Sept., 1905.] 



KNOWLEDGE & SCIENTIFIC NJEWS. 



--^ 



expected to fall, in order that they may be able to pro- 

 vide adequate means for the storm-water being carried 

 away without causing floods. For this purpose self- 

 recording rain gauges are of great value. These might 

 with advantage be also used by other observers with 

 whom " money is no object." 



Fig. 3 is a copy of the trace by the self-recording 

 rain gauge at the Fernley Observatory, Southport, 

 which shows the heavy rainfall which occurred on Sep- 

 tember loth, 1903, during the meeting of the British 

 Association in that town. 



Thunderstorm rains are often very heavy, but are 

 mostly of a local character; they are also occasionaJIy 

 accompanied by hail. The hailstones usually take the 

 form of little pellets or balls, and consist of compacted 

 ice and snow. During the exceptionally violent 

 thunderstorms which occurred at Harrogate and at 

 Richmond, in Yorkshire, on July Sth, 1893, hailstones 



and March loth, 1855, some of which are reproduced 

 in Fig. 4. 



Snow is much less dense than rain. A foot of snow 

 is, rouglily, equal to an inch of rain. Snow, however, 

 varies greatly in density; with very dense snow, seven 

 inches may equal one inch of rain, while with very 

 light snow as much as sixteen inches may equal only 

 one inch of rain. 



A " rainy day " in this countrv is that fin which a 

 hundredth of an inch (.01 in.) of rain has been 

 measured. The average number of rainy days in the 

 year at the Royal Observatory, Greenwich, is 157; 

 these are distributed over the months as follows : — 



January 

 February 

 March ... 

 April ... 

 May ... 

 June ... 



July ... 



August 



September 



October 



November 



December 



F'g- 3- — Record of Heavy Rainfall at Southport, September 10-11, 1903. 



from 2 to 3 inches in diameter fell, and caused great 

 destruction of property. These hailstones had several 

 alternate coatings of opaque and clear ice. These 

 coatings were no' doubt due to the re\olutio'ns accom- 

 plished bv the hailstones, which were probably several 

 times drawn in towards the vortex of the storm. 



Pig. 4.— Snow Crystals, by QIaisher 



When the aqueous vapour in the air is condensed at 

 a temperature below the freezing point it freezes and 

 falls in the crystalline form of snow. Snow crystals 

 are six-pointed stars, and are of great variety. The 

 late Mr. J. Glaisher, F.R.S., observed nearly 200 differ 

 ent varieties of snow crystals between February Sth 



From a long continued series of rainfall records it 

 is readily seen that there is a considerable variation in 

 the annual amounts. The London records of rainfall 

 show that from 1730 to 1750 there was a succession of 

 dry years, and most of the readers of " Knowledge " 

 will remember that there was also a succession of dry 

 years (with three exceptions) from 1883 to 1902. The 

 periods of successive wet years have been somewhat 

 shorter than those of dry years. 



With regard to the limits of fluctuation in the total 

 rainfall, Mr. Symons arrived at the following con- 

 clusions : — I. The wettest year will have a rainfall 

 nearly half as much again as the average. 2. The 

 driest year will have one-third less than the average. 

 3. The driest two consecutive years will each have one- 

 quarter less than the average. 4. The driest three 

 consecutive years will each have one-fifth less than the 

 average. 



These conclusions are of the greatest importance to 

 engineers when considering the question of water sup- 

 ply, for if provision is not made for " the driest three 

 consecutive years," the result will most likely be a 

 "water famine." There are many interesting sub- 

 jects connected with rainfall, such as the influence of 

 sunspots, periodicity, cycles of rainfall, &c., but these 

 have not been discussed, as they are outside the scope 

 of the present paper. 



