preface to fifteenth edition

2.78 SECTION 2
TABLE 2.12
Reduction of the Barometer to Sea Level—Metric Units
A barometer located at an elevation above sea level will show a reading lower than a barometer at sea level
by an amount approximately 2.5 mm (0.1 in) for each 30.5 m (100 ft) of elevation. A closer approximation can
be made by reference to the following tables, which take into account (1) the effect of altitude of the station at
which the barometer is read, (2) the mean temperature of the air column extending from the station down to sea
level, (3) the latitude of the station at which the barometer is read, and (4) the reading of the barometer corrected
for its temperature, a correction which is applied only to mercurial barometers since the aneroid barometers are
compensated for temperature effects.
Example. A barometer which has been corrected for its temperature reads 650 mm at a station whose
altitude is 1350 m above sea level and at a latitude of 30. The mean temperature (outdoor temperature) at the
station is 20C.
Table A (metric units) gives for these conditions a temperature-altitude factor of .................. 135.2
The Latitude Factor Table gives for 135.2 at 30 lat. a correction of ................................ 0.17
Therefore, the corrected value of the temperature-altitude factor is .................................. 135.37
Entering Table B (metric units), with a temperature-altitude factor of 135.37 and a barometric
reading of 650 mm (corrected for temperature), the correction is found to be .......................... 109.6
Accordingly the barometric reading reduced to sea level is 650 109.6 759.6 mm.
Latitude Factor–English or Metric Units. For latitudes 0–45 add the latitude factor, for 45–90 subtract the
latitude factor, from the values obtained in Table A.
Temp.—Alt.
Factor
From Table A
Latitude
0 10 20 30 45
50 0.1 0.1 0.1 0.1 0.0
100 0.3 0.3 0.2 0.1 0.0
150 0.4 0.4 0.3 0.2 0.0
200 0.5 0.5 0.4 0.3 0.0
250 0.7 0.6 0.5 0.3 0.0
300 0.8 0.8 0.6 0.4 0.0
350 0.9 0.9 0.7 0.5 0.0
A. Values of the temperature-altitude factor for use in Table B.*
90 80 70 60 45
Altitude
in
Meters
Mean Temperature of Air Column in Centigrade Degrees
16 8 4 0 6 10 14 18 20 22 26
10 1.2 1.1 1.1 1.1 1.1 1.0 1.0 1.0 1.0 1.0 1.0
50 5.8 5.6 5.5 5.4 5.3 5.2 5.1 5.0 5.0 5.0 4.9
100 11.5 11.2 11.0 10.8 10.6 10.4 10.3 10.1 10.0 9.9 9.8
150 17.3 16.7 16.5 16.2 15.9 15.6 15.4 15.1 15.0 14.9 14.7
200 23.0 22.3 22.0 21.6 21.1 20.8 20.5 20.2 20.0 19.9 19.6
250 28.8 27.9 27.5 27.0 26.4 26.0 25.6 25.2 25.0 24.9 24.5
300 34.5 33.5 33.0 32.5 31.7 31.2 30.7 30.3 30.1 29.8 29.4
350 40.3 39.0 38.5 37.9 37.0 36.4 35.9 35.3 35.1 34.8 34.3
400 46.0 44.6 43.9 43.3 42.3 41.6 41.0 40.4 40.1 39.8 39.2
450 51.8 51.3 49.4 48.7 47.6 46.8 46.1 45.4 45.1 44.8 44.1
500 57.5 55.8 54.9 54.1 52.9 52.0 51.2 50.5 50.1 49.7 49.0
550 63.3 61.4 60.4 59.5 58.1 57.2 56.4 55.5 55.1 54.7 53.9
600 69.0 66.9 65.9 64.9 63.4 62.4 61.5 60.6 60.1 59.7 58.8
650 74.8 72.5 71.4 70.3 68.7 67.6 66.6 65.6 65.1 64.6 63.7