WEATHER PREDICTIONS.
To predict with any great accuracy what the weather will be from day to
day is a somewhat complicated problem, and, as all of us have reason to
know, weather predictions made by those who have the matter in charge
and are supposed to know all about it often fail to come to pass. The
real trouble is that they do not know all about it. There are so many
conditions existing that are outside of the range of barometers,
thermometers, anemometers, and telegraphs that no one can tell just when
some of these unknown factors will step in to spoil our predictions.
In very many cases, perhaps in a large majority of them, the predictions
made by the weather bureau substantially come to pass. It has been
stated in former chapters that the changes of weather accompany the
movements of what are called cyclones and anti-cyclones, the cyclone
being accompanied by low barometric pressure and the anti-cyclone by a
higher one. The winds of the cyclone move spirally around the center of
lowest depression with an upward trend, the motions being in a direction
reversed to that of the hands of a clock. In the centers of high
pressure the current is downward instead of upward and the direction of
the wind around it is opposite to that around the low-pressure area. The
fundamental factor in predicting the weather is the direction of
movement of these areas of low pressure. In almost all cases the
direction of movement is from the west to the east, but not always in a
straight line. These movements, however, are classified so that after
the direction has become established one can predict with considerable
accuracy as to whether it will move in a curved or a straight line. By
movement we do not refer to the direction of the wind at any particular
point, but the onward movement of the whole cyclonic system, which is
usually from twenty-five to thirty miles an hour, but in some cases the
speed is much greater.
Not only does the upward movement of the whole system vary, but the
velocity of the wind around any given cyclonic center varies. There are
about eleven classes of cyclones that appear in the United States, each
class having its own path of movement and origin. A large number of
these appear to originate north of the Dakotas, and move directly east
to the Gulf of St. Lawrence. Three other classes originate on about the
same line, a little west,--say, north of Montana,--moving first in a
southeasterly direction, passing over the center of Lake Michigan and
bending northerly through Lake Ontario and finally landing in the Gulf
of St. Lawrence. Two other classes start at the same point, one of them
going as far south as Cincinnati, and the other as far south as
Montgomery, Ala., and both turning at these points northeasterly to the
Gulf of St. Lawrence. Two other classes originate in Colorado, one
moving in a northeasterly direction slightly curved, and the other
directly east. Still others have their origin farther south in the Gulf
of Mexico, and move in a northeasterly direction. Very rarely they
originate in the Atlantic east of Savannah, moving first in a
northwesterly direction, but finally bending to the northeast.
Every day there is a weather map made up showing the locations of the
high and low barometers, direction of wind, lines of equal pressure, as
well as those of temperature. By study from year to year all of these
phenomena have become systematized, so that by tracing an area of low
barometer from its origin in its progress easterly it is soon seen to
fall under one of these classes and we are able to predict about what
its course will be. Knowing the speed of its movement as well as the
velocity of wind and all the conditions attending it, taken in
connection with the weather conditions in the region for which the
prediction is made, an expert can ordinarily forecast with some degree
of accuracy. After all that can be said, however, weather predictions
based upon maps are and have been far from satisfactory. One who has
been a close student of local conditions for a number of years will
often predict with as great accuracy as the weather bureau. Areas of low
pressure are followed sooner or later by a fall of temperature; this is
especially true in the winter months. Sometimes this fall is very
marked, and then it is called a cold wave. These sudden changes of
temperature are not thoroughly understood, but are supposed to be due
partly at least to rapid radiation of heat into the upper regions, as
the clear atmosphere which usually attends areas of high pressure is
favorable to such a condition. Undoubtedly, too, there are dynamic
causes, forcing the colder air from the upper regions to the earth, when
it immediately flows off toward an area of low barometer.
Long-time predictions are purely guesses. They sometimes guess on the
right side, and this gives them courage to make another. It is an old
saying that "all signs fail in dry weather." In time of a drought it is
true that the indications which at ordinary times would be surely
followed by a rain are of no value. When a season is once established,
either as a rainy season or a dry season, it is likely to persist in
this character until a change comes that is produced by the movement of
the sun in its course northerly and southerly, and the change produced
from this cause requires several weeks of time.
If accurate weather predictions could be made for a long time in
advance, or for even a week, they would be of incalculable value. But it
is doubtful if ever this will be brought about, as there are too many
necessarily hidden factors which enter into the calculations. If
stations could be established all over the oceans with sufficient
frequency, and an equal number at a sufficient altitude in the air, I
have no doubt that much that is now mysterious might be made plain.