Tornadoes occur with greater frequency in the United States than in any other section of the globe. Tornadoes seldom occur in winter, except perhaps in the Southern States. They are more frequent in the month of May than at any other time during the year, although they occur sometimes in April, June, and July.
Between 1870 and 1890 about sixty-five destructive tornadoes occurred in the United States, involving great loss of life and property. When a tornado moves off the land on to the ocean it may become what is termed a waterspout. These probably never originate on the water, but after they have once formed may be carried over the water to a considerable distance. A tornado was never known to originate on the shores of Lake Michigan, but there are a few instances (the most notable one being the Racine tornado) when they have reached the lake after having traveled from some distant point inland.
The Racine tornado—so called because it destroyed a large portion of that city—happened fifteen or more years ago. The tornado originated about 100 miles southwest of Racine, Wis., in northern Illinois. The funnel-shaped cloud passed over the lake, but the tornado character of the storm was broken up before it reached the other shore.
When a tornado passes from land to water it becomes a waterspout only when the cloud-funnel hangs low enough and the gyratory energy is sufficiently great. There is a great pressure on the water outside of the funnel and almost a perfect vacuum inside. This latter fact contributes largely to the destructive power of the tornado. When a funnel is central over a building a sudden vacuum is created outside of it and it bursts outwardly from the internal air pressure.
CHAPTER XIII.
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.