Mower County transcript., 1890-11-26
ATMOSPHERIC RADIATION.


ATMQsPhERlc RADIATION.
An Interesting Explanation Furnishod by
hignsl-scrvice Onicer. 3'' r.
The discussion Of observations on the
nocturnal cooling Of the air at Madras,
India, leads to the conclusion that the
athermancy Of the air varies with its
relative humidity. Selecting as clear
nights those on which the cloud proper.
ton was one-tenth Of the expanse or
less, and rejecting all others, 1t was
found that the fall of temperature be-
tween sunset and sunrise was the great.
or the lower the tension of the vapor In
the air. But when, instead Of the va
por tension, the mean cloud proportion
was taken as the basis of the compar-
ison the part passu variation in the fall
of temperature was far more de-
cided; 80 much so, indeed, as to make
1t probable that the tendency to the
formation Of cloud is the real critical
condition OF athermancy, and that tho
apparent influence of vapor tension 10
only q secondary condition, owing to
the fact that, caeteris pari6us, the higher
the humidity Of the air the higher Is
the vapor tension. Estimating the
whole expanse Of the sky, in the usual
way, as 10, it was found that an average
OF 0.S5 of cloud reduced the nocturnal
fall Of temperature by more than one
fourth OF that which took place under q
perfectly cloudless sky.
The conclusion, then, which tho pres
ent state of our knowledge seems to
justify, is that both air and vapor, as
compared with other gaseous bodies,
are moderately good absorbers Of heat,
and in a nearly equal degree; but that
the varying diathermancy Of the
atmosphere depends chiefly on tho
quantity Of condensed vapor in it,
and also, it may be added, on solid
matter, dust, etc., suspended in it.
Condensed water exists in other forms
than as visible cloud masses. Tyndall
has demonstrated that the blue color Of
the sky is probably owing to condensed
vapor in very fine state OF division,
and that state which Roscoe spoke Of as
the opalescence Of the atmosphere may
possibly arise from q similar cause.
Then, again, suspended dust, such as
that which constitutes the constant haze
of the dry season in Upper India and
the Western plains Of the United States,
is great absorber Of heat as we'll as Of
light, and doubtless plays an important
part in that diurnal heating Of the lower
atmosphere which gives rise to the hot
winds of the summer months. As a
eneral rule, liquid and solid matters
re far more absorbenJ of radiation than
the gaseous matters Of the atmosphere.
1t is universal law that the readi-
ness with which substances part with
their heat by radiation stands in o di-
rect relation to their absorptive power.
Prof. Balfour Stewards experiments
have demonstrated that this law holds
good in detail, and that substances
which, like gases. -o,ored glass, or re-
fecting and colored bodies, absorb GER
tain kinds OF radiation select!vely, also
radiate those kinds selectively. Air,
then, which absorbs but little heat, as
compared with fluids and solids, radiates
anu loses heat but slowly, and hence
those elevated strata of the atmosphere
which are not in contact with mount
aims probably undergo but a small
change Of temperature between day and
night But rocks and soils, which
are greedy absorbers of heat and
in proportion as they absorb ii
arc also good radiators, play very
important part in the nocturnal cooling
II the lower stratum Of the air, which
loses heat by contact with the ground.
In like manner in tho. winter season
radiation from the ground is ono of the
chief causes Of the lower temperature
which then prevails over the land,
bringing about that greater density in
the lower atmosphere. On mountain
tracts the effect of this cooling is felt
in the currents OF cold air which drain
off the mountain slopes and flow down
the great valleys as cold winds during
the night and early morning, and in
the winter season frequently through
out the day. The deposition OF dew
and hoar frost on the surface OF leaves,
grass and other good radiators, are
other familiar effects of cooling by radi'
ation.-Lieutenant j. P. Finley, in Ma
tional Tribune.