W. B., No. 86.
X
Price, 10 cents.
U. S. DEPARTMENT OF AGRICULTURE. SB
WEATHER BUREAU.
INJURY FROM FROST
AND
781
58
METHODS OF PROTECTION.
PREPARED UNDER THE DIRECTION OF
PROF. WILLIS L. MOORE,
CHIEF OF BUREAU.
BY
W. H. HAMMON,
FORECAST OFFICIAL,
PARIMI
WASHINGTON:
WEATHER BUREAU.
1896.
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LETTER OF TRANSMITTAL.
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U. S. DEPARTMENT OF AGRICULTURE,
WEATHER Bureau,
Washington, D. C., February 6, 1896.
SIR: The following circular has been compiled by Mr. W. H.
Hammon, of the Weather Bureau, Forecast Official at San Francisco.
He has had the needs of the agriculturists of California especially
in mind, and I recommend its publication in the hope that its sug-
gestions may be of benefit to a large class of readers.
J. STERLING Morton,
Secretary of Agriculture.
WILLIS L. MOORE,
Chief of Bureau.
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INJURY FROM FROST AND METHODS OF PROTECTION.
It is the aim of this paper to describe both the methods of prevent-
ing or diminishing injury from frost, and the means of determining
when danger from this source is imminent.
THE FORMATION OF FROST.
In studying methods of preventing injury from frost, a clear
knowledge of the conditions under which it forms is essential. Frost
occurs when water vapor from the surrounding air is condensed upon
plants and other objects which have been cooled below the freezing
point by the radiation of heat. Radiation is most rapid through dry
air and when there is nothing to obscure the sky. Aqueous vapor,
although invisible, is always present, in greater or less quantities,
in our atmosphere, and can always be condensed into water if the
temperature be sufficiently lowered. If the condensation takes place
at temperatures above the freezing point of water, the moisture is
deposited as dew; if below the freezing point, the condensation is in
the form of frost.
The temperature at which condensation begins is called the "dew-
point," and varies with the amount of moisture in the air, being
higher the greater the amount of moisture present. It is always con-
stant for the same amount of vapor, no matter what the temperature
of the atmosphere may be.
The temperature of the dew-point can be readily determined by
the use of two ordinary thermometers, one of which has its bulb.
covered with thin muslin moistened with water. The evaporation
of the water about the wet bulb lowers the temperature and causes
this thermometer to read lower than the one with a dry bulb. From
the difference between these readings and the temperature of the air
itself the dew-point can be quite accurately determined from the table
given at the end of this paper (see page 11). This apparatus is known
as the wet and dry bulb hygrometer. A preferable form of the instru-
ment is the sling psychrometer, where the two thermometers are at-
tached to the same back and arranged with a cord, or handle, for
whirling, by which good ventilation is assured. Such an instrument
is extremely valuable to the orchardist or gardener in assisting him.
to know when frost is imminent.
The surface of the earth is continually losing heat by radiation
into space, but during the day it usually receives heat from the sun
more rapidly than it loses it by radiation, and consequently it grows
5
3
6
warmer. At night, however, heat from this source is cut off and the
continued radiation causes the temperature to fall. Under favorable
conditions this fall continues until the dew-point is reached when the
heat evolved by condensation retards or practically prevents further
cooling. The heat given off by the condensation of vapor is enor-
mous. The condensation of enough vapor to make a pint of water
will evolve enough heat to raise more than five pints of water from
the freezing to the boiling point; all this heat must be lost by radia-
tion in order that the formation of dew may proceed.
Radiation takes place most rapidly when there is nothing to ob-
scure the sky. Clouds or any other obstruction act as a screen in re-
tarding it. It takes place more rapidly from the surfaces of plants,
etc., than it does from the air about them, so that on still nights these
surfaces are frequently cooled several degrees below the temperature
of the surrounding air.
One more principle should be considered in a study of the condi-
tions under which frost forms and that is the increased density of the
air as its temperature is lowered. Owing to this principle the air, on
calm nights, arranges itself in accordance with its density. The
heavier cold air rests on the surface and surrounds the plants and
trees, thus increasing their liability to injury. On still nights this
fact is often very manifest. Frequently a thermometer close to the
ground will read 5° or 10° lower than one 8 or 10 feet higher. This
principle causes the air on slopes, as it becomes chilled by radiation,
to flow down into the valleys where it accumulates, thus frequently
causing severe frosts in the lowlands while the hillsides remain un-
injured. It is for this reason that frost does not so readily occur on
windy nights since the wind mixes the air to a more uniform tem-
perature throughout and causes that near the ground to be warmer
than it otherwise would be.
BEST LOCATIONS FOR ORCHARDS OR GARDENS TO AVOID
INJURY BY FROST.
