FROST
There is nothing in the study of the atmosphere that so intimately
concerns the horticulturist and the gardener as knowledge of the
conditions under which frost forms, and the methods that may be
pursued to gain immunity from its disastrous effects, or to lessen
the loss.
Frost does not necessarily form from air that has fallen to the
freezing point, as many suppose. On the contrary, the air ten feet
or less above the vegetation may be several degrees above freezing
when there is a heavy and destructive frost upon vegetation. The fact
is that vegetation radiates heat towards a clear sky faster than
does the air and may fall to the freezing point or below; while the
air, except the molecules actually in contact with the vegetation,
is considerably warmer. Frost is not frozen dew. The water vapor
is precipitated, or rather congealed, upon the vegetation without
passing through the liquid state at all. Frost is spoken of as light,
heavy, and killing. Tomato plants are killed by only a light touch of
frost, while fruit blossoms will stand several degrees of cold below
freezing. Therefore the tomato grower would consider as killing a
frost that to the fruit grower would only appear as light.
The radiation of heat from the earth is continuous both day and night
when there are no clouds to obstruct the passage of the heat rays.
The amount received from the sun during the day is greater than the
loss by radiation from the earth and the temperature of the air
rises. After the setting of the sun the radiation of the earth goes
on but there is no incoming heat from the sun to offset the loss and
the temperature of the air falls. As previously stated, the soil
and vegetation radiate faster than the air and the air in immediate
contact with the soil is cooled by conduction to it. Thus over a
level plain on a clear calm night there is found a relatively thin
layer of cold air near the ground, which increases in temperature
up to two hundred or three hundred feet, or which may be only five
or ten feet deep. Over sloping ground the force of gravity tends to
cause this thin surface layer of cold air to move down the slope and
to gather in depressions in somewhat the same manner as water would
move. Such movement is called Air Drainage. Of course this air is
slowly gaining heat by compression as it passes to lower levels, but
it is hugging closely to the cold earth and losing by conduction much
or all that it thus gains by compression.
After a study of the contour of the region with respect to air
drainage the writer purchased a considerable tract of land near
Rockville, Montgomery County, Maryland, and planted extensive
orchards thereon, with the result of harvesting nine successful crops
of fruit in a period of ten years after the trees became large enough
to bear. With the composition and the surface covering of the soil
the same, the low places in a field are always the ones that suffer
most when frost is possible. Figure 8 shows a minimum temperature
of 25° to have occurred at the base of a steep hillside when on
the higher ground at an elevation of but fifty feet the lowest
temperature was 44°, and at two hundred and twenty-five feet up the
mountainside the minimum was 52°.
[Illustration: FIG. 8.—Continuous records of the temperature from 4
P.M. to 9 A.M. at the base and at different heights above the base of
a steep hillside, showing the great differences in temperature that
sometimes develop on a clear, still night. Although the temperature
at the base was low enough to cause considerable damage to fruit, the
lowest temperature 225 feet above on the slope was only 51°. Note
that the duration of the lowest temperature was much shorter on the
hillside than at the base.—_Weather Bureau._]
In selecting a location for an orchard it is not so much a problem of
elevation above sea level as elevation above the surrounding region.
The direction in which the slope faces makes little difference. The
prime consideration is to get sufficient air drainage to gain the
greatest protection against frost without selecting land with such a
steep slope as to furnish excessive soil drainage and which would be
difficult to cultivate and move about upon in the spraying of trees
and in the picking of fruit. In the Maryland orchard the elevation
was only five hundred feet above sea level and only about two hundred
feet above the surrounding region, and the slope was so gradual as
almost to be imperceptible to one passing over it.
After nightfall the air on mountain peaks and on hills and ridges
soon becomes cooler than the air at the same elevation out over the
open valley, due to contact with the elevated earth, which radiates
heat and cools faster than the air.
Water vapor has a great capacity for heat. It is the most effective
of the various gases present in the atmosphere in obstructing
radiation of heat from the earth, as well as in absorbing incoming
radiation from the sun. The night temperature, therefore, falls
more slowly when the relative humidity is high than when it is low,
that is to say, when the air is nearer saturation, or nearer its
dew point. Drops of water that collect on the outside of a pitcher
of ice water on a warm day are formed through the chilling of the
air in contact with the pitcher; they begin to form as soon as the
temperature of the pitcher reaches the dew point of the air, which
temperature varies in accordance with the amount of water vapor
present in the air at the time. After sundown the temperature of
exposed objects falls, of some faster than others, depending on
their capacities for radiation. Vegetation radiates freely and often
falls to the dew point of the air, at which time dew begins to
form on it and continues to be deposited as long as the temperature
remains above freezing. Now, here carefully note that if the dew
point is above 32° the condensation of water vapor in the form of
dew liberates latent heat, which usually will be sufficient to check
the fall of temperature and prevent the formation of frost. If the
dew point of the air is 32° or lower frost forms. If the dew point
is very low the temperature may fall low enough to cause much damage
without the formation of any frost. As an example, if the dew point
be 20° and the temperature falls to 24° much damage might be done to
growing crops and no frost appear. This phenomenon is called black
frost; it seldom occurs. From the foregoing it might be assumed
that the possibilities of frost might safely be forecast from an
observation to determine the relative humidity taken early in the
evening, but unfortunately experience has shown that reliance cannot
be placed in such method of forecasting, as the humid air of early
evening may be displaced by much drier air before the hour of minimum
temperature the next morning.
One of the best locations to gain immunity from frost at the
critical period of plant growth is immediately to the leeward of a
considerable body of water. Wind blowing from a large body of water
is always heavily laden with moisture, which decreases the rate of
radiation both day and night, but especially during the period of
cold in the early morning when frost is liable to occur. Such winds,
largely affected by the temperature of the water over which they have
passed, modify the temperatures of both day and night.
