Page:Handbook of Meteorology.djvu/217

 The term “sea level” is differently interpreted in different localities, “Mean tide,” “mean low tide,” “mean high tide,” and “mean sea level” are used. If the local usage does not conform to that of established Federal usage, the nearest established Plane of Reference practicable should be sought as a starting point. The Weather Bureau has established a specific elevation for each of its stations; the nearest station practicable therefore may be taken as an initial point.

Altitudes by Comparative Barometric Observations.—Reasonably correct altitudes may be established by synchronous observations, one series at an established altitude, the other at the place whose altitude is to be determined. For this purpose the position of known altitude should be a Weather Bureau station or an observatory having a standard barometer and an observer of experience. If a mercurial barometer is used at the location whose altitude is to be determined, it should be allowed to “rest”—that is, to adjust itself to the altitude—for a few days, if possible.

The readings may be made hourly at the same time at both stations, the height of the mercury, time and temperature correction being noted. This may be repeated for several days until the reduced readings are constant. If the stations are not far apart, a single series of observations may suffice; if they are more than 25 miles apart differences in pressure other than those due to altitude may interfere.

For instance, six consecutive observations between the stations show a constant difference, and the lower pressure at the upper station may be assumed as a difference in pressure due to altitude.