1904.] on Some Asjjects of Modern Weather Forecasting. 



457 



In illustration of the application of these considerations to 

 practical meteorology, it was noted that rainfall is an indication of 

 the existence of rising air, and conversely the disappearance of cloud 

 may be an indication of descending air. It was further noted that 

 if the ascent and descent of air extended from or to the sm-face, the 

 actual paths of air along the surface, as traced from the direction and 

 speed of the winds, ought to show convergence in the case of rising 

 air and divergence in the case of descending air. 



The chart for April 16, 1903, was referred to for an obvious case 

 of dilatation or divergence of air from a centre corresponding with fine 

 weather, the centre of the area of divergence being specially marked 

 " no rain," and the actual trajectories or paths of air for two different 

 travelling storms were contrasted, to show how the rainfall might be 

 related to the convergence of the paths of air. The two occasions 

 selected were (1) the rapid travelling storm of March 24-25, 1902, 

 and (2) the slow travelling storm of November 11-13, 1901.* The 

 trajectories or actual paths of air for these two storms had been con- 

 structed from two hourly maps drawn for the pui-pose from a collection 

 of records of self-recording barographs, etc. Those for March 24-25 

 showed the paths to be looped curves with very little convergence, 

 whereas those for the storm of November 11-13 showed very great 

 convergence ; so much so that if four puffs of smoke could be 

 imagined starting at the same time from Aberdeen, Blacksod Point, 

 Brest and Yarmouth respectively, and travelling for 24 hours, they 

 would find themselves at the end of the time enclosing a very small 

 area in the neighbourhood of London. 



Corresponding to this difference of convergence as shown by the 

 paths, was the difference of rainfall as illustrated by two maps show- 

 ing the distribution of the rain deposited from the two storms. The 

 first, with little convergence, gave hardly anywhere more than half- 

 an-inch ; the second, with its great convergence, gave four inches of 

 rain in some parts of its area. 



[W. N. S.] 



* See Pilot Charts for the North Atlantic and Mediterranean, issued by the 

 Meteorological Office. February 1904. 



