APPENDIX 585 



well describes this sea of cloud in her book on the Sandwich Islands as 

 " all radiance above and drizzling fog below." 



The heavy bank of cumulus, that forms at noon on the south-west slope 

 at an altitude of 10,000 to 13,000 feet above the sea, and sometimes rises 

 above the mountain, is one of the most conspicuous of the cloud-phenomena 

 on the summit of Mauna Loa. Apparently extending from it, but in 

 reality moving towards it, are two lines of small cumuli that follow the 

 same level along either flank above the sea of cumulus, as is indicated in 

 the accompanying diagram. It was observed by Wilkes in mid-winter, 

 1840-41, but at a lower level. "Clouds would approach us (he writes) 

 from the south-west when we had a strong north-east trade wind blowing, 

 coming up with their cumulus front reaching the height of about 8,000 

 feet, spreading horizontally and then disappearing." During my sojourn 

 this bank formed a very striking feature in the landscape during the early 

 afternoon. On two or three occasions when I visited the south side of the 

 summit and descended for about a thousand feet I passed through this 

 bank, being then exposed to a driving mist coming up the slopes from the 

 south-west. Though its upper surface viewed from a distance is dazzling 

 white, below it is dark and nimboid. 



It is to an updraught of warm moist air on the south or south-west 

 slopes of the mountain, and to the prevailing cool north-east trade that 

 strikes the north side of the summit, that we must look for the explanation 

 of the development and situation of this bank. Although the trade-wind is 

 markedly stronger than the south-west updraught, some of the warm, moist, 

 southerly air-currents find their way, as shown by the observations at my 

 camp, along the sides of the summit, and a line of condensation is pro- 

 duced where they come into contact with the cool air of the north-east 

 trade as it sweeps past the flanks of the mountain. Sometimes at my camp, 

 when there was a light southerly breeze blowing, I have noticed the line 

 of small cumuli moving south along the mountain side towards the bank of 

 cumulus. ... I may remark that on a few days a small bank of cumulus 

 formed under similar conditions on the north-west side of the summit. 



From my study of the clouds I arrived at the conclusion that there 

 were three prevailing air-currents on the summit of Mauna Loa : 



(i) The updraught of warm moist air on the south and south-west 

 slopes of the mountain. 



(2) The north-east trade wind, the upper limit of this air-current being 

 probably not far above the summit. 



(3) An upper air-current from the south-east (E.S.E. — S.S.E.), which, 

 from the velocity of the clouds it carried, was often probably not over 

 a couple of thousand feet above the summit. It may be observed that on 

 the coast at the base of the southern slope of the mountain in the middle 

 of September, when the wind was N.E. and carried the lower clouds with 

 it, the upper clouds were, on several occasions, noticed travelling in 

 the opposite direction, namely, from the south. 



