Democratic messenger. [volume] (Snow Hill, Md.) 1869-1973, 1882-10-21
FROST INDICATIONS.
HOW FARMERS MAY BE FoREwARNED
AGAINST Low TEMPERATUREs-Av VAL.
UABLE CONTRIBUTION FROM THE UNI
TED STATES SIGNAL SERVICE.

UABLE CONTRIBUTION FROM THE UNI
TED STATES SIGNAL SERVICE.
The latest paper emanating from the
United States Signal Office is one that
calls for the strict attention of farmers
and all others dealing IN goods affected
by changes in temperature. 'To fore
tell frost is the title" cat all temper
atures," says the writer, Lieutenant
James Allen, seven the lowest, moisture
exists in the atmosphere in an invisible
state; the air is never absolutely dry.
The intervals between the particles of
air are partly filled with the vapor that
is constantly rising from the earth. This
capacity of the air for moisture has a
limit, aud when this is reached the air
is said to be saturated. A rise of tenn
perature increases the capacity of the
air for moisture. On the other hand q
fall of temperature diminishes the
capacity. But the capacity of the air
for moisture increases more rapidly than
the temperature. Thus. air can contain
at 80 degrees the 160th part of its own
weight, at SO degrees the S0th part and
at Sc degrees the 40th part, the law be-
ing that for every increase OF ~ degrees
the capacity is doubled" After show
ing how the amount of vapor in the air
may be measured by noting the temper
ature at which the moisture begins to
be condensed on a cold vessel the writer
gives full description of the dry and
wet bulb hygrometer, and illustrates
the manner in which the "dew point"
can be ascertained by its use. Regard
ing its usefulness to agriculturists he
says: "The ascertainment of the dew
point is of great practical Importance,
particularly to horticulturists, since it
shows the point near which the descent
of the temperature of the air during the
night will be arrested. For when the
air has been cooled down by radiation
to HIS point dew is deposited and latent
heat is given out. The amount of heat
thus set free being great the tempera-
ture of the surrounding air is imme-
diately raised. The same process con-
tinues to be repeated, and thus the tenn
perature of the air in contact with plants
or other radiating surfaces may be con-
sidered as gently oscillating about the
dew point. For if it raises higher the
loss of heat by radiation speedily low
ers it. Thus the dew point determines
the minimum temperature of the sur
face of leaves on the ground during the
night. The minimum temperature in
the instrument-shelter would be higher
than this.
"This suggests an important practical
use of the hygrometer. If the dew
point be ascertained by it the approach
of low temperatures or of frost may be
foreseen and provided against. Thus,
suppose on q fine, clear day, toward
evening, that the dry bulb is so degrees
and the wet bulb 40 degrees, the dew
point at the time is, therefore, 20 de-
grees Fahrenheit. Frost on the ground
may then be predicted with certainty,
and no time ought to be lost in protect.
ing such tender plants as may be ex.
posed IN the open air. If, on the other
hand, with sky quite as clear, the dry
bulb is so degrees and the wet bulb AZ
degrees, the dew point being thus As
degrees, no frost need be feared. As.
Stunning the sky to remain perfectly
clear of haze or clouds, the raising OF
depressing of the dew point during the
night USUALLY with a change of wind
are the only circumstances that can
happen to interfere with the predictions
founded on the hygrometer. Frequently
the presence of haze at high attitudes
during the night prevents the radiation
of heat from the earth and thereby the
frost which otherwise would have oc
Cured. Hoar frost is formed under the
same circumstances as dew, with the
exception of lower temperature. When
the temperature of the surface of plants
falls below 30 degrees the moisture o.
the air is condensed upon them in the
solid state and forms layer of snow
crystals like spongy ice. Hoar frost
therefore, is not frosen dew but the
moisture of the air is deposited in the
solid form, without having passed
through the liquid condition. Hoal
frost, like dew, is deposited chiefly upoI
those bodies which radiate best, such al
plants and the leaves of vegetables, an6
the deposit is made principally on those
p PARTS which are turned toward the sky
Since plants sometimes become cooled
by radiation from II to Is degrees be
low the temperature of the surrounding
air, frost may occur although ther
mometer few feet above the ground
in an instrument-shelter, may not sinh
to 80 degrees. During clear and still
night, when a thermometer six fee
above the ground sinks to SO degrees, (
heavy frost may be expected; slight
rost may occur when the same Their
mometer sinks only to At degrees.
Whatever prevents the radiation of heat
serves also to check the formation OF
hoar frost. During the cold nights OF
spring, plants which are sheltered by
trees are less liable to be injured by
frost than those which are fully exposed,
and thin covering of cloth or straw
will generally afford entire protection.
A garden may frequently be saved from
injury by kindling small smudge fire.
which viii envelop the plants in cloud
of smoke. Fogs and clouds also pro-
tect vegetation from the effects of frost. u
All persons desiring to use the n.y.
grometer will be supplied with them by
the Chief Officer of the Signal Service
at cost price, seven dollars. Fall in
structions as to the manner of reading
them will also be furnished free of charge
The hing of Sweden.