Page:Handbook of Meteorology.djvu/216

 A correction for latitude is required at Weather Bureau stations. This correction in the United States, Alaska excepted, varies from nothing at Lat. 45° to 0.05 inch in the southern part of the country. It is additive in latitudes higher than 45° and subtractive in latitudes lower. Being a constant, it may be included in the sea level reduction.

Except for weather bureau records, or for comparison with sea level records, reduction to sea level is not necessary. In general, station-altitude readings and the oscillations in pressure are of greater value to the observer than reduced readings. This is notably the case in aviation. It is often necessary to know whether one is entering a region of increasing or of decreasing pressure. The difference involves not only questions of plane support; it is also the distinction between clearness and cloudiness.

Obtaining Station Altitudes.—The altitude of a permanent station should be determined as closely as is possible with ordinary facilities. Two points, a “plane of reference” and a station fixed point are required. The first should be, if possible, a bench mark of the United States Coast Survey, the Lake Survey, the Mississippi River Commission, the Engineer Corps, or the United States Geological Survey. Of less precision are railway levels and city bench marks, and other surveys made by engineers; they will be found useful for reference even when their precision is doubtful. Railway station levels are reasonably precise. The top of the rail at a designated point within yard limits may be taken as a plane of reference.

If a precisely determined elevation is required it can be obtained best by a survey from the most accessible established bench mark. The station fixed point should be made on some object that is both fixed and durable. A young and rapidly growing tree is not a desirable object for a station mark; but a mark made on a full-grown tree is not subject to material change. A stone post set firmly in the ground, or a piece of painted scantling attached firmly to the corner of a building will answer the purpose. The mark should be of such a character that it will resist ordinary weathering. The final point in the determination is the station barometer, that is, the chain of determinations which begin at an established plane of reference and end with the ivory point within the cistern of the barometer.