< Page:Popular Science Monthly Volume 9.djvu
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THE POPULAR SCIENCE MONTHLY.
"A storm must be treated as an individual which is subject to development. This is difficult, on account of the nature of the subject, but it is possible and essential. We must take the storm at its earliest appearance, and not lose sight of it for one moment until we know it throughout its whole extent, in all its parts, from beginning to end."

This view of Prof. Blasius coincides with that of Sir William Herschel, who says:

"In endeavoring to interpret the weather, we are in the position of a man who hears at intervals a few fragments of a long history related in a prosy, unmethodical manner. A host of circumstances omitted or forgotten, and the want of connection between the parts, prevent the hearer from obtaining possession of the entire story."

Definition of a Storm.—But leaving methods and passing to results, our author defines a storm in general to be "the movement of the air caused by its tendency to reëstablish an equilibrium which has been disturbed; and we may call all such movements storms, whether they are gentle breezes or furious hurricanes, whether accompanied by more or less condensation of moisture or clouds, or even by none at all," as in deserts.

Classification of Storms.—As the result of his investigations in aerial movements in the northern hemisphere, Prof. Blasius presents the following classification of all storms:

1.Local or Vertical Storms. Stationary. Centripetal.—Produced by a tendency of the atmosphere to reëstablish in a vertical direction an equilibrium that has been disturbed. Characteristic cloud—cumulus.
2. Progressive or Lateral Storms. Traveling.—Produced by a tendency of the atmosphere to reëstablish in a lateral direction an equilibrium that has been disturbed. They are of two kinds:
(a.) Equatorial or Northeast Storms. Winter storms.—Produced by a warm current displacing a cool one to supply a deficiency toward the poles. Temperature changing from cool to warm.—Direction to the northeastern quadrant. Characteristic cloud—stratus.
(b.) Polar or Southeast and Southwest Storms. Summer storms.—Produced by a cool current displacing a warm one to supply a deficiency toward the equator. Temperature changing from warm to cool. Direction to the southern semicircle. Characteristic cloud—cumulo-stratus.
3. Loco-Progressive or Diagonal Storms. Traveling locally. Rotary—tornadoes, hailstorms, sandstorms, water-spouts, etc.—Produced by a tendency of the atmosphere to reestablish the equilibrium of a polar storm which has been disturbed in the plane of meeting by a peculiar configuration of the ground.—Direction, the diagonal of the forces of the two opposing currents transversely through the polar storm.—Characteristic cloud—conus.

In order that the significance of the above classification may be clearly understood, it will be well to notice in brief outline the general movements of the atmosphere surrounding the globe, more especially those in the northern hemisphere.

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