The National tribune., 1890-09-11
THE WEATHER.
Absorption By the Atmosphere.
BY LIEUT.. J. p. FINLEY, U. q. SIGNAL sEnvrc

BY LIEUT.. j. ", FINLEY, u. b. SIGNAL BERVICE-
correspondence invited, suggestions desired
and Information cheerfuly furnished Every ef
fort employed to make this Bureau OF practical use
to the readers of THE NATIONAL TRTHuNE. All
communications should be addressed-IPe Feather
TIE NATIONAL TRIBUNE Office. Washington, D. O. |
This is subject which has received consid-
cradle attention from physicists, and is one of
the highest Importance to metoorology. But
rs yot our knowledge OF It is very imperfect
and the subversOn of the hitherto popular
doctrine of the relative athefmancy OF water
vapor to air, which has played so conspicuous
part in most DG the recent writings on this
branch of motoorolOgy, obliges us to fall back
on the individual observations of Forbes, JAMS
sen, Rewmayer and others who have investi-
gated this question of atmospheric absorption
by observations on the atmosphere in the mass
These observations are of Two kinds, viz, those
which have aimed at diTorential measure
ment of tho solar radiation before and after it
has been sifted, and partially by different
thicknesses of the atmosphere, and those which
have compared the cooling of the lowest
stratum of air and of the earth on which it
rests, in different states of the atmosphere, on
tho assumption that under cloudless sky the
cooling will proceed the faster tho more dia-
thormanous the superintendent air mass.
Ot the former class perhaps the most valua-
bio and most direct in the information they
afford are the actinometrio observations made
simultaneously on mountains and on the plains
at their foot. The conclusions arrived at from
these observations aTe that the absorption Of
the solar rays by the strata Of air TO which we
have immediate access Is considerable in
amount, for even q moderate thickness; that
tho absorption almost certainly reaches limit,
beyond which no further loss will take place
by an increased thickness of similar atmOs-
pberic ingredients, and that the residual heat
may amount to from half TO a third Of that
which reaches tho surface of th6 globe after q
vertical transmission through n clear atmos-
phere; that the law of absorption in a clear and
dry atmosphere, equi valcnt to between one and
four times the mass Of air traversed vertically,
may be represented (within those limits) by an
intensity diminishing in q goOmetrical pro-
erosion, plus n constant quantity, which Is the
limiting value already mentioned
Taking tho value Of tho oxtra-atmospherlc
radiation as 73o (of the arbitrary scale OF the
actinometer) tho limiting value Of the solar
radiation, after passing through an indefinite
atmospheric thickness is 1920. The absorption
in passing through vertical atmosphere OF such
thickness as to exert pressure of 20022 (re-
dnced) baromotric inches, is such as to reduce
the incident hoat from 1 to 0.533
{, Tho most extensive aud important observa-
tiou on tho varying rate OF cooling in different
states of tho atmosphere hitherto published
arc probably those of Dr. Noumayer Mt Mel
b"uAho th A9JsF oo5 Alt plaCcu bpiIlb bucI-
momcter in the focus of q parabolic mirror,
directed 30 tho zenith, aud resting on "a bed of
n-conducting material, while il was screened
on all sides around from currents Of air. The
observations, 1876 in number, were mado at
all times Of the day and night, but only when
tho zenith was perfectly free from visible cloud
and the shade temperature of tho air and that
Of evaporation were observed simultaneously.
The radiation observations having been re-
duced and tabulatcd in series first, according
to tho increase Of the vapor tension, then ac-
ooTdng TO temperature, aud lastly according
to tho relative humidity of tho air, led to tho
conclusion that the absolute quantity Of
aqueous vapOr in air is alone in itself not sufi-
oiont as Il criterion for the degree Of radiation.
The relative humidity Of tho air greatly in
fuonces radiation in such manner that tho
greater tho degree Of relative humidity tho less
radiation is noticeable In other words the
athermanCy of tho air varies with It's relative
humidity. The nearer il Is TO that state In
which Il begins to deposit cloud or fog. the moro
opaque IL is 70 dark heat, aud il is this, aud not
merely the absolute quantity Of the yapor diff
fused through It, that is the determining con-
dition of athermancy.