551 



ON THE FORMATION OF A MACKEREL 8KY. 

 By A. H. 8. Lucas, M.A., B.Sc. 



On Friday, April 20tli of this year, Sydney was favoured with 

 a remarkably extensive and regular displa}^ of alto-cumulus clouds, 

 forming a very complete example of that phenomenon which is 

 very rarel}" seen in these latitudes, a well-developed Mackerel 

 Sky. 



I am indebted to Mr. H. C. Russell, F.R.S., the Director of 

 the Sydney Observatory, for the following description of the 

 clouds, taken from his notes made at the time of the appearance. 



" In the forenoon a batch of well defined narrow stratus 10° wide 

 and extending from S.S.W. to N.W., appeared low down W.S.W., 

 and rose graduall}^, and at noon began to show signs of the cirro- 

 cumulus character. At 2 p.m. this was much more decided, and 

 by 3 p.m. the whole W. sky from horizon to zenith, and from N. 

 to S., was covered by the alto-cumulus of the Photograph No. 31. 

 At the time we w^ere in the middle of a spell of fine weather with 

 W. winds and clear skies. An anticyclone resting over Australia 

 with its centre in lat. 30'. Photograph No. 32 was taken 30 

 minutes after No. 31. At this time the clouds seemed to have 

 grown larger or neai-er. The clouds passed gradually over to the 

 E., and at night, with the moon })ehind them, looked if anything 

 finer than when the Photographs were taken. No immediate 

 change of weather took place ; it was fine and clear before and 

 after. The cloud band was not a feature of the anticyclone then 

 lying over Australia." 



The unusually close and regular banding of the clouds, the long- 

 parallel rows, rank behind rank, covering more than half of the 

 visible f-ky, naturally and strongly fixed the attention, and the 



