The National tribune., 1890-11-06
THE WEATHER.
BY LIEUT.. J. p FINLEY, U. 8. SIGNAL SERVICE.

Correspondence invited, suggestions desired,
and information checrfuly furnished. Every ef-
fort employed to make this Bureau of practical use
to the readers of THE NATIONAL TRIBUNE. All
communications should he Addressed Weather,
THENATIoNAL TRIBUNE Office Washington, ID. O. !
One of tho most important applications of
the laws of dynamic heating and cooling in
meteorology is that of convection currents.
By convection currents are meant ascend
ing and descending currents which arc set
up in consequence of portion OF mir having
become, by change of temperature either less
or more dense than other parts of the same
stratum, while exercising the same elastic
pressure. In such case it Obeys the law of lo-
taton, and the heated air rises, while cooler air
descends and takes its place. That this kind
Of action is of high Importance, aud, together
with its inverse action (the cooling and sink
ing of heavy air., is the principal cause of
winds is fact which can scarcely be considered
open to question. But n consideration of tho
laws OF dynamic heating aud cooling and their
consequences, will show that the conditions
under which con vecton currents can be set up
arerigorously limited. Observation tells US that
in a column OF dry air, or Que much below satu-
ration, these conditions arc very rarely fulfilled
except on u small scale aud within n short dis
lance of tho ground This fact has been insult
ficiently insisted on in some of our elementary
treatises on moteorology, and much erroneous
The land and sea breezes prevalent in coast
regions are the result of convection currents.
The monsoOns of India are the consequence OF
convection currents on vast scale, which dur
ing the rainy season arc ascending and during
the cold season descending currents in the in
terror of the country.
Dry air in ascending uses up heat at the rate
Of about one degree in every 1873 feet, and IT.
the temperature of the atmosphere through
which it ascends does not decrease ut an equal
Or s0me greater rate, any mass which has been
so heated as to expand and form an ascending
convection current, will cool more rapidly than
the strata through which it ascend5 and will
soon be reduced 10 the temperature aud density
Of the air ut the same level, when its further
motion will be arrested. Moreover, If imme-
diately about it's initial position the vertical
decrement OF temperature is not moro rapid
than one degree IN 1873 feet, it will not ascend at
all, and We may assert, generally, that no con-
vection current arising from "A disturbance OF
the vertical equilibrium of temperature can
exist In a dry atmosphere in which the vertical
decrease Of temperature is less than one degree
in lS3 feet; unless the ascending air Is contin-
ually receiving heat from some external source.
The same law holds good, mttafs mutdndis. for
convection currents produced by the cooling
and sinking of air of an upper stratum No
descending current can be set up in q dry at
mosphere unless the temperature of tho air
around and through which 1t sinks increases
dOwnwards more rapidly than one degree in
1873 feet, or unless the current is continually
parting with the heat generated by its descent.
Observation shows that in warm, dry regions,
and during the hottest part of the day, the
temperature decrement with altitude is never
so rapid AS one degree in lS3 feet, except in the
lowest strata near the earth, and therefore
merely local convection currents in q dry at
mosphore. arising from disturbance of vertical
equilibrium, are of rare occurrence The only
important instances are those afforded by dust
storms so common on arid plains in hot wca-
Their.
To the reverse action, viz: the heating of q
descending current, DIR. Ham attributes the
hot, dry winds felt in the Alps and some other
mountainous countries and locally known as
the Fohn" similar descending mountain
current in Montana. Idaho, Washington. Ore.
gon, and British Columbia ia called the "Chi-
nook" term taken from the region inhabited
by the Chinook Indians.