Devereux. — Direction of Motion of Cirrus Clouds. 19 



^y inspection of this table we find tlie following : — - 



(a.) Cirrus is most frequently observed in November, with, the least 

 frequency in August. 



(6.) The seasonal variation shows maxima in the spring and autumn. 

 In the former season the greater frequency is undoubtedly accounted for 

 by the known greater energy and magnitude of the antarctic disturbances 

 which transit the Dominion ; whilst in the autumn season the greater 

 frequency can be attributed to those monsoonal systems which affect all 

 that portion of the North Island lying between about longitudes 173° and 

 178° B. and north of the parallel of 38° S. 



(c.) Cirri are usually moving from some westerly point. From due 

 west, 23 per cent, of the total number observed ; from W.N.W., 14 per 

 cent. ; from N.W., 24 per cent. ; and from W.S.W., 11 per cent. : these 

 four points accounting for 72 per cent, of total observations. The mean 

 direction computed from all observations is W. 7° 16' S., or nearly west. 

 In Mr. Quayle's investigations of the movement of cirri at Melbourne the 

 mean direction of motion is shown to be a little north of west; but 

 his investigations cover a period of sixteen years, or four times the scope 

 of this paper, so that the longer period would naturally give a truer mean, 

 and would in a measure eliminate any variation due to cyclical move- 

 ments. Both results, however, demonstrate the west-to-east drift of the 

 upper currents in these latitudes. Melbourne and Waihi lie almost on 

 the same parallel of latitude. 



It is perhaps necessary to refer to the few observations of cirrus with 

 -an easterly component. During the passage of low pressures to the east- 

 ward of the Bay of Plenty, those which approach from the northward or 

 north-east, cirrus is frequently observed ; but owing to the prevalence of 

 lower clouds at these times the true direction of motion of the cirri is often 

 very difficult of determination : their direction is usually from the east- 

 ward. 



In the " Transactions of the New Zealand Institute," vol. 37, p. .563, 

 190.5, Captain Edwin refers to cirri-movements during the passage of 

 cyclones to the northward or eastward of East Cape, and the drift from the 

 eastward. 



General Remarks as to Cirrus Formations. 



Frequently the first appearance of cirrus is pecuJiar. A patch is 

 projected above the horizon a little south of west, in shape not unlike a 

 fan-shell, the radiations answering to the radial lines on the latter. The 

 formation is sometimes striated, and when this happens a westerly wave- 

 depression not infrequently follows, with wintry weather and electrical 

 disturbances. 



If a wave-depression is of greater extent than usual, detached cirrus 

 plumes are frequently observed travelling eastward, with heavy showers 

 of thunder type following, often accompanied by hail. They may be 

 described as miniature cyclones, or swirls, traversing the country^, integrals 

 of an extensive system. Their passage is marked by frequent rise and fall 

 of the barograph-trace, the amplitude being about j^^ in. 



Cirrus and cirro-stratus may often be observed low down on the W.S.W. 

 horizon, of a distinct " smoky " coloration, the ends being frequently 

 brushed back, as it were, and recurving on themselves. If cirrus main 



