B. The Diurnal and Cyclonic Variation of Pressure in Different Seasons.—This problem is satisfactorily solved only by a study of the curves traced by the barograph, or by plotting, as a curve, hourly or half-hourly readings of the mercurial barometer. The diurnal variation of the barometer is the name given to a slight double oscillation of pressure, with two maxima and two minima occurring during the 24 hours. This oscillation is in some way, not yet understood, connected with the diurnal variation in temperature. It is most marked in the tropics and diminishes towards the poles. Fig. 15 illustrates, in the May curve, the diurnal variation of the barometer at
Cambridge, Mass., during a spell of fair spring weather, May 18-22, 1887. The maxima are marked by + and the minima by 0. The cyclonic variation of pressure is the name given to those irregular changes in pressure which are caused by the passage of cyclones and anticyclones. The second curve in Fig. 15 shows the cyclonic variations in pressure recorded by the barograph at Cambridge, Mass., during a spell of stormy weather, Feb. 23-28, 1887. These curves serve as good illustrations of these two kinds of pressure variations.
Study your barograph tracings, or your barometer readings, as illustrating diurnal or cyclonic variations of pressure. Note the character and the amount of the diurnal and cyclonic variations, and their dependence on seasons.
Over the greater part of the Torrid Zone the diurnal variation of the barometer is remarkably distinct and regular. Humboldt first called attention to the fact that in those latitudes the time of day may be told within about 15 minutes if the height of the barometer is known.
C. The Relation of Local Pressure Changes to Cyclones and Anticyclones, and thus to Weather Changes.—Make a detailed study of the relation of the local pressure changes at your station, as shown by the barograph curves, or by frequent readings of the mercurial or aneroid barometers, to the passage of cyclones and anticyclones, and to their accompanying weather changes. Classify the simple types of pressure change, so far as possible, together with the general weather conditions that usually accompany these types. Apply the knowledge of local weather changes thus gained when you make your forecast on the basis of the daily weather maps.
CHAPTER XXVI.
METEOROLOGICAL TABLES.
The tables which follow are those which are now in use by the United States Weather Bureau. They were first published in the Instructions for Voluntary Observers issued in 1892, and were reprinted in 1897. The following instructions will be found of service in the use of the tables:—