Disturbances of the atmosphere are classified as follows: Cyclonic, or
low area storms, or anti-cyclonic, or high area storms.
[Illustration:
Fig. 31
]
The word “cyclone” to most people immediately means a terrific storm,
whereas in weather observing the cyclonic storm is not really a cyclone
or hurricane at all. It is a storm with an atmospheric pressure below
average. Particularly important is the wind that blows about this area,
which is always spirally inward, due to the rotation of the earth on its
axis. This is probably why it is given the name of cyclonic storm, for
it bears one of the important characteristics of a real cyclone. As the
wind is deflected and moves into the storm center, it turns to the right
and in the form of a whirlwind, spirally, moves around the storm center.
(See Fig 31.) It is this whirling process that has given it the name,
cyclonic storm.
As the air rises over the point of low storm area, or, in other words,
the area of low pressure, and travels into the atmosphere, it is not
permitted to rise to any great height, because it is always acted upon
by the force of gravity and is being pulled back to earth again. We
assume that because of this fact, this rising air which has been pulled
back to the earth again piles up in certain places, causing the
barometer to rise. Such a center as this is known as a high barometric
center or the anti-cyclonic area. Here the circulation of the air is
exactly opposite to that of the cyclonic area.
[Illustration:
Fig. 32
]
We are all more or less acquainted with these anti-cyclonic storms,
because in winter these great masses of air rise up from the warm areas,
pile up, and form high pressure areas over the mountains of Canada, and
soon this high pressure works down upon us as blizzards and cold waves.
We have described quite minutely the movement of the wind about these
points of high pressure and low pressure and have shown you the map and
have illustrated the high pressure and low pressure areas, but there is
still another feature that is of great importance to us, and that is the
movement of the storms and the fact that storms have a progressive
movement from west to east.
These storms move more rapidly in the United States than elsewhere, and
are more rapid in their movement in winter than in summer. Their speed
is almost one half again as great. The average velocity of the low area
storm in the United States is about twenty-five miles an hour in June,
July, August, and September, and from October on they continue to
increase.