The rate at which the wind blows varies in different parts of the whirl, but is never very great. In the northern hemisphere the rotary movement is in the direction opposite to that in which the hands of a watch move, while in the southern hemisphere it is with the hands of a watch. AsTHE EFFECTS OF A TORNADO.
The iron windmill was blown across the cellar and protected the people who had fled there for safety.
these are areas of ascending air, they are storm areas. The extent of the precipitation varies in different parts of an area according to the direction from which the ascending air has come. Note the direction of the wind and the rainfall area as shown on the map (section 85).
Air which comes from the continental interiors is dry, while that from the oceans contains much moisture, some of which it deposits when made to ascend. These whirls move in a general eastward direction with varying velocities, but averaging about 20 or 30 miles per hour. To these is due the larger part of the rain which falls inREMAINS OF FARM BUILDINGS DESTROYED BY A TORNADO.
middle latitudes. Areas of high pressure in which the air moves spirally downward and outward from the center as has already been stated (section 74) are called anti-cyclones. These are areas of dry, cold weather.
84.Paths of Cyclonic Storms across the United States. -Themap on secton 85 shows the paths of a large number of cyclonic storms across the United States. It will be seen from this that although these paths vary considerably, yet the general direction is a little north of east. The movement of the cyclone is in the direction of the prevailing winds of the middle latitudes.
In summer time the average rate of motion of the cyclone across the continent is about 500 miles per day, while in winter it is 800. The velocity of the wind in the cyclone is also much less in summer than in winter, as the difference in pressure between the low and high areas is much less. The changes in temperature as the storms pass are greater in winter than in summer since the regions from which the northerly and southerly winds flow inWATERSPOUT SEEN OFF THE COAST OF NEW ENGLAND.
toward the center of low pressure vary more in their temperatures.
85.Sudden Weather Changes. -In middle lati- tudes there often occur, particularly in winter, sudden changes in the temperature of 20° or more in a few hours. In our own country, if the temperature falls 20° or more in 24 hours, reaching a point lower than 32° F. in the north or lower than 40° in the south it is known technically as a cold wave, and there is a special flagFig. 82.
(Fig. 82) displayed by the Weather Bureau to indicatethe approach of such a change.
When these waves extend over the southern part of the country, they are very destructive to the orange groves and delicate crops and are known as "freezes." A notable freeze of this kind occurred in 1886 and did tremendous damage to the orange groves of Florida. So great was the effect upon this important industry throughout the orange belt that for years afterward the "freeze" was the date from which events were reckoned.
If the northwesterly wind which brings on the cold wave is accompanied by snow, it is called a blizzard, and on the plains and prairies, where the wind has a clear sweep, it is much dreaded. Cattle and men, when caught in it, frequently perish. In southern Europe the coldest winds are from the Siberian plains and are therefore northeasters. In theCYCLONES AND ANTI-CYCLONES.
United States the cold area is at the southwest and rear of the cyclone, whereas in Europe it is at the north and front.
When, instead of the strong, cold northwest winds which blow into the rear of a cyclonic area and in the colder seasons may produce a cold wave, there is a prolonged movement of highly heated air from the south into the front of the low pressure, as sometimes occurs during the warm months, the "hot spells of summer" are caused. The air issultry, exceedingly hot and oppressive. Sunstrokes and prostrations from heat are common. The "hot winds" of Texas and Kansas, the Santa Ana of lower California and the siroccos of southern Italy are intensified examples of these winds. All sudden weather changes of this kind are due to atmospheric conditions related to areas of low pressure.
86.Weather Forecasting. -The data necessary for forecastingthe weather are telegraphed to the Weather Bureau Stations every day, and a record of them placed on the weather map. The observations re- corded on these maps furnish the forecasters with all the information obtainable as to what the weather of the future is to be. It has already been stated that the dominant cause of our weather conditions is the eastward movement of cyclones and anti-cyclones.
If the direction and rate of motion of these can be determined the weather of those places which are likely to come under their influence can be foretold with a good deal of accuracy. If a cyclone were central over the lower Mississippi valley with an anti-cyclone to the west of it, we should expect that the southerly and southeasterly winds and rains to the east and southeast of the Mississippi would gradually change to fair weather and westerly winds with increasing cold, as the cyclonic area was replaced by the anti-cyclonic.
The rate at which the change would take place would depend upon the rapidity of the movements of the two areas of high and low pressure, and the order of change in the direction of the winds would depend, for any place, upon the directions taken by the centers of these areas. The direction of movement and the rapidity of movement of the cyclonic areas are, therefore, two of the chief factors which enter into the prediction of the weather. There is usually an increase in the intensity of the storm as the Atlantic coast is approached.