Avoid, as far as possible, placing the tenderest plants on low ground,
especially the bottoms of narrow valleys with high hills on either
side. In addition to the loss of their own heat by radiation, these
valleys will become filled, on frosty nights, by the air which has been
cooled by the rapid radiation on the slopes and has then flowed down-
ward into the bottoms. Bottom lands opposite the mouths of canyons
should be especially avoided for the same reason. The converse of
this is true that plants on gentle slopes are less liable to injury than
bottom lands.
Slopes facing the south are preferable to northern slopes, because
they receive the rays of the sun more directly and for a longer period.
Slopes facing the west are to be preferred to those with an eastern
¿
7
exposure, since they receive heat longer in the afternoon, and being
shaded for a time in the morning from the direct rays of the sun, the
slower disappearance of the frost seems to cause it to be less injurious.
The fact that frosted plants are more seriously injured by being sud-
denly warmed in the dry air than when the frost disappears more
slowly seems well established.
Moist soil, or localities that can be easily flooded for the purposes
of protection, are to be preferred to dry sections of otherwise similar
location; for the evaporation of the moisture from the soil on dry,
cold nights, will tend to raise the dew-point of the air, and thus
diminish the probability of frost.
The irrigation of the hillsides about a valley in which protection is
desired, and the growing thereon of plants or trees with a large
amount of foliage will, by the evaporation from the soil and verdure,
tend to raise the dew-point of the surface air as it flows downward
into the valley. When the dew-point is thus raised above 32° F., the
probability of frost is annulled.
METHODS OF FROST PREVENTION.
From a study of the foregoing principles under which frost forms,
it would seem that there would be the greatest probability of success,
in preventing frost or diminishing its severity, by working along the
following lines, namely:
(1) Diminishing the radiation of heat.
(2) Raising the dew-point, and thus making sensible the latent
heat of condensation at a temperature above the danger point.
(3) Adding heat to the air.
(4) Draining the cold air away from the section which needs pro-
tection.
(5) Mixing the air so as to prevent the cold air from sinking to
the surface.
One method of diminishing radiation, which is of considerable
value, especially in a level country, is to obscure the sky by
means of the smoke of smudge fires. This method has been used
with success in the level wheat fields of the Dakotas and Manitoba,
and should be of about equal value in the broad interior valleys of
California. It is not so successful in the narrow valleys of a hilly
country, for, while it retards the radiation of heat in the valley, the
smoke bank is usually of low elevation, and radiation proceeds unin-
terruptedly from the hillsides, whence the cooled air flows down into
the valley, underneath the smoke, and chills the plants. Damp
straw, tar, turpentine, old hay, anything that will result in the
greatest amount of smoke will serve as fuel for these fires. Have
the fuel on the ground in advance and start the fires while the tem-
perature is several degrees above the danger point.
8
It is believed that decidedly better results will be attained if damp
fuel is used or if the fire be sprayed with water, for this will add
vapor to the air, which, in condensing, will assist in checking radia-
tion by obscuring the sky with fog or cloud, and, at the same time,
the dew-point will be raised. This plan should result in absolutely pre-
venting injury if the temperature be much above the danger point, for
the condensation of the vapor will continue to distribute heat through-
out the space occupied by the mist. In the case of smudges, the fire
warms and expands the air near it, causing it to rise. This establishes
an upward current of warm air from the fire, which conducts the heat
of the fire upward and beyond the space needing protection, and cool
air flows in from the sides to take its place. Thus the heat of the fire
has but little effect in diminishing the intensity of the frost, almost
the entire protection being gained by the blanket of smoke produced.
By spraying the fire, on the other hand, a large portion of the heat of
the fire is consumed in evaporating the water, which on rising from the
fire, is quickly condensed as it comes in contact with the surrounding
air. The heat of condensation thus becomes manifest in the lower
air. The heat of the fire is thus, in a measure, trapped and distrib-
uted throughout the lower stratum of the air and greatly aids in
protecting the plants. Every quart of water thus evaporated and
again condensed in the surrounding air contains heat sufficient to raise
the air temperature 1° Fahrenheit throughout a space about 50 feet.
square and deep. The method of spraying fires, therefore, brings into
operation the first three principles of protection enunciated above,
namely, retarding radiation, raising the dew-point, and adding heat
to the air.
In places where irrigation can be used it will be found of great
value in protecting. Let the water be turned on until the soil is
thoroughly moistened. The evaporation of the water from the damp
soil will tend to raise the dew-point. Since evaporation takes place.
near the surface this method is especially valuable in protecting low
plants and shrubs, but has also been found very valuable in protecting
citrus groves from freezing weather. The irrigating should be done
at as early an hour as possible, preferably on the day preceding the
night when frost is anticipated, and the ground kept thoroughly wet
until danger from frost is passed.
