The National tribune., 1890-11-13
THE WEATHER.
Convection Currents in Moist Atmosph

Convection Currents in n Moist Atmosphere.
BY LIEUT.. J. p. FINLEY, U. 8. SIGNAL SERVICE
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THE NATIONAL TRIBUNE Office. Washington, ID. o.
As soon as air is saturated so that on further
cooling it begins to deposit q portion of its
moisture in the form of cloud or fog the evo-
luton of latent heat introduces now and i'm
portant element into the problem Of determin-
ing the change in temperature with elevation.
1t admits of convection currents being main
tanned in an atmosphere in which tho vertical
decrement of temperature is much less rapid
than in the case of dry atmosphere, or with
moist unsaturatod air. The dynamic cooling
OF saturated air varies according to the initial
baromotrie pressure aud temperature. With the
pressure Mt 80 inches and tho temperature Mt
freezing the air will cool one degree in nearly
277 feet of ascent. At lower pressures and
higher temperatures the change is less rapid.
If the air in n cumulus cloud 1000 feet above
the earth has temperature of &C degrees, and
is under pressure of 20 inches, il will have TO
rise to a hight of 5,065 feet in order to cool 10
degrees. Where the change in elevation is only
few thousand feet the initial ratio of cooling
varies little from the mean rate
Tho reason, then, that an ascending current
Of saturated air may be maintained with YER
tical decrement of temperature not more rapid
than that which Is shown by observation TO be
tho normal average decrement in the atmos-
phere, whereas an ascending current of dry air
cannot BIO so maintained, Ia that in the former
tho heat which is constantly converted into
work is supplied by tho latent heat emitted
by the cOndGnsing vapor; whereas the latter
has no such source OF energy to draw upon.
descending current might equally be main
tanned in an atmosphere laden with fog or
cloud, since in that case the heat developed by
the compression Of the descending air would
DO used up as latent heat in evaporating the
cloud, but such q condition rarely occurs in
nature
Where air is not allowed to expand there is
no work done at the expense of the heat sup
piled, and tho whole work goes toward heat
ing the air. The increase of temperature,
therefore, given to air by a certain amount Of
heat communicated TO il, where il is not allowed
TO expand, is to that in tho case of expansion
under constant PRESSURE in the ratio Of unity
to O.7l53, or as l.402 10 unity. Conversely, the
heat required to raise tho temperature of q
given quantity of air through ono degree I'M
der constant volume, is to that required in the
case OF expansion, under constant pressure as
unity to 1.i02.
In the compression or expansion Of confined
air the work expended in tho one case, or done
in the other, is not proportional to the amount
OF contraction or expansion of the volume,
since tho force or resistance overcome Is con-
stantiy changing. becoming continually greater
in case Of compression and less in that of ex-
pansioD, so that the amount of work can be
computed accurately only by means of log
arithmic formula. Where the ascending air
Ia saturated tho cooling arising from expansion
continually diminishes the capacity of the air
for moisture, and the lower the temperature the
less the amount Of yapor which can exist in
the air. Hence, as tho saturated air Rscends
aud cools the aqueous Vapor is condensed, and
the latent heat given out az condensation tax os
plaza raises the temperature of the air above
what IL would otherwise be, aud thus causes
tho rate OF cooling of the ascending saturated
air to be much loss than in the case of dry or
unaaturatcd air