Page:Weather Facts and Predictions.djvu/28

 [The directions of all others are influenced by the Earth's rotation.]

The backing of the wind when it extends beyond S. or E. indicates cyclones.

Shiftings of limited extent, as N.W. to S.W., or E.N.E., to N.N.E., are often only the return of the vane to its original position owing to the Equatorial or Polar current, as the case may be, regaining its supremacy.

When a calm succeeds a storm, the pressure is unusually low, consequently the foul air imprisoned in the mineral of coal pits escapes more readily into the air, accompanied by a buzzing sound, which miners regard as prognosticating a storm or heavy rain. Accordingly, it is when the barometer is low that explosions of fire-damp are most common.

Storms are invariably vorticose, that is to say, travel across a region in curved paths which do not return on themselves.

Squalls occurring during storms are thought to indicate the approaching cessation of the storm.

Just prior to a tempest the atmosphere is unusually still. [Because its great rarefaction (low-barometer) enfeebles its ability to transmit sounds.]

After a gale from S.E. or S.W. to N.W. a lull of a day or two may follow with symptoms of a continuance of bad weather, then a rapid backing of the wind through W. and S. to S.E. After this, another gale may spring up which may be even more violent than the former one.

If the wind howls, or veers about much, rain will follow.