A modified form of water protection which is valuable in orchards
is to spray the trees with water. This plan is probably even more
valuable in protecting, citrus fruits and other plants which are not
injured until the temperature has fallen several degrees below the freez-
ing point; for, in these cases, the water will tend to freeze before the
fruit is injured, and, in freezing, will make sensible a large amount of
heat, thus preventing further cooling of the air. For this method to be
successful, the spraying must be continued until the temperature rises.
9
Of these several methods of prevention and protection it is believed
that the best results will usually be obtained from the modified method
of smudges where the fires are used to vaporize water. The irrigating
and spraying should be also used when possible, especially in protect-
ing citrus fruit and trees from severe freezing weather.
All the preceding methods are subject to one serious defect when
applied to the protection of narrow valleys between high hills or
mountains. In such cases the cooling by radiation from the hill-
sides continues, and the denser air then flows into the valley and
displaces the air which has been kept warm by the various means of
protection. To make it more efficient it may, therefore, be necessary
to extend the protection to the hillsides. It may be possible that
this danger to the orchard or garden, could, in many instances, be
permanently removed or greatly diminished if windbreaks were so
erected as to drain off this cold air from the locality. The breaks
could be in the form of a high, close fence or levee. Perhaps a hedge
would be of use; preferably there should be a levee 4 or 5 feet high
with the ditch on the side next the hill into which, if possible,
water should be turned. On the top of the levee a fence should be
placed. The evaporation of the water from the stream would tend
to raise the dew-point of the air. The levee should be so placed
along the slope near its base as to intercept the cold air and lead it
beyond or around the orchard. It should have a continuous slope
downward, the steeper the better, so that the cold air will not over-
flow it.
}
WHEN TO PROTECT.
The experience of the past two seasons has shown that forecasts of
sudden and decided changes in temperature over a large territory are
among the most accurate made by the Weather Bureau; conse-
quently it is reasonable to expect that if suitable arrangements are
made warnings may be received of those otherwise unexpected cool
waves which will result in frost. There are instances, however, when
the general forecasts of the Weather Bureau cannot be expected to
be sufficiently specific to provide for the different conditions that
may prevail in various sections. The temperature frequently remains
for several days near the critical point, and a change of a very few
degrees or a local clearing or clouding of the sky will cause or pre-
vent injury. Again, the conditions in certain localities are such as to
make them more susceptible to frost than the surrounding region.
Prof. Willis L. Moore, Chief of the Weather Bureau, states that
while forecast official in Wisconsin, he observed that a frost occurring
immediately after a rain was not as injurious as when the ground
and plants were dry. It is, therefore, necessary that the orchardist
and gardener be able to judge, at times, for themselves when danger
from frost is imminent. For this purpose they should be.provided
No. 86-2
10
with a wet and dry bulb hygrometer or psychrometer, mentioned on
page 11, by which the dew-point of the air can be determined.
If, in the afternoon, the dew-point is near the critical temperature,
arrangements should be made for protecting, if necessary. If, at a
later hour, the dew-point is constant or lower, the sky clear or clear-
ing, and the air calm, it is reasonable to expect that the temperature
will fall to the dew-point during the night. The efforts to protect
should be based on this dew-point. If it merely approximates the
danger point (and no warning of more severe temperatures has been
received), but little protection will be necessary and action may be
delayed until the temperature is but a few degrees above the danger
point. However, if the dew-point be several degrees below that liable
to cause injury, or if it be falling, or if a change for the colder is
anticipated, efforts to protect should be undertaken earlier. The
ground should be irrigated during the day, smudge fires started when
the temperature is several degrees above the critical point, and, if the
temperature continues to fall, the fires should be sprayed until dense
fog or mist envelops the entire space. If the temperature should
then fall to the danger point, the trees and plants should be thor-
oughly sprayed, at the same time keeping up the smudge and vapor-
izing fires.
It would seem that these precautions should be sufficient to prevent
injury, unless it be in the case of narrow valleys, where the cold air
from the unprotected hillsides displaces that which has been kept
warm, and, should wind-breaks be found successful in removing this
danger, it is believed that there are few, if any, localities where injury
could not be avoided.
It is evident that in attempting to protect one ranch the owner
will, in a measure, protect his neighbor; therefore, if some arrange-
ments for cooperation among the individuals in the same locality
were made, the greatest protection could be provided at the minimum
expense. By such a cooperation of all the residents of the valley, a
system of wind-breaks or air-drainage dikes (if found valuable) could
be laid out and built in such a manner as to result in the greatest
general good at the least cost. Arrangements could be made which
would insure the distribution of a frost warning from the Weather
Bureau throughout the entire district. Some person, provided with
a telephone, could receive the warnings from the Bureau and, in turn,
telephone them to all others having such instruments. All so receiv-
ing them could display the frost signal, and thus the warning would
be quite thoroughly distributed. Then, by cooperation in protection,
the best results would be obtained.
