THUNDERCLAPS. 



Bv KEGINALl) KY\'ES, l-.R.Mr.i .Soc. 



FlGl'RE 418. 



While the approach of a thunderstorm at night is 

 heralded from a great distance by lightning without 

 thunder, in the day time the growling of the distant 

 thunder is often heard before the liglitning is noticed, 

 e.xcept by those who are in places where they can 

 see for mai)\- miles in the direction of the approach- 



i n g storm. 

 As the storm 

 comes nearer, 

 the growling 

 develops into 

 a rumbling, 

 ) u t it may 

 still be diffi- 

 cult to dis- 

 tinguish one 

 ) I' a 1 fro m 

 another, es- 

 pecial 1\" if 

 there is a 

 great deal of 

 cloud in the 

 sky. As soon 

 as the peals are distinctly and separately heard 

 we may begin to measure the distances and, if the 

 storm approaches slowlv and is well-detined and 

 of small size, it may sometimes be possible to make 

 a fair guess at the size of the storm b)- noting the 

 distances of the furthest and of the nearest flashes. 

 These distances are measured, roughly, by counting 

 one mile to each four and a half seconds which 

 elapses between the flash and the thunder. The 

 first part of the peal is that due to the nearest part 

 of the flash. When a big and very active storm is 

 approaching the flashes are sometimes so frequent 

 that incessant growling in the far distance is followed 

 by incessant rumbling as the storm comes nearer, 

 and even when the storm is quite near it may be 

 impossible to distinguish one peal from another or 

 even to tell whether the crash which follows several 

 seconds after a flash of lightning is caused by that 

 flash or by some earlier and more distant one. 

 Storms of this character sometimes occur in the 

 British Isles, and they not infrequently accompany 

 severe cyclones in the tropics or mark the advance 

 of the monsoons. These great currents, however, 

 by no means always advance upon the countries 

 which they traverse in the fashion which English 

 writers love to describe : they often begin with 

 ordinary showers or ordinary thunderstorms succeed- 

 ing one another by longer or shorter intervals, and 

 accompanied by modeVate or even scanty rains. 



Any very big circular disturbance, advancing with 

 great masses of cloud and accompanied by smaller 

 and more violent whirls, or areas of c\xlonic type, 

 ma\' bring \iolent lightning and very hea\y rain over 



a fairl}- broad belt of country ; but very often the 

 intensity of the storm depends upon the state of the 

 atmosphere over the country to which it comes. 

 The local condition of the atmosphere, the physical 

 form or topography of the country and the small size 

 of each satellite or subsidiary whirl, carried along 

 with the main " depression " or c}clonic area, are 

 the causes of the great differences in the local inten- 

 sity of a storm, even when it is a big one. For 

 instance, the great Derby Day storm burst with 

 terrific force over Epsom Downs partly, no doubt, 

 because one of the most intense of the minor whirls 

 passed that way, parti}- because of the configuration 

 of the country just there, and parti}' as the result of 

 the electrical strain which already existed over the 

 Downs. This condition of the atmosphere was 

 shown for an hour or two before the storm came, by 

 the occasional falling of very large drops of water, 

 through the dry air, and from a sky only thinly 

 clouded over at a great height, .-^t Dorking there 

 was moderate rain, in the Holmwoods hardly any, 

 a little further south at Ockle}- the storm was very 

 fierce accompanied b}- severe hail. Again, the terrific 

 violence of the storm at W'esterham on the main 

 ridge of the country, may be compared with the 

 relativel}- mild experiences in neighbouring jilaces. 



/-^: 



Figure 419. 



Measuring the Distance. 



When a thunderstorm of an ordinary character has 

 approached within a mile or so, or when it is actually 

 passing overhead, it is usually possible to trace each 

 peal of thunder to its flash and, by counting seconds 

 between the flash and the first crack of thunder, to 

 measure, at the rate of four and a half seconds per 

 mile, the distance of the nearest part of the flash. 

 It is a mistake, however, to suppose that by the 

 loudness of the crash, and allowing for its distance, 

 we can judge of the size or force of the flash of 

 lightning. We are usually in error when we turn to 

 one another and say " That was a tremendous flash" 

 or " Something must have been struck." The 



377 



