The Cambria freeman. [volume] (Ebensburg, Pa.) 1867-1938, 1882-11-03
FROST INDICATIONS.
HOW FARMERS MAY BE FOREWARNED
AGAINST Low TURES-A
VALUABLE CONTRIBUTION FROM
THE UNITED sTATES SIG
I-sERVI ICE.

THE UNITED STATES SIG
L-sERVI CE.
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. ITO fore
tell frost is the title. > ''At all temper
atures, - says the writer, Lieutenant
James Allen, ''even the lowest, moisture
exists IL the atmosphere in an invisible
state: the air is never absolutely dry.
The intervals between the particles of air
are partly filed with the vapor that is
constantly rising from the earth. This
capacity of the air for moisture has a
limit, and when this is reached the air
is said to be saturated. a rise of temper
ature increases the capacity of the air
for moisture. On the other hand a fall
of temperature diminishes the capacity.
But the capacity of the air for moisture
increases more rapidly than the tempra-
tuTe. Thus, raiT can contain at 30 de-
grees the 160th part of it's own weight,
at 56 degrees the S0th part and at sc de-
grees the 10th part, the law being that
for every increase of 27 degrees the ca
pacity is doubled s" After showing how
the amount of vapor in the air may be
measured by noting the temperature
at which the moisture begins to be con-
densed on a cold vessel the writer gives
full description of the dry and wet
bulb hygTOmeter, and illustrates the
manner in which the "dew pOint'' can
be asceTtaiued by its use. Regarding
its usefulness to agriculturists he says
1.-The asceTtainment of the dew point
| is of great practical Importance, par
ticularly to horticulturists, since it
shows the point near which the descent
OF the mperature of the air during the
night will be arrested. For when the
/ air has been cooled down by radiation to
this point dew is deposited and latent
heat is given out The amount of heat
thus set free being great the temperature
OF the surrounding air is immediatly
raised. The same process continues to
be repeated, and thus the temperature
OF the air in contact with plants or 9TH
er radiating surfaces may be considered
/ as gently OScillating about the dew point
For if it raises higher the loss of heat by
radiation speedily lowers it. Thus the
dew point determines the minimum
temperature of the surface of leaves on
the ground during the night. The mi
nimum temperature in the instrument
shelter would be higher than this.
This suggests an important practical
use of the hygrometer. Ifthe dew point
be ascertained by it the approach of low
temperatures or of frost may be foreSeen
and provided against. Thus suppose on
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 degrees Fahren-
heit. Frost on the ground may then be
predicted with ceartainty, and no time
ought to be lost in protecting such ten
der plants as may be exposed in the open
IL
quite as clear, the dry bulb is 90 degrees
and the wet bulb AZ degrees. the dew
point being thus 43 degrees, no frost need
be feared. Assuming the sky TO remain
perfectly clear of haze or clouds, the rails
ing or depressing Of the dew point dur
ing the night usually with a change Of
Wlnd) are the only circumstances that
can happen to interfere with the pTedic-
tiOns founded on the hygIOmeter FIRE.
quently the presence of haze at high al
titudes during the night prevents the ra-
diation of heat from the earth and there
by the frost which otherwise would have
occurred. Hoar frost is formed under
the same circumstances as dew, with the
exception of a much lower temperature.
When the temperature of the surface of
plants falls below 80 degrees the mois-
ture of the air is condensed upon them
in the solid state and forms a layer of
snow crystals like SpOngy ice. Hoar
frosts, therefore, is not dew bnt the
moisture of the air is deposited in the
solid form, without having passed thro'
the liquid condition. Hoar frost, like
dew, is deposited chiefly upon those bod
les which radiate hest, such as plants
and the leaves of vegetables, and the de-
posit is made principally on those parts
which are turned toward the sky. Since
plants sometimes become cooled by radi-
alton from II to Is degrees below the
temperature of the surrounding air, a
frost may occur although thermometer
few feet above the ground, in an instru-
ment-shelter, may not sink to 30 degrees.
During clear and still night. when
thermometer six feet above the ground
sinks to SO degrees, a heavy frost may
be expected slight frost NAV occur
when the same thermometer sinks only
to AZ degrees. Whatever prevents the
radiation of heat serves also to check the
formation of hOar frost. During the
cold nights OF spring, plants which:aTe
sheltered by trees are less liable TO be in
jUred by frost than those which are fully
exposed, and thin covering of cloth or
straw will generally afford entire protec-
tiOn. a garden may frequently be saved
from injury by kindling a small smudge
fire, which will envelop the plants in
cloud of smoke. Fogs and clouds also
protect vegetation from the effects Of
frost.
All persons desiring to use the hygro-
meter will be supplied with them by the
Chief Officer of the Signal Services at
cost price, seven dollars. Full instruc-
tiOns as to the manner of reading them
will also befurnished free of charge.