Commodore Wilkes was encamped on the east side of the crater, and there (December and January) he experienced strong south-west winds, on at least three days having the force of a gale. These are the prevailing winds in this season over the group; whereas in August, the time of my sojourn, south-westerly winds are quite out of season, this being in the midst of the period of the N.E. trades.
It will be gathered from the foregoing remarks that the mere record of the winds is insufficient for the purpose of obtaining any definite notion of the air-currents at this elevation (13,600 feet). It is to close observation of the clouds that we must look for data of importance.
The Clouds.—The clouds on the summit of Mauna Loa were an unending source of interest to me, and I will give briefly the results of my observations. The highest clouds were wispy cirri, often arranged as in a mackerel sky, and evidently at a great altitude. They were only observed on four or five days. (The lower clouds are indicated in the accompanying diagram.) Below them and at no great height above the mountain were to be not infrequently observed isolated woolly clouds that were carried in a few minutes across the sky and had a brief existence, often forming and melting away as one gazed at them. Next, there was a heavy bank of cumulus, which formed on the south-west slope near the top of the mountain, from which lines of cloud extended along each flank. Lowest of all was a broad belt, or rather a sea, of cumulus that was developed on both sides of the mountain about one-third way down its slopes, and during the day-time isolated the peak from the world below. It is with the last two cloud formations that we are most concerned, and I will first describe the sea of cumulus.
The sea of cumulus, as in the case of similar cloud-formations of most other isolated mountains, when viewed from above, as from the mountain-top, presents a cloud-field of dazzling whiteness, sparkling in the sun. Seen from below, as from the coast, it has the dark lowering appearance of the rain-cloud and indicates the rain-belt. Disappearing during the night, this broad belt begins to form again between 8 and 9 a.m., and by 10 or 11 a.m. the lower regions are completely hidden and the mountain’s summit, cut off from the world, rises above the level of the sea of clouds like an island in an Arctic ocean. As the day progresses the clouds become more compact and dense. The usual altitude of this broad belt of cloud is between 7,000 and 8,000 feet. This level is indicated by the burying of the Kohala mountains, which rise to a height of 5,500 feet in the distant north-west corner of the island, and by the usual emergence of the highest summit of Hualalai, which rises, still nearer, to an elevation of 8,275 feet. On some days, however, it attains a height of nearly 9,000 feet. On such occasions the highest peak of Hualalai kept reappearing and disappearing during the day, but the distant summit of Haleakala in East Maui, 10,032 feet in elevation and 80 miles away, was always visible.
Words fail to describe the magnificent aspect of this sea of cloud which shuts off the spectator from the world below. From the summit of the mountain he gazes down on its surface lit up by a sun shining in a typically cloudless sky. At one time it appears as an undulating Arctic land covered with snow of dazzling whiteness. At another time it looks like a hummocky frozen Polar sea sparkling in the sunshine. Through occasional rifts, however, one can discern a dark dismal region of mist and rain-cloud beneath. Miss Bird, who passed a night on the summit in June, 1874, well describes this sea of cloud in her book on the Sandwich Islands as “all radiance above and drizzling fog below.”
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Diagram illustrating the prevailing cloud-formations of Mauna Loa during August, 1897.
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 produced 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.