Page:Weather Facts and Predictions.djvu/9



The barometer is subject to a diurnal variation, comprising two maxima and two minima. The maxima occur within an hour (before or after) of 10 a.m. and 10 p.m. The epochs of minima are, under the same conditions, 4 a.m. and 4 p.m.

The amount of these diurnal variations diminishes from the equator towards either pole. [For the reason that they depend on the influence of the sun's heat on the atmosphere, which is indeed the cause of the diurnal variation at any one place.]

There is an annual variation in the height of the barometer, but in England it is of inconsiderable amount.

One of the greatest known changes occurred on September 6th, 1865—a fall of 1.69 inches in 1h. 10m.

In the Northern hemisphere the barometer falls with S.E., S., and S.W. winds: with a S.W. wind it ceases to fall, and at W. begins to rise; it rises with a W., N.W., and N. wind, and with a N.E. wind it ceases to rise, and at E., or towards S.E., begins to fall.

When the wind is on the W. side of the compass changes of barometer accompany changes of weather, but with the wind on the