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National Geographic : 1905 Jun
Contents
FORECASTING THE W tion, and yet extends upward but four or five miles. The whole disk of whirl ing air four or five miles thick and i, 500 miles in diameter is called a cyclone or cyclonic system. It is important that a proper conception of this fundamental idea be had, since the weather experi enced from day to day depends almost wholly on the movement of these trav eling eddies, cyclones, or areas of low pressure. That one may gain a clear under standing of the difference between the movements of the air in the cyclone and the movement of the cyclone itself, or its translation from place to place, let him picture in his mind the solar system, with all of its planets and their satellites, turning each upon its own axis and pursuing its orbit about the sun, and then imagine the sun also as rotating and as moving forward in space without change in the relation of the planets to the sun, or the satellites to the planets, and he will have less difficulty in com prehending the various phases of the translation of a cyclonic system and the sequence in which the force and the di rection of the wind changes; how the wind must blow into the front of the storm in a direction partly or wholly contrary to the movements of the storm itself and into the rear of the storm as it passes away ; how the wind increases in velocity as it gyrates spirally about the center and approaches nearer and nearer the region where it must ascend ; how centrifugal force, in causing the higher layers of air to move away from the center, tends to cause an accumu lation of air about the outer periphery of the storm, which in turn presses downward and impels the surface air inward. This whole complex system of motion moves forward the same as does the sun and his system. The black round disk indicates that the weather is cloudy at the moment of the observation, and the open disk clear sky. S. and R. stand for snow and EATHER AND STORMS 26I rain: The large figures in the four quarters of the cyclone show the aver age temperature of each quadrant. The greatest difference is between the south east and northwest sections. This is due in part to the fact that in the south east quadrant the air is drawn north ward from warmer latitudes, and in the northwest quadrant the air is drawn southward from colder latitudes. Chart II, constructed from observa tions taken :2 hours later, shows that the storm or cyclonic center, as indi cated by the word " low," has moved from central Iowa since 8 a. m. and is now, at 8 p. m., central over the south ern point of Lake Michigan. The shaded areas show that precipitation has occurred during the past 12 hours in nearly the entire region covered by the cyclone. Unfortunately for the science of forecasting, precipitation does not show that relation to the configuration of the isobars that temperature, wind velocity, and wind direction do. Note that none has fallen in the south ern portion of Ohio, in northwest Mis souri, and in West Virginia and eastern Kentucky, although they are near the storm center, while a fall has occurred in New England, quite remote from the center of barometric depression. These facts illustrate how a forecast of rain or snow may fail for a portion of a state or for a whole state, even though the storm pass over the state and the wind and temperature change precisely as pre dicted. However, all the places men tioned as failing to receive precipitation were showered upon during the further progress of the storm, except northwest Missouri, as will be seen by referring to chart III of the following morning. The cyclone has continued its course toward the northeast, and has brought the rain area eastward to include nearly the whole Atlantic coast region. The weather has cleared on the west side of the storm. Charts II and III contain red lines,
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