Hailstones in Hawaii

Hailstones are a rare sight in Hawaii due to the high temperatures (averaging 22°C near the coast in February) and a steady trade wind layer shallower than 3 km. However, in winter, midlatitude fronts hit the islands and cumulonimbus associated with them often produce snow at the tops of Mauna Kea and Mauna Loa (4202 km and 4172 km in height, respectively). The combination of well-developed cumulonimbus and low temperatures at the surface sometimes leads to the observation of hailstones at ground level even in Hawaii. 

The beauty of these hailstones were observed and cataloged extensively on 30 January 1985 over a large area of the eastern side of the island of Hawaii. Since cloud-nucleus concentrations are low in this area, the cloud-drop growth rate is high in Hawaiian clouds (Takahashi, 1981). Raindrops and snow crystals probably coexist in the Hawaiian cumulonimbus. The hailstone structure may therefore be different from continental hailstones where snow crystals and small droplets dominate in the cloud. Graupel are usually found to be the "embryos" of hailstones over the central continents (List,1958; Knight andKnight,1970). Frequently, however, hailstones are observed to have a circular clear center, probably a frozen drop. Hailstone shapes can be spherical, ellipsoidal, oblate, or conical. A lobed structure is sometimes observed in large hailstones (Browning, 1966). Although they are extremely rare, Briggs (1968) observed spiked hailstones at Oak Ridge, Tennessee. This storm in 1985 brought a variety of fairly unique hailstones that are worth exploring.

On 30 January 1985, a cold front hit the Hawaiian island chain. This front contained tall cumulonimbus with a cloud-top temperature: around  -70°C, and hailstones fell over many areas of eastern Hawaii. Thunder and lightning accompanied the hail-stones. Temperatures in Hilo dropped to 18°C. Since it is very rare to observe hailstones in Hawaii, people ran outside when they heard hail fall on their roofs. Many people collected hailstones and kept them in their freezers. In response to requests through the local newspaper, many people offered their hailstones for study. These hailstones were carefully kept in a jar chilled with dry ice and stored in the Cloud Physics Observatory freezer. Over the next two weeks, the collected hailstones were analyzed. Researchers cut the hailstones with a small saw and then made them thinner with a small heated iron. The structures of hailstones were examined both with ordinary transmitted light and with polarized light. 

This process revealed that hailstone shapes differed greatly with location. At both the southern and northern parts of the island, hailstones were spherical. The Kapapala hailstones had two “arms,” while Kalopa's hailstones were conical. Because they fell at higher elevations, hailstones at Ocean View, Kapapala, Kalopa, and Puuhue kept their original shapes with a minimum of melting. 

At Ocean View (840-m elevation), spherical hailstones with an average diameter of 14 mm were collected. Most of the ice had a milky appearance except in the center portions where rather-transparent ice was seen. At the center, bubbles were often observed and the diameter of the central clear ice was typically 3.3 mm. Several transparent and clear rings were seen near the center and then opaque ice developed in the rest of the hailstone. It is interesting to note that the structural centers deviated from the geometric centers of the hailstones. Single crystals dominate the central area and crystals elongate outward, showing graupel-like growth structure. 

At 7:45 A.M. hailstones fell for about five minutes at Kapapala. Two kinds of hailstones were collected there. One type was spherical (averaging 12 mm in diameter) and the other was a type of hailstone with spikes (27mm x 14mm on the average). In both cases, the hailstones were clear and air bubbles were often observed at the center. In the spherical hailstone a central region existed that contained numerous air bubbles. "Cross- polarized" pictures show polycrystalline ice. Crystals were large and extended outward from the hailstone center. The spiked hailstone was most interesting, with one or two spikes developed especially well. At the center of these hailstones, circles were always observed. Bubble-rich narrow rings were seen surrounding this center crystal. Air bubbles lined up along the centers of the ice lobes and a wavy-icicle structure was also seen. Surprisingly, the ice was single crystal. 

Between 4:00 A.M. and 4:15 A.M., 7-mm conical hailstones fell in a rancher's yard in Kalopa. It was interesting to observe air bubbles at the center of a clear circular area of 1.8 mm (average diameter). From the clear center, ice extended outward and ice grew asymmetrically when the ice sphere grew to 5 mm. The conical appearance was thus formed. The center was mostly single-crystal ice and small ice crystals grew at the periphery. 

Meanwhile, at Puuhe, spherical hailstones averaging 18 mm in diameter fell. The hailstones were transparent and most had air bubbles at the centers. Clear rings alternated with transparent rings. Hailstones were of polycrystalline ice.Both elongated and small ice crystals grew alternately outward from the center. The hailstone centers coincided with the structural centers. 

After these detailed observations of hailstones from different locations, several unique features became clear. First, Hawaiian hailstones always had a clear ice center. Frozen drops may be the origins because of the air bubbles at the centers of most hailstones. However it is difficult to determine the original embryo size by appearance only, because in the wet-growth regime, ice can grow continuously without changing the appearance of the frozen drops. However in a dry-growth regime, ice may show a change in structure at the frozen raindrop boundary. If Kalopa hailstones fit within this case, the original raindrop size would average 1.8 mm in diameter. 

Another interesting characteristic of the Hawaiian hailstones was the elongated type of hailstone collected at Kapapala. Unlike the Oak Ridge hailstones (Knight and Knight,1970), here only one or two spikes developed a lobed appearance. Lobed structures were seen even when hailstones were as small as 2 cm, unlike lobed hailstones observed only at larger sizes (Browning, 1966). The alignment of air bubbles in the spikes suggested that the growth pattern was similar to that of icicles. Single-crystal ice in hail-stones suggested the existence of a high liquid-water content in the cloud and subsequent growth of ice in warm air. With relatively large cloud drops and warm air, protrusions of ice were formed in the wet growth regime (Levi and Aufdermaur, 1970). A tentative explanation of large spike formation is that the water collected at the front and then flowed along the hailstone surface. The remaining water, after much shedding, froze slowly at the rear of the ice body. Turbulence behind the main hailstone body might assist the cooling of the spike surface so that the water could freeze. When a stable axis was found, a long icicle tail then formed. The upside-down rotation by 180° to another stable axis could then grow another icicle. 

Spherical hailstones with a polycrystalline structure collected at the same location may have grown in a rather-low liquid-water content in the cloud. The clear ice appearance and large crystal size indicated that the environmental temperature may have been around -5°C. Hailstones were spherical in both Ocean View and Puuhue. However, the Ocean View hail was asymmetric and opaque while the Puuhue hail was symmetric and transparent in structure. The opaque appearance may be due to the accretion of small cloud drops while the transparent appearance may be caused by accretion of large drops. Graupel-like growth with a wide apex angle is caused by the oscillation of hail. 

When a cloud's supercooled-drop size is large, hail may tumble and a symmetric growth pattern from the center, as in the Puuhue hail, may be the result. In this case, large drops of water that hit the hailstone must have momentum large enough to tumble the hailstone.

The hailstone, a thin section of the hailstone, and the thin section through a polariscope

These observations of only one storm from 1985 show the endless varieties of stones and the amazing clues to environmental interactions that can be found in hail. We look forward to your own observations and hail stories!

This article has been adapted specifically for the AMS Weather Band from a longer article by Tsutomu Takahashi. Any errors or omissions may be attributed to AMS staff. Copyright remains with the AMS