The National tribune. [volume] (Washington, D.C.) 1877-1917, 1890-09-18
THE WEATHER.
BY LIEUT.. ). p. FINLEY, U. s. SIGNAL SERVICE
Atmospheric Radiation.

BY LIEUT.. J. p. FINLEY, u, s. SIGNAL SERVICE.
The discussion Of observations on the noe
turnal cooling of the air at Madras, India, lead
TO the conclusion that the atbermacy of the air
varies with its relative humidity. Selecting
as clear nights those on which the cloud pro-
portion was one-tcnth Of the expanse or less,
and rejecting all others, IL was found that the
fall OF temperature between sunset and sun
rise was the greater the lower the tension Of
the vapor in the air. But when, instead of
the vaPor tension, the mean cloud proportion
was taken as the basis Of tho comparison, the
pdri passa variation IN the fall Of temperature
was FAR more decided; so much so. indeed as
TO make Il probable that the tendency 10 the
formation OF cloud is the real critical condition
Of athcrmacy, aud that the apparent influence
Of vap0r tension IS only secondary condition,
owing TO the fact that cdeteris pdri5tis the
higher the humidity Of tho air the higher ia
the vapor tension. Estimating the whole ex-
panse OF the sky, in the usual way, as 10, it
was found that an average Of O.SJ of cloud re-
duced the nocturnal fall of temperature by
more than oue-fourlh OF that which took place
under perfectly cloudless sky.
The conclusion, then. which the present
state Of our knowledge seems TO Justify. is that
both air and yapOr, AS compared with other
gaseous bodies, are moderately good disorders
Of heat. aud IN a nearly equal degree; but that
tue varying diAthcrmacy Of the atmosphere
depends chiefly on the quantity Of condensed
vapor in il, and also, IL may be added, on solid
matter. dust etc, suspended in il. Condensed
water exists Ia other forms than as visible
cloud masses. TyHdali has demonstrated
that the blue color Of the sky is proba-
bly, owing 10 condensed vap0r in a very
fine state Of division. and that state which
Roscoe speaks Of as the 0pGLescnice Of the at
mospheTe may possibly arise from a 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 ahsorber Of
heat as well as Of light, and doubtless plays an
important part in that diurlal heating Of the
lower atmosphere which gives rise 10 the hot
winds Of the Summer months. As general
rule, liquid aud solid matters are far more alb
sorbent OF radiation than the gaseous matters
Of the atmosphere.
il is a universal law that the readiness with
which substances part with their heat by radi-
alton stands IN q direct relation 10 their alb
gorptive power Prof. Ballour Stewar's expe.
Timents have demonstrated that this las holds
good ln detail, and that substances which like
gases, cOiOred glass, or rejecting aud colorcd
bodies, absorb certain kinds Of radiation select
lively, also radiate those kinds sclcctiyely. Air,
then, which absOrbs but little heat, ~ com
paicu win nuns anu bOilAb, gallatcd ana loses
heat but slowly, and hence those elevated
strata Of thc atmosphere which are not in con-
tact with mountains probably undergo but "A
small change Of temperature between day aud
night. But rocks and soils, which are greedy
absOrbeTs Of heat, aud in proportion AS they
absorb it are also good radiators, play very
important part in the nocturnal cooling Of the
lower stratum of the air, which loses heat by
contact with the ground. Ia like manner in
int Lott
il one Of the chief causes Of the lower tenn
perature 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 iOw down the great valleys AS cold winds
during the night and early morning. and In
the Winter scasOQ frequently throughout tho
day. The deposition Of dew and boar frost on
the surface Of leaves, grass aud other good radi-
atoTs, are other familiar effects Of cooling by
radiation.