It may be believed that the trouble and expense necessary to carry
out these plans are greater than the benefits accruing from the pro-
tection; but this seems hardly true as a general rule. The supplies
11
needed in the various methods are inexpensive. Any old rubbish is
suitable as fuel for the fires. The apparatus used in spraying trees
to avoid injury from insect pests is suitable for spraying to avoid
frost, and the only other material necessary is water. Really, the
trouble, inconvenience, and labor are practically the only drawbacks,
while the gain may amount, at times, to many thousand dollars.
THE USE OF THE PSYCHROMETER.
To obtain the dew-point from the wet and dry bulb hygrometer or
psychrometer, moisten the muslin on the wet bulb and then whirl or
fan the instrument, when the temperature will fall. Continue the
ventilation until the wet-bulb thermometer ceases to fall, when the
two thermometers should be read.
Subtract the reading of the wet-bulb thermometer from that of the
dry. Find this difference in the column at the left of the table.
The dew-point will then be found at the intersection of the line
site this difference, with the column which is headed by the number
nearest the air temperature (dry-bulb reading).
Example.
Dry-bulb thermometer.
oppo-
55
Wet-bulb thermometer
44
Difference
11
Dew-point from table.
30
Second example.
Dry-bulb thermometer.
43
Wet-bulb thermometer
38
D
Difference.
5
30
Dew-point from table, between 28 and 33, about.
Dew-point table from Weather Bureau Instructions to Voluntary Observers (1892).
Difference
of readings
of dry and
wet bulbs.
150
Temperature of air-Fahrenheit.

200 250 300 350 400 450 500
550
60° 65° 700
O 1200 TH LO
11
17
6
13
4
73∞∞
22
8
5
ROP=D
27
32
19
25
30
15
22
11
18
5
14
22
10
11
12
18
14
15
G`OGO IRRID
6
9
7
2
13
8
***** **∞
33
18-
9
***** *2000
38
43
48
35
41
46
39
44
30
36
42
28
33
40
25
31
37
22
28
34
18
25
13
22
8
18
13
8
19
14
***** *88* ***8-
***** 5*88* 32**
53
52
57
50
48
53
46
51
43
49
41
47
39
45
36
43
33
40
30
28
24
32
20
29
10
32885 42795 4395!
25R85 95999 83885*
63
69
62
67
60
66
59
64
57
55
61
59
51
57
55
51
42
49
40
47
45
$4462 23452 82848
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12
INJURIOUS TEMPERATURES.
!
Table of temperatures at which the following plants are liable to
receive injury from frosts, compiled from information received from
horticulturists, orchardists, and gardeners throughout the entire Pa-
cific Coast.
The temperatures given are as nearly as possible those of the air
in contact with the plant itself.

Plants or fruits.
In bud.
In blossom.
In setting fruit.
At other times.
Plants or fruits.
In bud.
In blossom.
In setting fruit.
At other times.
Almonds
Apples
Apricots
Asparagus
Bananas
Barley
Beans
Beets...
• ·
Cabbage
+
Cantaloupes
Cauliflower
►
•
•
。 ANGAM
30
30
29
30
30 31
32
29 29
29
31
31
。 AAXON
о
о
О
28
Onions
26
30
26
32 31
Oranges +
Parsnips
Peaches.
30 31
31
...
29
Pears
31
•
Peas
25
•
•
15-27
... •
222324
32
30-31
Plums
Potatoes:
Irish
20-27
Sweet
31
Celery..
28
•
Prunes
·
•
**** AMO
29
28
29
29
30
30
31
30
31
30
31
8887 AM
9897 SHO
30 30
29
30
31
30 30
31
31
31
31
Cucumbers
·
Cymlings, or squash.
Flowers *
Grapes
Grape fruit
Lemons..
Lettuce
Mandarins
Oats..
Okra
·
31
31
31
32
Radishes
25
• 2*3*2*** MARK
29
27


29
29
31
31
31
30
• •
Shrubs, roses, or trees 26-30 28-32
30-26
31
31
31
30
Spinach...
21
31
31
30
28
Strawberries
28
28
28
30
··
•
30 31 31
28
Tangerines
....
31 31
31
28
30
31
31
28
Tomatoes..
31
31
31
31
12-28
Turnips
26
1. •
31
31
31
28
Watermelons
28-31
31
•
Wheat..
31 31
31
Walnuts, English
30
31
31
28
•
Olives
30
31
31
18+1
248
* Depends on variety. †Injured at 2° higher if continued four to six hours. Ripe. § Green.
.