Forecasting then would seem to be very easy; for one would only have to
know the place of origin of the storm and the rate of travel, to
foretell exactly the time of arrival. Unfortunately these are only the
average paths; and as with most mean values, represent a value not often
experienced in fact. These paths then are not paths which any given
storm will follow. One must recall the story of the operating surgeon
who gave the average age of his patients in the operating room as 35.
There were but two patients, one 69 years old and the other 1 year old.
[Illustration: FIG. 9. ZEPHYROS--THE WEST WIND]
As a matter of fact the path of any individual depression depends upon
several factors, some of which are:--the prevailing eastward drift of
the air; the extent and motion of some anticyclone advancing before the
"LOW"; the duration and speed of relatively dry cold tongues of air from
the north; and the supply of water vapor brought from southern waters by
south winds. A depression can make little headway if to the north or
east the normal path is blocked by what is known as a stagnant "HIGH."
So therefore, if the anticyclone is a slow mover, a Texas storm, which
would normally pass not far from southern New England, may be deflected
farther north than when the HIGH moved rapidly east. So too, with the
storms which originate in the western part of the country. A slow moving
HIGH will prevent the LOW following it, from moving east at a normal
rate along the usual path.
Anticyclones then, are the real weather controls. There are various
types, but all drift from the north or west. Occasionally they enter the
country from the Pacific, but the great majority come from Alberta and
move leisurely southeast, often reaching the South Atlantic States; but
more frequently recurving and passing to the north.