Page:Handbook of Meteorology.djvu/225

 Interpretation of Barograph Records.—The usual eight o’clock observations of pressure furnish nothing more than changes in pressure at twelve-hour intervals. For the purpose of scientific study the continuous barogram is the best object lesson. The West India hurricane, the ordinary storm, the heavy downpour, the thunder-storm and the cold wave—each leaves its individual earmarks on the record sheet. In no other way can the observer work out his position and time of diurnal inequality so well as by the use of the record sheet.

A discussion of the specific features made by the barograph pen, noted in the preceding paragraph, is not necessary. Each should be noted on the record sheets, at the time of its occurrence. At times the specific features in a record sheet enable an observer to make forecasts that would escape notice if the observations were made with an ordinary barometer. Thus, if the trace of the pen in the progress of a falling barometer is convex, an increasing violence of an approaching storm is indicated. In the same manner, a concave trace of the pen with a rising barometer indicates an increasing force of the wind—frequently the onset of a cold wave.

A close study of the trace of the barograph pen will enable an observer to discern conditions, leading to fairly certain predictions, which otherwise would pass unnoticed. Continuity of record constitutes, to a great degree, the value of pressure records, and while the records of hourly observations intelligently graphed, might lead to similar conclusions, few meteorological laboratories are so equipped as to make such observations possible. But the barograph catches and records minute pressure alterations that might escape the notice of the most careful observers even if hourly observations were recorded.

There is not much difficulty in comparing the graphs of metric charts with those of inch charts. A base line for either may be drawn on the other, and distances or ordinates are readily measured with dividers or with a graduated scale. A record sheet ruled on tracing cloth affords a convenient method of comparison. The best method of comparison is the one which best suits the observer.