Now I have told you what the local observers at each station watch and record and note in their reports sent to Washington. What you naturally desire now to know is how do the officials at the central office make their deductions as to the probable weather throughout the country. How do they know that a cold wave is advancing eastward, or that a severe storm is travelling up the coast, and that cautionary signals are to be set between Cape Henry and Passamaquoddy, or some other points? One of the principal ways in which the observers can tell the path of a storm is by watching the rainfall ahead of it. They have found that there is a sort of advance guard of rain, behind which is the lowest barometric area; and they regard that part of the country where the barometer is lowest as the centre of the storm. The reports from various stations show the path of the advancing rain, and the weather observers know that a low barometer is likely to follow it. They cannot tell exactly how fast it will advance, for areas of clear weather stand in the way of the storm, and local causes sometimes prevent them from yielding quickly.
COURSE OF CIRCULAR STORM SHOWING ITS TWO MOVEMENTS.
The chief reliance of the observers, however, is on a general acquaintance with the laws of storms. Years of observation and recording have proved that storms have ways of their own, and when you know where a storm has come from you can come very close to telling just where it is going. At any rate, it cannot get lost so long as it is in the United States, for the weather men are always on its track. The greatest originating place for storms is the equator, and, in our hemisphere, that part of it which is near the West Indies. Most of our cyclones, or revolving storms, originate there. These storms have two kinds of motions. In the first place, the storm-wind blows in a circle, like a gigantic whirlwind; and in the second place this whole thing advances over the land and sea, very much as a top, while spinning on its own centre, will move slowly along the floor. A cyclone starting down near the equator will begin by moving westward; then it curves around and goes northward, its diameter increasing and the velocity of its rotation decreasing, and finally it edges off over the New England States, and goes out to sea. (See diagram.) In the southern hemisphere these storms follow a similar track to the southward. In both hemispheres the storms advance at from two to forty miles per hour, and it is this movement which is uncertain and which requires close watching.
The storms which come from the far West are less understood. One theory is that they go around the world; and some of them have been traced all the way around, except in Asia, where there are no observers. These storms cross the United States in three ways. Sometimes they come in by way of Alaska, sometimes further down the Pacific coast, and again by Lower California. They usually lose some of their force when they reach the middle of the continent. From that point they are very likely to move to the Lake region, where they acquire a fresh supply of vapor and energy, and finally go off to the Atlantic by way of the St. Lawrence River. The observers keep posted as to their path by watching the premonitory rainfall and the succeeding low barometer.
Cold waves also have ways of their own, and the observers have learned them. The waves come in from three different points—northwest, west, and southwest. Those from the northwest often move directly east, and in that case the cold weather is not likely to extend south of the Ohio River. Sometimes, however, they move in a southeasterly direction, and then the whole country east of the Mississippi is affected. Those which come in from the southwest usually extend in a north-easterly direction. In these cases there are large decreases in temperature at Shreveport, St. Louis, and such places, before Chicago is affected.
Thus I have given you the outlines of the data from which the Weather Bureau predicts what kind of a day it will be to-morrow. The observers could tell more than they do now if they could only keep track of the storms when they are out on the ocean. But unfortunately there is no method by which stations can be maintained on the face of the great deep. The weather students are compelled to do the best they can with such information as they can obtain from ship captains, and this is not constant or systematic, and is therefore far from satisfactory. The value of the information which the service furnishes to the sailors is, on the other hand, very great. The steamers of the regular lines, of course, sail as they are advertised to do, without considering the weather; but they know what to expect, and can be prepared for it. Sailing-vessels, however, often avoid heavy weather and even danger at sea by heeding the warnings of the observers. You and I just take our umbrellas with us when the probabilities are rain, but the sailor stays in his harbor and lets the cyclone get well out to sea ahead of him before he sets sail.
"FARMER" DUNN'S HOME.