HARDWICKE'S SCIENCE-GOSSIP. 



227 



HINTS TO LOCAL METEOROLOGICAL 

 OBSERVERS. 



I. Hailstorms. 



AS hailstorms are essentially local phenomena, it 

 is to local observers that we must look for any 

 addition to our knowledge of them. 



A reference to any Encyclopaedia article will show 

 the sadly confused state of our information on the 

 subject. Flat contradictions will be found on every 

 page. It will be seen that while some authorities 

 assert that hailstorms occur most frequently in 

 summer, others maintain that winter is pre-eminently 

 the hail season. There must be an error some- 

 where. As a matter of fact, false statements have 

 been so persistently reiterated that by mere repetition 

 they have come to be regarded as truths. 



Let me allude to one fertile source of error. I have 

 referred to it before.* It is the confusion of hail 

 proper, or French grilc, and winter hail, or grcsil — 

 two entirely different phenomena, Gi'isil is the 

 small round powdery snow which often falls towards 

 the end of a snowstorm and in the early part of a 

 very frosty night. I suspect that Dalton and other 

 observers did not distinguish between the two kinds 

 of hail, and spoiled their results in consequence. 



The following points are of primary importance, 

 and should not be overlooked in taking down an 

 account of a hailstorm : — 



1. Period. — The date and hour at which a hail- 

 storm occurs, though apparently small matters, are 

 of the utmost importance. I am inclined to think 

 that while each country has distinct maximum and 

 minimum periods of hail-fall, the distribution of the 

 quantity over the year varies materially. For 

 example, in this country the maximum occurs in 

 summer, and is very strongly marked, while in 

 Germany the maximum is in spring and is not so 

 decided. If the periods of maxima and minima all 

 over the world could be determined, a comparison of 

 them would doubtless throw much light upon the 

 nature and cause of hailstones. 



2. Area. — When a hailstorm is not purely local it 

 usually assumes the form of the tornado and sweeps 

 over the country in one, or sometimes two, narrow 

 bands. In the former case it is sufficient to ascertain 

 the area covered by the storm, but in the latter it is 

 necessary to determine {a) the length, [b) breadth, 

 {c) direction of motion, and [d) rate of progression of 

 the storm-band. Such particulars can only be 

 obtained by the co-operation of numerous local 

 observers. Our organized observatories are quite 

 incompetent for such work, being too sparsely dis- 

 tributed over the country. 



3. Physical features of the locality .—'Y\i& proximity 

 of mountains seems to induce the fall of hail, while 



' Nature," vol. xxiv. pp. 1S7-90. 



that of forests has the opposite effect. Progressive 

 storms often diverge from their course on encounter- 

 ing a river or valley, and foUow that of the depression. 

 It is therefore useful to note if any of those physical 

 features be in the neighbourhood of the storm, and if 

 so, their effect upon it. It is also of importance, 

 especially in the tropics, to determine the elevation 

 of the country above the sea level. It is frequently 

 asserted, on what authority I know not, that hail 

 never falls in the tropical regions at a less elevation 

 than 2000 feet. It is worth while testing the state- 

 ment, 



4. Temperature. — The fluctuations of temperature 

 during a hailstorm are often very remarkable, and 

 should be carefully observed. A reading of the 

 thermometer may be taken shortly before the storm 

 begins and another directly on its cessation, 



5. Barometrical readings should be taken, if pos- 

 sible. 



6. Wind. — Its {a) direction near the earth's sur- 

 face, (b) direction in the higher regions as indicated 

 by the cloud motion, and {c) force, are important 

 points. Some observers have noticed that the 

 clouds move in various directions while a hailstorm 

 is in progress. Kiimtz actually went the length of 

 attributing the formation of hail to the conflict of 

 opposing winds ; and Beccaria says, " While clouds 

 are agitated with the most rapid motion, rain gene- 

 rally falls in the greatest plenty ; and if the agitation 

 be very great, it generally hails." Howard, in 1809, 

 noticed the wind change from E. to S., then to W., 

 back again to E., and finally to W., during a hail- 

 storm. 



7. Raiit.—'K^va. sometimes falls before hail, some- 

 times after it. The area of a hailstorm is generally 

 fringed with rain, and in the case of a moving storm, 

 rain falls along both edges of the track. Rain 

 before hail is somewhat rare, and its occurrence 

 should be carefully noted. 



8. Clozids.—YidJLl clouds are invariably cumulus. 

 Volta and other theorists have assumed that there 

 are always two strata of clouds at different elevations. 

 Arago pointed out that they are generally of an ashen 

 hue. Their aspect, apparent thickness, and height 

 above the earth may be noted. 



9. Electrical phenomena. — It is frequently stated 

 that thunder and lightning always accompany the 

 fall of hail, but such is by no means the case. When 

 there is lightning it is important to observe the rela- 

 tion between the discharges and the fall of the hail— 

 whether the lightning precedes the hail, or vice versa. 

 If possible, the electricity of the air before and after 

 the storm should be ascertained by means of an 

 electrometer. 



10. Duration. — The duration of the storm at one 

 spot may be noted, 



11. Preliminary sound.- — Kahn, Tessier, Peltier, 

 and others affirm that they have heard a peculiar 

 rumbling or pattering sound in the air immediately 



