ATMOSPHERIC HUMIDITY
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
-----------+-------------+-------------
| Weight of |
| vapor per |Weight of
Temperature|cubic foot of|cubic foot of
of the air |saturated air|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.| 1.0 | 2.0 | 3.0 | 4.0 | 5.0 | 6.0 | 7.0 | 8.0 | 9.0 |10.0 |11.0
_t_ | | | | | | | | | | |
-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+----
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.| 12.0 | 13.0 | 14.0 | 15.0 | 16.0 | 17.0 | 18.0 | 19.0 | 20.0
_t_ | | | | | | | | |
-----+------+------+------+------+------+------+------+------+-----
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.
=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.
[Illustration: FIG. 157.--Hygrometer of U. S. Weather Bureau type; for
determining atmospheric humidity.]
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.
[Illustration: FIG. 158.--The hygrodeik. A form of hygrometer in which
relative humidity is found directly from a diagram.]
=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.
[Illustration: FIG. 159.--Psychrometer of U. S. Weather Bureau type;
for accurate determination of atmospheric humidity.]
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.
[Illustration: FIG. 160.--Dial hygrometer.]
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.
[Illustration: FIG. 161.--Swiss cottage “Barometer.” This device is
arranged to show the condition of atmospheric humidity by the movement
of the images. It is not really 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
------+-------+-----+-----+-----+-----+-----+-----+-----+-----
Air | Vapor |Depression of wet-bulb thermometer (_t_-_t´_)
temp.| press.|-----+-----+-----+-----+-----+-----+-----+-----
_t_ | _e_ | 1.0 | 2.0 | 3.0 | 4.0 | 5.0 | 6.0 | 7.0 | 8.0
------+-------+-----+-----+-----+-----+-----+-----+-----+-----
30 | 0.164 | 27 | 25 | 22 | 18 | 14 | 9 | +3 | -5
31 | 0.172 | 29 | 26 | 23 | 20 | 16 | 11 | 5 | -2
32 | 0.180 | 30 | 27 | 24 | 21 | 17 | 13 | 8 | +1
33 | 0.187 | 31 | 28 | 25 | 22 | 19 | 15 | 10 | 3
34 | 0.195 | 32 | 29 | 27 | 24 | 20 | 16 | 12 | 6
| | | | | | | | |
35 | 0.203 | 33 | 30 | 28 | 25 | 22 | 18 | 14 | 8
36 | 0.211 | 34 | 31 | 29 | 26 | 23 | 20 | 15 | 11
37 | 0.219 | 35 | 32 | 30 | 27 | 24 | 21 | 17 | 13
38 | 0.228 | 36 | 33 | 31 | 28 | 26 | 23 | 19 | 14
39 | 0.237 | 37 | 34 | 32 | 29 | 27 | 24 | 21 | 16
| | | | | | | | |
40 | 0.247 | 38 | 35 | 33 | 31 | 28 | 25 | 22 | 18
41 | 0.256 | 39 | 37 | 34 | 32 | 29 | 26 | 23 | 20
42 | 0.266 | 40 | 38 | 35 | 33 | 30 | 28 | 25 | 21
43 | 0.277 | 41 | 39 | 36 | 34 | 31 | 29 | 26 | 23
44 | 0.287 | 42 | 40 | 38 | 35 | 32 | 30 | 27 | 24
| | | | | | | | |
45 | 0.298 | 43 | 41 | 39 | 36 | 34 | 31 | 29 | 26
46 | 0.310 | 44 | 42 | 40 | 37 | 35 | 32 | 30 | 27
47 | 0.322 | 45 | 43 | 41 | 39 | 36 | 34 | 31 | 28
48 | 0.334 | 46 | 44 | 42 | 40 | 37 | 35 | 32 | 30
49 | 0.347 | 47 | 45 | 43 | 41 | 39 | 36 | 34 | 31
| | | | | | | | |
50 | 0.360 | 48 | 46 | 44 | 42 | 40 | 37 | 35 | 32
51 | 0.373 | 49 | 47 | 45 | 43 | 41 | 39 | 36 | 34
52 | 0.387 | 50 | 48 | 46 | 44 | 42 | 40 | 37 | 35
53 | 0.402 | 51 | 49 | 47 | 45 | 43 | 41 | 39 | 36
54 | 0.417 | 52 | 50 | 49 | 47 | 44 | 42 | 40 | 38
55 | 0.432 | 53 | 52 | 50 | 48 | 46 | 43 | 41 | 39
56 | 0.448 | 54 | 53 | 51 | 49 | 47 | 45 | 43 | 40
57 | 0.465 | 55 | 54 | 52 | 50 | 48 | 46 | 44 | 42
58 | 0.482 | 56 | 55 | 53 | 51 | 49 | 47 | 45 | 43
59 | 0.499 | 57 | 56 | 54 | 52 | 50 | 48 | 46 | 44
| | | | | | | | |
60 | 0.517 | 58 | 57 | 55 | 53 | 51 | 49 | 47 | 45
61 | 0.536 | 59 | 58 | 56 | 54 | 52 | 51 | 49 | 46
62 | 0.555 | 60 | 59 | 57 | 55 | 54 | 52 | 50 | 48
63 | 0.575 | 61 | 60 | 58 | 56 | 55 | 53 | 51 | 49
64 | 0.595 | 62 | 61 | 59 | 58 | 56 | 54 | 52 | 50
| | | | | | | | |
65 | 0.616 | 63 | 62 | 60 | 59 | 57 | 55 | 53 | 51
66 | 0.638 | 64 | 63 | 61 | 60 | 58 | 56 | 54 | 53
67 | 0.661 | 65 | 64 | 62 | 61 | 59 | 57 | 56 | 54
68 | 0.684 | 67 | 65 | 63 | 62 | 60 | 58 | 57 | 55
69 | 0.707 | 68 | 66 | 64 | 63 | 61 | 60 | 58 | 56
| | | | | | | | |
70 | 0.732 | 69 | 67 | 66 | 64 | 62 | 61 | 59 | 57
71 | 0.757 | 70 | 68 | 67 | 65 | 63 | 62 | 60 | 58
72 | 0.783 | 71 | 69 | 68 | 66 | 65 | 63 | 61 | 60
73 | 0.810 | 72 | 70 | 69 | 67 | 66 | 64 | 62 | 61
74 | 0.838 | 73 | 71 | 70 | 68 | 67 | 65 | 64 | 62
| | | | | | | | |
75 | 0.866 | 74 | 72 | 71 | 69 | 68 | 66 | 65 | 63
76 | 0.896 | 75 | 73 | 72 | 70 | 69 | 67 | 66 | 64
77 | 0.926 | 76 | 74 | 73 | 71 | 70 | 68 | 67 | 65
78 | 0.957 | 77 | 75 | 74 | 72 | 71 | 69 | 68 | 66
79 | 0.989 | 78 | 76 | 75 | 73 | 72 | 70 | 69 | 67
| | | | | | | | |
80 | 1.022 | 79 | 77 | 76 | 75 | 73 | 72 | 70 | 69
------+-------+-----+-----+-----+-----+-----+-----+-----+-----
------+-------+-----+-----+-----+-----+-----+-----+-----+-----
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
------+-------+-----+-----+-----+-----+-----+-----+-----+-----
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 |
-----+----+----+----+----+----+----+----+----+----+----+----+----+----
=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.