The Iola register., 1882-12-29
The. Formation Of Dew.

! rLLEtN4 u
The formation of dew, one of natures
oft-recurring phenomenon, although an
object Of absorbing interest, has not
ELECTED that thoughtful, investigation by
the casual observer to which it is entitled;
indeed, to many its nocturnal origin is
| yet involved in mystery. In the econ-
omy of nature it is an important factor.
from its benign influence upon the hui
midity and atmospheric temperature.
1t is a established fact that the
atmosphere derives its aqueous vapor
from evaporation, and as evaporation is
| the result from heat, whereby the gas
eous molecules ofthe atmosphere become
distended, and their capacity for sup
porting vapor augmented, so must the
quantity of vapor contained in the atmos-
phere be dependent upon the temperature
of the air. When the atmosphere is sat
urated with vapor, by adding moisture,
the elasticity of the vapor already present
will not be increased, but will condense
into minute drops and appear in the form
of mist or dew. 'f-f.
The temperature at the moment of this:
conversion of vapor into dew is termed
the dew point, by which we are enabled
to determine the elasticity of vapor for
each degree of temperature, and the
quantityof vapor present at any time in
the atmosphere evaporation commences
at sunrise and continues until the diurnal
| temperature has attained its maximum
between two and four o'clock in the,
afternoon. But with the increase of
atmospheric temperature there is also an
increased capacity for supporting vapor
| in a state of insensible suspension, con-
sequently the air itself and objects sur
rounded by it feel comparatively dry
from what they appeared to be in the
morning. although the quantity of vapOr
has greatly increased.
As the sun recedes and loses in in
tensity her rays diffilse less heat upon
the earth's surface, and consequent re-
duction Of the temperature begins and
continues in three ways: First. b. evapo-
ration. which proceeds from the effect of
surface heat and carries of caloric in a.
latent state; second, by an amount of
surface heat being conducted downward
into the earth; and third, by radiation
which continues and becomes the most.
effectual agent of cooling.
In this way the decrease of tempera-
ture begins at the surface of the earth.
and when diminished until the vapor of
the lower temperature commences to
saturate the air it becomes fresh and
moist. This condition often occurs be-
fore sunset. and generally commences
before that time in valleys and depressed
locations, because the solar heat had
ceased to act on them before its disap-
pe,rance from the exposed table lands.
As radiation is the principal esuse of the
reduction of the temperature. it pro-
gresses more rapidly from surfaces COW
cred with grass, trees and undergrowth
| than from open. unobstructed surfaces.
l. Radiation proceeds more slowly from
water, surfaces, because when the super
ficial stratum imparts its heat to the rising
vapor it sinks, and the one Immediately
below comes up to the surface.
As the decrease of temperature pro-
crosses more rapid from four to nine
O'clock in the evening than during the.
remainder of the night, it is evident that
large proportion of vapor suspendedin
the heated air during the day is precipi
tated as evening dew, and frequently by
ten o'clock the deposit on the foliage of
trees becomes so copious that it may be
heard falling in drops from leaf to leaf.
Another period in which dew is copiously
deposited is after midnight and at the
dawn of day. 'H
After nine O clock the rate of decrease
in atmospheric temperature is consider
ably diminished, and with it the forma-
ton of dew likewise retarded, but when
the temperature has arrived at its mini
mum. the deposition of dew is augment
ed. The appearance Of dew only in the
morning proves that the temperature of
the evening was not reduced to the tenn
perature of the dew point.
But dew will not form on every even
ing, for the following reasons: First
when the sky is. covered with clouds
radiation from the earth is in a measure
compensated by radiation from the clouds
and, therefore, the surface stratum OF
air is not reduced to the temperature OF
the dew point; or humid south or
southwest wind may interpose the requis-
ite reduction of temperatures another
condition unfavorable to the formation
of dew is that of dry northwest wind,
which may absorb the liberated vapor-
A hazy atmosphere or large amount
of invisible vapor will retard radiation,
hence serene, eloudless nights, without
deposition of dew, are regarded as indi-
cations of coming rain.
According to the theory of Professor
Henry. the amount of the deposition OF
dew depends upon the stiffness OF the
atmosphere, for if brisk winds are blow
ing at the time, the different strata of
air will be mingled together, and that
which rests upon the surface of the
ground will be so quickly displaced as
not to have time to cool down sufficiently
to produce deposition,
Various ingenious methods have been
devised to ascertain the hygrometric-
state of the atmosphere. The delicate
hair hygrometer by Saussure, and that
by De Luc made of the fiber of whale
bone, have had their admirers, yet the
results from their reductions are far
from being satisfactory. The dry and
wet bulb Thermometers have superseded
all other instruments for practical scien-
tific purposes, The psychrometer is, at
the present day, an indispensable adj
junct to every msseorologist! The i'm
mersed wet bulb indicates the depression
of temperature if the air is not already
saturated with moisture, whereby evap-
Grafton is prevented; the rate OF evapor-
alton and the consequent depression of
the wet bulb will be greater in proper
ton. as the air is further from the point
of saturation.
With the aid of valuable tables pub
lished by the Smithsonian institution,
and United States Signal Office. calcula-