534 



METEOROLOGY. 



a larger volume of new air enters the progress- 

 ing cyclone in front, and increases the amount 

 of precipitation. Espy showed many years 

 ago that, on account of mechanical heating by 

 compression, no descending air can he accom- 

 panied by precipitation ; and an explanation is 

 thus afforded why there are no, or but little, 

 cloud and precipitation in the rear of rapidly 

 moving cyclones. On the other hand, in order 

 that a cyclone may advance rapidly, there must 

 be a rapid decrease in pressure, and consequent- 

 ly a rapid removal of the air, in front of the 

 advancing depression. Since, according to the 

 normal circulation of a cyclone, there are an 

 inward movement near the earth's surface, 

 and an upward and outward movement near 

 the top, the upward and outward movement is 

 necessarily increased in unusually rapidly mov- 

 ing cyclones, and with it also the cloudiness 

 and precipitation are increased. Observations 

 at Blue Hill Observatory indicate that velocity 

 of storm movement, and especially variability 

 of weather, are intimately connected with the 

 velocity of movement of the general atmos- 

 sphere. Hence, the author concludes that the 

 main cause of rapid cyclone progression is 

 an unusually rapid drifting of the atmosphere 

 over large regions ; and the unequal distribu- 

 tion of rain around the cyclone is due to its 

 rapid progress. 



An attempt has been made, by Mr. E. Doug- 

 las Archibald, to find a basis of reconciliation 

 between Faye's theory of storms and the the- 

 ory to which it is thought to be opposed. M. 

 Faye's theory, to express it in brief, considers 

 that air-whirls around a vertical axis, includ- 

 ing cyclones, typhoons, tornadoes, and water- 

 spouts, originate in the upper currents of the 

 atmosphere, and are propagated downward by 

 a descending motion, accompanied by gyration 

 round a vertical axis. The opposite theory is 

 not, as M. Faye describes it, that the move- 

 ments rise from the ground in an ascending 

 current that borrows a gyration from that of 

 the earth itself; but, as developed by Ferrel 

 and Sprung, it makes the action begin in a 

 slight upward motion in unstable air, usually 

 near the lowest cloud-stratum, and possessing 

 a gentle gyratory motion relative to some 

 central point, " which is never wanting in a 

 cyclonic area." Once the motion is started, 

 and the air that feeds it is nearly or quite 

 saturated, the action will go on and be prop- 

 agated downward, not by a descent of the 

 air, but by the transference of the physical 

 conditions which favor the continuance and 

 maximum development of the ascending cur- 

 rent. The increasing rapidity of gyration of 

 the air as it approaches the axis, however gen- 

 tle it may be at starting, only allows it par- 

 tially to feed the initial and continuously re- 

 produced vacuum, which is thus compelled to 

 draw its supplies chiefly from the non-gyrating 

 air at the lower end of the aerial shaft. As 

 this is drawn upward, the centrally aspiring 

 surrounding air is made to gyrate more rapidly 



(partly by the friction of the superjacent rotat- 

 ing layer), and thus the gyratory and other 

 conditions are propagated downward until a 

 balance is struck between supply and demand. 



Thunder-Storms The Council of the Royal 

 Meteorological Society has appointed a com- 

 mittee to collect volunteer observations on 

 British hail- and thunder-storms. The objects 

 sought are: A knowledge of the nature and 

 causes of the different kinds of thunder-storms ; 

 a discovery of the localities where hail and 

 thunder are most frequent and destructive ; 

 and, if possible, to obtain an increased power 

 of forecasting hail and thunder, whereby it is 

 hoped that eventually damage to persons, 

 stock, and property may be lessened. 



As one of the most certain prognostics of 

 thunder, Mr. B. Woodd-Smith mentions the 

 formation of parallel streaks or bars, in cirrus 

 and cirro-stratus, and on the surface, apparent- 

 ly, of nimbus clouds. In cirrus they give often 

 almost the first intimation of coming change 

 after settled weather, and are most usually 

 followed within twenty-four or thirty-six hours 

 by thunder. When they appear on nimbus the 

 interval is much less, but they have not been 

 seen on the thunder-cloud itself. These small 

 patches of definitely marked " parallel bars " 

 should be distinguished from the more general 

 parallel arrangement which is often seen on a 

 much larger scale, but is not known to have 

 any very distinct value as a weather prog- 

 nostic. 



The typical course of the meteorological in- 

 struments at the advent of a thunder-storm 

 has been studied by Ferrari on the basis of the 

 records at Bern, Santis, and Rome. Previous 

 to the storm, the pressure and relative moist- 

 ure diminish, while the temperature increases, 

 so that, at the outbreak, the first two have 

 reached a minimum and the last a maximum. 

 At this moment pressure and moisture increase 

 very rapidly, and the temperature falls at a 

 corresponding rate, so that at the end of the 

 storm the first two elements have reached a 

 maximum and the last a minimum. The force 

 of the wind, which was slight before the storm, 

 augments very speedily when it begins, and is 

 quickly stilled after it. The minimum of press- 

 ure and relative moisture and the maximum 

 of temperature .are thus simultaneous with the 

 beginning of the storm, and the course of the 

 temperature is opposite to that of the two 

 other elements. The same course continues 

 in the after-storm, but is less pronounced. 

 The force of the accompanying wind and the 

 violence of the shower increase with the ve- 

 locity of advance. The storms seem to origi- 

 nate in a limited region, whence they spread 

 on one side. The ordinary form of the de- 

 pression is an ellipse, the major axis of which 

 is perpendicular to the axis of the shower. 

 The same is the case with the depression of 

 temperature that follows the storm. Storms 

 of short course, or local showers, are accom- 

 panied by light winds, extensive storms usu- 



