abstracts: meteorology 243 



the scattering which takes place in the distribution of the several 

 groups of lows in latitude. Thus, the North Pacific group, which is 

 essentially a storm moving in high latitudes, has on the average for 

 Januarjr, Ijeginning with latitude 55°, along the 95th meridian and pro- 

 ceeding southward by five-degree steps, seven, six, seven, and four lows, 

 respectivelj^ The corresponding numbers for July are three, one, 

 four, and one. The senior author concludes the paper with a number 

 of precepts collected by him during his service as a forecaster. 



W. J. H. 



METEOROLOGY. — The thunderstorm and its phenomena. W. J. 

 Humphreys. Monthly Weather Review, 42:348-380. 1914; 

 Journal of the Franklin Institute, 178: 517-560, 751-776. 1914. 



The Simpson theory, that the great amount of electrical separation 

 in a thunderstorm is due to mechanical rupture of raindrops, the best 

 supported and most consistent theory of this phenomenon, is explained 

 in detail. From this theory it follows, as was already evident from 

 other considerations, that thunder and lightning are neither storm- 

 originating nor storm-controlling factors, but themselves the result of 

 those violent winds that obtain beneath and within large cumulo- 

 nimbus clouds. 



Hence those conditions essential to the production of cumulus clouds, 

 that is, abundant humidity and that temperature distribution which 

 induces strong vertical convection, are also essential to, and likewise 

 sufficient for, the genesis of the thunderstorm. This in turn explains 

 why, over land areas, these storms are more frequent in the afternoon 

 than at night, more frequent in summer than in winter, and more fre- 

 quent in equatorial regions than in higher latitudes. 



According to the distribution of barometric pressure and surface 

 temperatures, thunderstorms are divided into five distinct classes, and 

 each class illustrated by a group of three meteorological charts. One 

 of each group gives the typical conditions in question. The other two 

 show respectively the 12 hour antecedent and the 12 hour subsequent 

 conditions. 



The abrupt and marked changes in temperature, pressure and wind 

 velocity that accompany the onset of a thunderstorm are all discussed 

 in detail, as are also the downrush of cold air and the uprush of warm, 

 the genesis of hail, the formation of "thunder heads," roll scud, and 

 other details of the storm. 



The various forms of lightning, streak, rocket, ball, sheet, beaded. 



