Forecasting the Weather and Storms 263 



mer wanes, the first general rainstorms 

 usually occurring during the latter part 

 of September. This has given rise to 

 the erroneous idea of an 4 ' equinoxial 

 storm." 



HOT WAVES 



For some reason there come, in sum- 

 mer, periods of stagnation in the drift of 

 the highs and the lows. At such times, 

 if a high sluggishly rests over the south 

 Atlantic Ocean between Bermuda and 

 the coast of the United States and a 

 low over the northern Rocky Mountain 

 region, there will result what is popu- 

 larly known as a warm wave, for the 

 air, on account of its slightly greater 

 specific gravity, will slowly and steadily 

 flow from the southeast, where the press- 

 ure is greater, toward the northwest, 

 where the pressure is less, and, receiv- 

 ing constant accretions of heat from the 

 hot, radiating surface of the earth, with- 

 out any cyclones to mix the upper and 

 lower strata, will finally attain a tem- 

 perature almost unbearable to animal 

 life. This superheated condition of the 

 lower stratum in which we live continues 

 until the high over the ocean dies out 

 or drifts away to the east and the low- 

 pressure area in the northwest begins 

 to gyrate as a cyclone and moves east- 

 ward, mixing in its course strata of un- 

 equal temperatures and precipitating 

 the cool and welcome thunder showers. 



COLD WAVES 



Chart IV shows the beginning of a 

 cold wave in the northwest on the morn- 

 ing of January 7, 1886. Observe that 

 the heavy, black isobar passing through 

 Montana is marked 30.9, while the iso- 

 bar curving through southern Texas is 

 marked 29.8, a difference of 1.1 inch in 

 the air pressure between Montana and 

 Texas. The red isothermal line in Mon- 

 tana is marked 30 degrees below zero, 

 while the isotherm on the Texas coast 

 indicates a temperature of 50 degrees. 



The people of the Gulf states, with 



a morning temperature of 40 to 50 de- 

 grees, knew nothing of the great volume 

 of extremely cold air to the northwest 

 of them ; but from the distribution of 

 air pressure shown by chart IV the 

 forecaster anticipated that the cold air 

 of the northwestern states, on account 

 of its great weight, would be forced 

 southward to the Gulf and eastward to 

 the Atlantic Ocean, or, more accurately 

 speaking, that the conditions causing 

 the cold in the northwest would drift 

 southward and eastward. He therefore 

 issued the proper warning to the threat- 

 ened districts. 



Now turn to chart V of the following 

 morning, and it will be seen that the 

 cold wave has covered the entire Missis- 

 sippi Valley. The 10-degree isothermal 

 line has been forced southward almost 

 to Galveston, where the temperature the 

 preceding morning was 50 degrees. 



The low shown on the preceding chart 

 as being central in southern Texas has 

 moved northeastward to Alabama and 

 on chart V appears as a fully developed 

 storm. The difference in pressure be- 

 tween the central isobar of the low and 

 the central isobar of the high is now 1.4 

 inches. 



The low is lower and the high is 

 higher — conditions that argue ill for the 

 coast line toward which the low is mov- 

 ing. Next look at the arrows at the 

 coast stations from Key West, Florida, 

 to Eastport, Maine ; they are found to 

 have short bars at one end, which indi- 

 cate that every port, large and small, 

 between these two places is flying dan- 

 ger signals, and that every promontory 

 or island along this vast stretch of sea- 

 shore will exhibit the warning lights of 

 the Weather Bureau as soon as night 

 falls. 



Twenty-five years ago mariners de- 

 pended on their own weather lore to 

 warn them of coming storms ; then, al- 

 though the number of ships plying the 

 seas was much less than it is now, every 

 severe storm that swept across them left 



