Page:The National Geographic Magazine Vol 16 1905.djvu/323
Forecasting the Weather and Storms 287
statement that storms of August and September may form southeast of the Windward Islands, cross the Caribbean Sea, recurve in the Gulf of Mexico or near the South Atlantic coast, and pass northeastward over the Atlantic Ocean and be lost in the interior of Europe or Asia. The history of these storms and of all others of the oceans is learned by collecting and charting the daily observations from thousands of moving ships in connection with the observations of island and coast stations.
THE TRANSLATION OF STORMS
The air expands upward to an altitude of 50 miles or more. It is so elastic and its expansion is so rapid as it recedes from the earth that nearly one-half of its mass lies below the three-mile level. Our storms and cold waves are simply great swirls in the lower stratum of probably not more than five miles in thickness, which more than likely are caused by the flowing together, on about the same level, of masses of air of widely different temperatures. An elaborate system of cloud observations, made during recent years, shows that the atmosphere, in the middle latitudes of both hemispheres, flows eastward over these agitations of the lower air without being disturbed by them.
In the temperate zones cyclones and anti-cyclones drift toward the east at the usual rate of 600 miles per day, or about 37 miles per hour in winter and 22 miles per hour in summer ; but there is no definite rule on which the forecaster can rely. Sometimes they move at twice this speed, and again at less than half of it, or, what is more embarrassing to the prophet, remain stationary for one or two days and die out. It is safest to assume that the velocity of translation of a storm will be the average of the two immediately preceding it, unless the distribution of air pressure over the continent is markedly different in the several cases. Cyclones
and anti-cyclones usually alternate, but not always. At rare intervals a rainstorm or a cold wave may be followed by an atmospheric action similar to itself, with only a narrow neutral area between. The most difficult weather map to interpret and make a forecast from is one that contains several partly developed cyclones and anti -cyclones, each of small area and little force. The most that can be said then is that the weather will be unsettled, no definite type of weather lasting more than a few hours.
Four -sevenths of all the storms of the United States come from the north plateau region of the Rocky Mountains and pass from this sub-arid region eastward over the Lakes and New England, producing but scant precipitation. The greater number of the remaining three- sevenths are first defined in the arid southwest states or territories. These nearly always can be relied on to cause bountiful precipitation as they move northeastward over the lower Mississippi Valley and thence to New England. Drouths in the great wheat and corn belts and elsewhere eastward are broken only by cyclones that form in Arizona, New Mexico, or Texas. Storms move faster in the northern part of the United States than they do in the southern portion, and their tracks migrate with the sun.
After the forecaster has spent many years in studying the courses of storms, he learns that, at times, through a gain in force that is not shown by observations taken at the bottom of the air, storms suddenly develop unexpected energy or pursue courses not anticipated in his forecast, or that the barometer rises at the center of the storm without premonition and dissipates the energy of the cyclone. Fortunately, such cases are exceptions.
Chart XII illustrates the courses of summer storms in the United States. The lines show the origin and the tracks