 70° in temperature.

The amount of moisture in air at 20°F. and at 80 per cent. humidity is only 1.58 grains to the cubic foot. If this air is now raised to 70° the moisture will still be 1.58 grains where there should be 4 grains of water to make 50 per cent. humidity. It therefore will require the addition of practically 2.42 grains of water for each cubic foot of entering air in order to bring it up to 50 per cent. humidity.

In a case with the above conditions of atmosphere, suppose it is desired to know the amount of water that would be taken up in humidifying the air for a school-room of size to accommodate 40 pupils. The prescribed quantity of air for this purpose is 30 cubic feet per minute for each pupil. The air is to be maintained at a humidity 50 per cent. saturated. The problem will be one of simple arithmetic. If each pupil is to receive 30 cubic feet of air per minute or 1800 cubic feet per hour, the 40 pupils receiving 1800 cubic feet per hour will require 40 × 1800 = 72,000 cubic feet of air per hour. To each cubic foot of the air is to be added 2.74 grains of water, 72,000 × 2.42 = 164,240 grains of water. Reducing this to pounds, 164,240 ÷ 7000 = 23.46 pounds or 2.77 gallons of water per hour.

In practice the room will show a higher amount than 50 per cent. humidity with this addition of the amount of water, because of the water vapor that is exhaled from the lungs of the pupils. That a considerable amount of water vapor is added to the atmosphere by breath exhalation is made evident from the moisture condensed by breathing on a cold pane of glass. In any unventilated room occupied by a considerable number of people the humidity is thus increased a very noticeable amount.

The change in humidity of the air in a closed room filled with people is very pronounced. The constant exhalation of moisture from the lungs is sufficient to saturate the air in a short time. The heavy atmosphere of overcrowded, unventilated rooms is due to moisture exhalation, body odors and increased carbonic acid gas. As the humidity of the atmosphere is increased a sensation of uncomfortable warmth is the result of the lesser evaporation.