PROCEEDINGS OF THE ACADEMY AND AFFILIATED 
SOCIETIES 
PHILOSOPHICAL SOCIETY OF WASHINGTON 
886TH MEETING 
The 886th meeting was held in the Cosmos Club Auditorium on Saturday, 
May 5, 1923. It was called to order at 8:15 p.m. by President Wuits with 
35 persons in attendance. 
The first paper, Free-air pressure maps and their accuracy, was presented 
by Mr. C. LeRoy Metstnerer. The paper was illustrated by lantern slides 
and was discussed by Messrs. WHITE, LIrTLEHALES, TUCKERMAN, HawkEs- 
worTH, HuMpPHREYS, PAWLING, and GREGG. 
Author’s abstract: The variations of barometric pressure from day to day 
are of fundamental importance to the weather forecaster; but before the 
barometric readings can be compared, they must be reduced to some common 
level. At present, sea-level is the only reduction level in use, and for stations 
of no great elevation above this level the barometric indications correspond 
closely with surface weather conditions. But in high elevations the sea- 
level reductions are less satisfactory. The physical advantage of reduction 
upward to a free-air level is that horizontal barometric gradients correspond 
closely to the air movement at the same level. Moreover, the mean tem- 
perature of the air column (an important term in the hypsometric equation) 
is a real quantity in contrast with its fictitious nature when the reduction is 
downward. 
The difficulties of securing current observations of the average tempera- 
ture of the air column at a sufficiently large number of stations and of re- 
ducing them in time to be of current usefulness to the forecaster, necessitate 
the use of some regularly-observed surface weather element as an index to 
this quantity. 
It was found upon analyzing a large body of aerological data that the 
difference between the mean temperature of the air column and the surface 
temperature varied markedly with surface wind direction and the season. 
A classification of these data in this manner enabled one to ascertain the 
geographical distribution and further to interpolate for non-aerological 
stations in the construction of tables for barometric reductions. 
During the three months, December 1922 to February 1923, inclusive, 
daily post-card reports of 8 a.m. (75th meridian time) pressure at 1 and 2 
kilometers (3281 and 6562 feet) above sea-level were made by 29 stations 
within the area embraced by the six aerological stations of the Weather 
Bureau. A statistical investigation of the accuracy of the reductions was 
made from two classes of data: 
(1) Computed free-air pressures compared with pressures measured by 
kites at the aerological stations showed that 73% of the computations agreed 
within 0.05 inch.. (Isobars on the daily weather map are drawn for inter- 
vals of 0.10 inch.) ‘ 
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