Concurrent ice nucleus counts made twice daily at Mauna Loa Observatory (3.4 km.) and near sea level at Hilo, Hawaii, from December 1, 1961, to February 5, 1962, and at intervals of a few minutes during 24 hours on July 10–11, 1963, indicated the wide fluctuations observed elsewhere, with counts varying not only from day to day, but by as much as an order of magnitude within minutes.
Background levels at Mauna Loa Observatory ran well below those at Hilo and averaged under 10 ice crystals in 10 liters at −24°C., values comparable with the lowest reported from other isolated localities. At both sites diurnal variations, evidently reflecting the diurnal mountain circulation and its effects, saw afternoon counts at Mauna Loa Observatory tending to increase with the influx of more turbid air from lower elevations and those at Hilo to decrease from dispersion of the accumulated contaminants by convection and onshore winds.
Although the investigation was not designed to look into fundamental questions, the relatively small distance between the two stations, their large difference in elevation, and the Observatory's isolation for much of the day from the underlying atmosphere permitted inferences concerning such matters as the source and vertical distribution of the ice nuclei and the possibility of extraterrestrial (meteoritic) influences. It would appear that the ice nuclei observed at Hilo did not ordinarily or in major part come from aloft, but rather from the lower atmosphere, and that—in general, and in the presence of the trade inversion—the number of ice nuclei decreased with height in the Hawaiian area.
Major oscillations in the four ice nucleus count-sequences (Mauna Loa Observatory and Hilo, morning and afternoon) appeared to be closely synchronous and to accompany the air mass changes or other synoptic events implied by the deepening and thinning of the moist layer on the Hilo sounding.
*Present address Weather Bureau Research Station, Las Vegas, Nev.