The National tribune. [volume] (Washington, D.C.) 1877-1917, 1890-07-03
THE WEATHER.
Vertical Circulation in Moist AtmOsphc,
by LIEUT.. j. P. FINLEY. U. g. SIGNAL SERVICE.

by LIEUT.. j. z. FINLEY, u. 8. SIGNAL SERVICE.
correspondence invited suggestions desired
and information cheerfully furnished. Every Of.
fort employed 10 make this Bureau of practical use
to the readers of THE NATIONAL tribune All
communications should be addressed Weather.
THE NATIONAL TRIBUNE Office. Washington. ID. O.
In tho case of moist atmosphere. with the
unstable state for dry air, we have the same
energy for originating and maintaining YER
teal circulation as in the case Of dry air, with
the additional energy of all the latent heat of
the aqueous vapor set FREQ in its condensation
in the ascending current, and this latter Is a
continuous source of energy as long as moist
air is being drawn in from all sides TO supply
this current. For example, suppose the air is
saturated and is in the unstable state for dry
air; then at the first upward start of the air it
becomes warmer than the surrounding air and
the tendency is to continue on. As soon as the
circulation is fully established, the vertical
gradient in this ascending current becomes
about half q degree Fahrenheit for each 100
meters of ascent, being slightly greater or less,
according to the season of the year and the alti-
tude.
With temperature Of 300 Mt the earth's sur
face the decrease Of temperature is about 0.37o
for each 100 meters OF ascent up TO an altitude
of 1000 meters, and even up to much greater
attitudes the rate would vary but little from
this. With temperature of 100 below zero at
the earth's surface the decrease Is about 0.7io
for each 100 meters Of ascent. If tho atmos-
phere were not completely saturated, but had
depression OF the dew point Of 50, then the air
at the earth's surface would have TO ascend
about ] 900 meters before condensation would
commence and in this part Of the ascent tho
rate of cooling would be 10 instead of 0.37o for
each 100 meters, after which the latter rate
would take place.
IL may happen that before the air rises mp to
tho altitude where condensation takes place the
whole air-colnmn may be heavier than n similar
one OF tho surrounding part OF tho atmosphere.
In this case there must DO more than n nero
initial and temporary impulse or very slight
temperature disturbance merely, 10 start the
vertical ascent Of tho air The initial temper
ature Of the air before ascent commences must
be so much greater than that of the surround
ing air. that 1t does not become cooled down in
it9 brst ascent, before condensation takes place.
TO lower temperature, on the average for the
whole column, than that of the surrounding
air.
Where the air Is saturated at all attitudes
condensation takes place at once at all levels
as soon as an ascending current IL from any
cause induced. and the eloud or fog Is formed
at all attitudes down 20 the earth's surface
The dTier the air the higher arc tho clouds
whore the conditions arc favorable 90 the pro-
dpction OF ascending currents. Without as
cending currents therefore there can DO no
S'ouus' IN Ail lhe region AROUND that OF the
ascending air. whore there IL gradual descent
OF air to supply tho in-draught of these cur
rents, the air clear. for even If there were CON-
plete saturaton and cloud from a previous as
cent of the air as II began to descend the air
would 1T once become warmer and unsaturated
and the cloud would soon disappear by evapora.
ton. In these changes 10 should be under
stood that the air cools by expansion only. The
higher il ascends the greater the expansion
II tho air is cooled in other ways as by noe
turnal radiation during n clear night or by
passing from lower TO higher ATTITUDES, or from
warmer 10 a colder surface as from the ocean
TO the land In the Winter, or vice versa In Sum-
mcr, there may be cloud called fog at and near
tho earth's surface, without any ascent of air TO
higher attitudes 10 give rise 10 cooling by OK.
pansion.