326 



composite origin of agglutinated hailstones. Some examples 

 of this shape were fully 2 inches in length. 



(c) Irregular in outline: some of this type appeared to 

 have been formed by the coming together of ice fragments 

 that united at various angles, forming a surface of uneven 

 but rounded outlines. Fig 4, pi. xvi., may be taken as a 

 type of this kind ; a similar form is also included among the 

 figures in the text. 



During a hailstorm that occurred in Adelaide two months 

 later than the notable storm described in this paper, the 

 writer picked up a hailstone, half an inch in length, the 

 main body of which consisted of four symmetrical, lobulate 

 segments, arranged maltese-cross pattern, with two blunt 

 spines of uniform diameter throughout, and placed at the 

 opposite extremities of the hailstone (pi. xvi., fig. 8). This 

 occurrence is interesting, as showing a similar form to some 

 of those which fell in the storm of May 12. 



Thunderstorms. 



The very common association of hail with thunderstorms 

 suggests that there is a certain genetic relationship between 

 these two things. The conditions favourable for the develop- 

 ment of a thunderstorm are : a warm and humid layer of 

 atmosphere in the lower portion with a colder stratum of 

 air above. These are the conditions that give rise to the 

 cumulus cloud, and therefore it is this form of cloud that 

 commonly develops into a thundercloud. An essential feature 

 is the existence of a marked temperature gradient, which 

 gives rise to a violent upcurrent of air, which, with a gyratory 

 movement, carries the moist, warm air to high altitudes, 

 where the aqueous vapour undergoes rapid condensation ; the 

 effect of this uprush is seen in the turbulent and gyratory 

 movements of the storm cloud. The warm current rises in 

 front of the storm, and in proportion to the violent movement 

 of uprise there is a corresponding downrush of cool air which 

 underrides the warm in front. This explains the violent 

 gusts of wind which commonly accompany thunderstorms. 



Much light has been recently thrown on the electric 

 phenomena of thunderstorms by the observations and experi- 

 ments of Dr. G. C. Simpson, of the Indian Meteorological 

 Department, and these have been confirmed and elaborated 

 by W. J. Humphreys/ 1 ) Professor of Meteorological Physics, 

 Washington. Dr. Simpson's experiments show that the rain 

 which falls during a thunderstorm carries electric charges, 

 which may be either positive or negative, more frequently 



U)"The Thunderstorm and its Phenomena," Monthly Weather 

 Review, U.S. Dept. of Agriculture, vol. 42, No. 6, June. 1914. 



