Mower County transcript. [volume] (Lansing, Minn.) 1868-1915, 1888-10-03
TO FoRETELL FROST.
How Every Intelligent Man Can DO II In
g.A Simple and Practical Way. n.

How Every Intelllgant Man Can DO It}ln
simple and Practical Way.
Frost is condensed vapor from the
air. In order, therefore, to predict
frost, is is requisite to know the laws
governing this condensation.
They are simple. Why, let. us first
inquire, should the vapor condense at
allp Why not remain always in the
ARP Because the air can only contain
certain amount, and all in excess Of
this is condensed. The air is like q
sponge. which will begin to drip when
charged with more water than it has
capacity for sustaining. But here comes
IN an important item. It appears that
the air will hold more moisture when
at q high temperature than when at q
low temperature. At 80 deg. Faren-
heit the air will hold the 16Oth part Of
It's own weight, at SO deg. the 80th
part and at SO deg. the 10th part. The
law Is that at every increase of 81 de-
grees the capacity is doubled. If at
noon the thermometer stood at so deg.
and at night at ~ deg. equal to 50
deg. XII deg then the air at evening
would be able to hold twice as much
vapor without condensation. write,
Is able, for the vapor may not be there.
Suppose, however, that these tenn
peratures be reversed; that at noon
the air has q warmth of II deg. and
at evening of so deg. Then at noon
the air can hold twice as much moist
ure as at evening, If, then, it be
charged to its full extent at noon, what
must happen then as the cool of the
evening comes on, and it's capacity for
vapor is reduced one-half by the fall in
temperature? Evidently the extra va
por or moisture will be discharged
from the air. This, in fact, is just
what happens, the cool of night pro-
ducing that condensation which is
called dew. BUT dew will fall only on
condition that there be vapor in the
air. Ii the air be dry, however cold
the night, no dew will fall.
1t is easy to see now that the ques-
ton whether there will or will not be
dew on the given night depends on
two factors-the amount of vapor in
the air at that time and the tempera-
ture of the night. Calling the amount
of vapOr X, we know that if the tenn
perature falls low enough it will
finally reach point where the air will
no longer sustain vapor, and conden-
sation will begin. But how shall we
ascertain at just what point this will
happen? By cooling the air expert
mentally, on the afternoon before the
given night, until dew does fall, and
noting the temp erature at which this
happens. An ice pitcher condenses
dew by cooling the air. We place
a thermometer in a bowl of water and
drop in ice bit by bit until the first
traces of dew appear on the bowls ex-
terror, then reading the thermometer,
we know that if the temperature falls
to that point called the dew-point, there
will be dew during the ensuing night.
The dew-point will, of course. differ
on different days, varying with the
varying quantity of vapor in the air;
if the quantity be small the dew-point
is low, if great, it is high. Briefly de-
fined, the dew-point is the temper
allure at which dew begins to fall, the
air at that temperature being no longer
able to support the amount of vapor
present in it. It has already been
stated. that as the temperature de-
creases, the capacity of the air for
sustaining vapor grows less and less.
An interesting and important fact
now must be pointed out. It is this:
that when the fall of dew begins the
night ceases to grow cooler. And since
the dew-point is the temperature at
which the fall of dew begins, if we
know the dew-point. for a given night,
we know the lowest temperature of
that night. The reason why the night
ceases to grow cooler Immediately the
dew-point it reached is too
complex to give here; the
fact, however, is we'll established.
Knowing, therefore, the dew-point of
any night, we know it's minimum
temperatures; and this Is how frOsts
are predicted. If the dew point be
80 deg. (the freezing point) or below,
and the night be clear, a frost is al
most certain. If the dew point is not
below Al deg, there will almost GER
tainly be no frost. The range between
80 deg. and AZ deg. is uncertain ground,
the probability of frost being slight at
AZ deg. and increasing as 80 deg. is
approached. s =.7
The method of ascertaining the dew
point given above with bowl and
fragments of ice is a clumsy and mac
curate one, A much more convenient
and reliable instrument is that known
as the hygrometer. The United States
signal service Office at Washington is
so much interested in inducing farm
ers to protect themselves from local
frosts that it offers to send any one a
hygrometer at cost ($7) with a
pamphlet; containing full directions for
it's use. The instrument requires in
telligence and care in it's use; to such