Cyclone

a whirlwind on a vast scale. As now used by scientists, the term is to be distinguished with care. The "western cyclone," a violent, destructive whirlwind of the Mississippi Valley, should be called a tornado; the still larger, violent, destructive whirlwind of the West Indies and the Atlantic is a hurricane; the very similar whirlwind of the East Indies, the Philippines, and the Pacific is called a typhoon. In the sense that a cyclone is a whirling and revolving wind, it cannot be denied that all three of these winds, the tornado, the hurricane, and the typhoon, are cyclones; but it is convenient and now customary to reserve the term cyclone for vast circular movements of air, hundreds of miles in diameter, that are taking place all the time, and are matters of every day experience.

Confining our discussion to North America, we may say, to start with, that our ordinary breezes, calms, and winds are but parts of a cyclone, and that a cyclone is simply an ordinary wind. The prevailing movement of air in the north temperate zone is from the west. If the surface of the United States and southern Canada were a low, flat plain, with the lines of equal heat running uniformly and permanently along the parallels of latitude, we should have, so far as local conditions govern, monotonous westerly winds the year around. As a matter of fact, the air in our belt travels steadily in an easterly or northeasterly direction, but it does so in vast swirls that follow each other eastward at intervals of from three to seven days. They travel across the continent at a rate of from 500 to 1,000 miles a day. These swirls or cyclones account for the different directions of wind. As the air moves in a whirl it is evident that an east wind in one edge of a cyclone must be a west wind in the opposite edge of the same cyclone. If the wind in the east edge of a cyclone is from the south, the wind in the west edge of the same swirl must be from the north. If a locality be in the pathway of the center of a cyclone, it is evident that the wind for that locality must change in direction as the cyclones advances.

The writer recalls a personal observation of this sort at Fargo in the Red River Valley one midwinter. As the evening of a fine day drew on, a furious south wind sprang up and brought a heavy fall of damp snow. The flakes fell in rags, such as Villon describes as falling in Paris. The town went to sleep in a warm, blinding snowstorm. The next morning the air was calm, the sun was shining, and the world lay still and white. At eventide again, a high wind was raging, and the air was filled with fine icy crystals, this time from the north. It seemed as though the wet, clinging, flaky snow from the south, and the fine, icy snow from the north, with a calm between, were the eastern edge, the western edge, and the central portion of a cyclone moving toward the Atlantic coast. By getting reports from all parts of a cyclone, the United States Weather Bureau is able to forecast the weather within the area occupied by the cyclone. It is essential to understand that the direction of a cyclone as a whole is a different thing from the direction of a particular part of the cyclone. The four quarters of a wagon tire are moving upward and forward, downward and back-ward, but the wheel as a whole is moving forward. One edge of a swirl in a stream is moving downward, and the opposite edge is moving upward, but the swirl as a whole is going down stream.

If the reader has gained a notion of circular winds on a large scale traveling eastward, he is ready for the notion that the direction of a cyclone current is spiral, rather than circular. The origin of a cyclone is not altogether clear, but, however that may be, the center of the whirl is an area of low barometric pressure, an area in which the air is less dense and toward which the air is rushing to even up the pressure. This combination of a circular direction and a direction toward the center gives the cyclonic current more or less of a spiral movement toward the center. This increase of density is likely to be accompanied by rainfall or snowfall. The area in which this precipitation occurs is known as a cyclonic storm. The storm area is much smaller than the entire cyclone. If the observer will stand with his back to the wind--this for a wind or a breeze in the northern hemisphere--the center of the cyclone will be on the left hand. If he remember that the center is traveling eastward, he can locate the entire cyclone in a general way. Corresponding to cyclonic movements of air about centers of low pressure, we have also cyclonic movements around centers of great density--centers of high pressure. Under these conditions the currents wind spirally from the central area outward. Such an area of cyclonic movement is called an anti-cyclone. An area of high pressure, the density of which is undergoing decrease by reason of an anti-cyclonic movement, is an area of fine weather.

Put in another way, we may say that as the density of the air in an area increases, its capacity to hold moisture decreases. The central area of a cyclone is an area then of probable precipitation. On the other hand, the capacity of air to hold moisture increases with increasing rarity. The central area of an anti-cyclone, therefore, is likely to be an area of fine weather. As a falling barometer indicates the approach of a cyclonic center, it also indicates a probability of rain. In like manner, a rising barometer indicates the approach of an area of anti-cyclonic air, and probable fair weather. Our cyclones originate in the southwest and in the northwest, or else they come from the Pacific.