The all-important condition for the formation of frost is an
atmosphere already cool, with a gentle northwest wind and a clear
sky, which condition, with more or less coolness, always accompanies
the high barometric areas that follow the low-pressure areas of
warmth, cloudiness, and moisture.
At an expense of two millions of dollars per annum the Government
maintains some two hundred observation stations of the Weather
Bureau, and twice daily telegraphs observations to all the large
cities of the nation, but unfortunately in many cases these are not
published for the benefit of the people who could make valuable use
of them. The Bureau’s own deductions from these observations, in the
form of forecasts and warnings, are extremely valuable, but an even
greater service could be rendered the public by neatly lithographing
an evening weather map and mailing it from all large cities each
night, so that every intelligent person whose business is affected
by the weather could, through a study of the chapter on Forecasting
in this book, judge for himself as to the effect that the coming
weather may have on his particular interests. One could then watch
the movements of the high barometric areas and the low areas and
become weatherwise himself, and he who studied these charts the most
diligently would have an advantage over less progressive competitors.
Evaporation goes on at all temperatures, even below freezing and from
solid ice, its rate, of course, being diminished by low temperatures.
At times, in spring or fall, the temperature of the air over rivers,
when there is little wind, falls so far below the temperature of
the water that the water vapor rising from the river by evaporation
is quickly condensed in the form of fog, which may cover a part or
all of the low contiguous land, checking radiation and preventing a
further fall in temperature.
In valleys near the ocean, fog sometimes drifts in from the water
when frost is imminent and prevents its formation. On nights with
fog, contrary to the usual condition, the hillsides are always colder
than the lowlands, unless the fog extends high enough to cover them.
In 1891-1894 the writer, in studying the conditions under which
frost forms on the cranberry bogs of Wisconsin, was impressed with
the fact that the occurrence of frost on a given field depended as
much on the character of the surface and its covering as it did on
the temperature of the air a few feet above, one place receiving
an injurious frost, another a light frost, and still another none
at all, while each had the same conditions as to temperature, wind
velocity and direction, and all were at the same elevation, so that
the differences could not be accounted for by air drainage.
In one case the marsh was cleanly cultivated and covered with sand,
in another there was clean cultivation but no sand, and in still
another case there was a thick growth of vegetation. As the result
of a long series of observations conducted by Professor H. J. Cox,
working under the directions of the writer, minimum thermometers were
placed among the vines over newly sanded surfaces in two marshes,
one at Cranmoor and one at Mather, Wisconsin. The locations selected
for this inquiry represented the best results that could be secured
from sanding, draining, and cultivating. Comparison was made at each
marsh between the readings taken close to the vines of the clean part
of the marsh and those taken close to the surface over the unsanded
peat bog. The average lowest night temperature over the sand for the
four months was 5.9° higher than over the peat at Cranmoor, and 4.2°
at Mather. On one night the minimum over the surface at Cranmoor was
12° higher than over the peat, while at Mather a difference of nine
degrees was recorded on another night.
Through cultivation the marsh may be kept free from weeds, moss,
or other rank growth, thus permitting the sun’s rays to reach the
soil and increase its temperature during the day, while a growth of
thick vegetation screens the soil from the sun’s rays, and there is
consequently less heat in the latter soil to be given out during
the hours of low temperature at night. Drainage lowers the specific
heat of the soil and decreases the cooling effect of evaporation.
Therefore, under sunshine, the dry soil becomes warmer than the wet
and, whether or not it has a greater quantity of heat to give off
at night, it has a higher temperature and therefore radiates more
freely to the air above. A covering of sand likewise lowers the
specific heat of the surface and thereby causes it to gain a higher
temperature during the day than an unsanded surface receiving the
same solar rays. It therefore radiates more rapidly at the critical
time when heat is needed to prevent the temperature of vegetation
from falling to the freezing point and gaining a deposit of frost.
[Illustration: FIG. 9.—Continuous records of the temperature 5
feet and 35 feet above ground on a tower in a pear orchard. Note
the large difference in temperature at the two levels before the
orchard heaters were lighted at 4 A.M. By 5 A.M. the temperature
was practically the same at the two levels, showing that the heat
from the burning oil had been nearly all expended in raising the
temperature of the air within 35 feet of the ground. This point is
further illustrated by the fact that at 5 A.M. when most of the
heaters were extinguished, the temperature at the 5-foot level fell
rapidly, while it remained practically stationary at the 35-foot
level.—_Weather Bureau._]
In many orchards in the Rocky Mountain States, where fruit growing is
highly profitable and the injury from frost more than probable every
year, an extensive use is made of oil and other fuel-burning heaters
between the rows of trees. Those who wish further information with
regard to this matter should send to the Weather Bureau, Washington,
D. C., for Farmers’ Bulletin No. 1096. At first thought it would
seem that heat so applied would be blown away or instantly escape
upward. But on frosty nights there is not much wind; if there is,
there is little danger from frost. And then, as previously stated,
on such nights there is what is called temperature inversion, and
the temperature actually rises with the first few feet of ascent,
and the heated air soon reaches air of its own temperature, when no
further ascent occurs. When the air forty feet from the ground is
ten degrees warmer than it is around and in contact with vegetation,
as often occurs on frosty nights, the heat from the fires is nearly
all expended in raising the temperature of the air within this forty
feet. Figure 9 furnishes the result of an experiment illustrating the
correctness of the foregoing theory.
[Illustration: FIG. 10.—Average dates of last killing frost in
spring.]
[Illustration: FIG. 11.—Average dates of first killing frost in fall.]
Figures 10 and 11 show the average dates of the last killing frost in
spring, and of the first killing frost in fall.