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 observa- 

 tions of island and coast stations. 



THE TRANSLATION OF STORMS 



The air expands upward to an alti- 

 tude of 50 miles or more. It is so elas- 

 tic 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 dur- 

 ing recent years, shows that the atmos- 

 phere, in the middle latitudes of both 

 hemispheres, flows eastward over these 

 agitations of the lower air without be- 

 ing 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 fore- 

 caster can rely. Sometimes they move 

 at twice this speed, and again at less 

 than half of it, or, what is more embar- 

 rassing to the prophet, remain station- 

 ary 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 preced- 

 ing 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 rain- 

 storm or a cold wave may be followed 

 by an atmospheric action similar to it- 

 self, 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 pla- 

 teau region of the Rocky Mountains 

 and pass from this sub-arid region east- 

 ward 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 Missis- 

 sippi 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 noi thern part of the United 

 States than they do in the southern por- 

 tion, 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 observa- 

 tions taken at the bottom of the air, 

 storms suddenly develop unexpected 

 energy or pursue courses not antici- 

 pated in his forecast, or that the barom- 

 eter 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 



