Wood County reporter. [volume] (Grand Rapids [i.e. Wisconsin Rapids], Wis.) 1857-1923, 1903-08-25
ON
THE WEATHER
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every particle of vegetation capable
of being killed by a freezing tempera-
ture.' Vapor can always be condensed
into water if the temperature be
sufficiently lowered and as soon as
the process of condensation begins the
evolution of heat takes place which
tends to prevent further falling
temperature. The heat involved or
rendered sensible by the condensation
of vapor is enormous, as is evident
from the fact that the condensation
of sufficient vaporS to make pint of
water will evolve enough heat to
raise more than five points from the
freezing to the boiling point. All
this heat must be lost by radiation, in
order that the formation of dew may
proceed or the temperature fall. 1t
is, therefore, a very important fact
that when the dew point, or the tenn
perature at which condensation begins
is reached, the heat evolved by this
process prevents Or at least materially
retards further fall Of temperature.
Condensation and the consequent
evolution of heat begins at a higher
temperature than when the air con-
tanned larger percentage of aqueous
vapor, and therefore, any process by
which the amount of moisture in the
atmosphere can be increased will
lessen the liability of frost. Radia-
tion proceeds most rapidly under a
clear sky and more rapidly from the
surface of the plants, than from the
air about them, so that under favor
able conditions the surface of plants
is frequently several degrees colder
than the surrounding air. One more
elemental factor enters into the
problem, viz: The tendency of the
air to arrange itself in layers or strata
in accordance with it's density. On a
still night the colder air being more
dense and consequently heavier rests
on the surface and surrounds the
plants, thus increasing their liability
to frOStS. Owing to this fact a
thermometer close to the ground will
frequently read from to 10 degrees
lower than one s or 10 feet higher.
There are then three essential cOndi-
tions favorable to the formation of
froSts. A clear sky, dry air and still
night, and when all are present at
the critical period the result should
not be in doubt. To these of course,
must be added the proper date with a
general air temperature sufficiently
low to make froSts within the range
of possibility. The average date of
the last killing frost in the spring in
this section of the state falls on May
25, but killing froSts have occurred
as late as the last of June,
so that June may be considered
the critical spring month. The aver
age date of the first fall of frost is
September IL, but frosts have been
known tC occur the latter part of
August. While our records in the
cranberry district are hardly of
sufficient length to determine posi-
lively the average date, it would seem
from what data we have at hand that
there is at least tendancy for frost
to occur both earlier in the fall and
later in the spring in this portion of
the Wisconsin Valley than elsewhere
at the same latitude. A clear sky is
essential for rapid radiation. Clouds
obstruct the passage of the heat rays
emitted by vegetation at the surface
and in measure reflect back the heat
which quickly prevents further cooling
or even causes rise in temperature.
The effect of even the tlnieSt cloud
is appreciate and those of you who
have never made this observation
will find it an interesting experiment..
The temperature has frequently been
observed to rise two or even five
degrees within as many minutes while
a cloud was passing over, and fall as
quickly usually to lower point than
before the appearance of the cloud.
A dry atmosphere is favorable to
frost for the reason that the tempera-
ture must reach a lower point before
the condensation of vapor begins,
with it's consequent evolution of heat,
than when the air is more fully
saturated. There is a distinction,
however, between dry soil and dry
air, in regard to the liability to frosts,
while dry atmosphere promotes the
formation of froStS, as just explained,
dry soil is unfavorable as a rapid
loss of heat is promoted by active
evaporation of moisture. 1t is evident
that other conditions being equal
frost is more likely to occur on moist
than on dry ground provided, however,
the ground be not too moist in which
case the moisture evaporated, and
added to the air will tend to retard
radiation and consequently prevent
frosts. The practical lesson to be
gained from this law is this: When a
marsh is dry and frost is indicated,