4 o’clock, morning and afternoon. These oscillations in pressure are probably due to the waves of temperature which ceaselessly follow the sun with the rotation of the earth.
The semi-diurnal maxima and minima are greatest in equatorial latitudes; they decrease in higher atitudes. According to the observations of General Greely, the oscillations of the barograph pen were scarcely noticeable in polar regions. The amplitude of oscillation is greater by day than by night; it is greater at the equinoxes than at the solstices. The day amplitude is greater over the continents than over the sea; the night amplitude is the reverse. According to Humphreys, the whole atmospheric shell vibrates in waves which happen to be in 12-hour wave lengths. Records made by P. R. Jameson on the East African Coast near the equator show a maximum of about 0.025 inch above and the same minimum below normal pressure.[1] In tropical regions the irregular variations in pressure are infrequent; the semi-diurnal oscillations, on the other hand, are very regular. The claim that an observer can tell the time of day by the barometric pressure is not without foundation.
Other Variations in Pressure.—Pressure ranges exceeding 1.5 inches during a week—the record of a barograph sheet—are not uncommon. Weekly and monthly ranges are usually much greater in winter than in summer and much greater in mid-latitudes than in low or high latitudes. Professor Mohn has summarized as follows:[2] The barometer is high when the air is cold, when it is dry, and when an upper current flows into a given area. It is low when the lower air is heated, when it is damp, and when it has an upward movement.
The variations in pressure with which weather science is chiefly concerned are the daily highs and lows which cross the continents in mid-latitudes from west to east and, for the greater part, are lost in mid-ocean. These great billows of the atmosphere are comparable to the billows of the sea; but, as is shown in Chapter XIII, the lows are usually storm centers and the highs are rapidly moving masses of cold air. The former are the cyclones of the forecaster; the latter, the anticyclones.