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Transoctions. 



Art. II. — The Direction of Motion of Cirrus Clouds. 

 By H. B. Devereux, F.R.Mot.Soc. 



[Read before the Auckland Institute, 11th December, 1912.] 



Over four years of systematic observation of cirrus clouds at Waihi, and 

 their direction of motion, has shown not only their value as an adjunct 

 to amateur forecast work, but an invaluable key to the study of those 

 westerly waves of low pressure which are the predominant type of cyclonic 

 systems which transit the Dominion in rear of the anticyclones. 



By a strange coincidence, the writer was engaged on this paper when 

 the Australian Monthly Weather Report for December, 1910, came to hand, 

 containing therein a paper by Mr. E. T. Quayle, B.A., on the " Annual and 

 Seasonable Variation in the Direction of Motion of Cirrus Clouds over 

 Melbourne." In this paper the term " cirrus " means cirrus and cirro- 

 stratus. The daily observations of these high-level clouds have not been 

 confined to any particular hour, as the appearance has been generally found 

 to be coincident with time about or after the early morning or afternoon 

 diurnal barometric minimum. 



In these investigations the first seven months of 1912 are included, as 

 this year has been unusually favourable for cirrus observations, fifty-eight 

 being recorded, or over 20 per cent, of the total under review. 



The following table shows not only the principal points of the compass 

 from which cirri moved, but the seasonal variation. 



Table showing for each Month the Total Number of Days on which Cirrus Clouds were 

 observed moving from the Principal Compass-points during the 4:^^ Years 1st Januiry, 

 1908, to 31st July, 1912. 



In the above table no observations have been entered of cirri whose 

 direction of motion was not determinable. 



