A single unit of
Hailstones may be spheroidal, conical, or generally irregular in shape. The spheroidal stones often exhibit a layered internal structure, with of ice containing many air alternating with of relatively . These probably correspond to and and are called rime and glaze, respectively. The conical stones fall with their bases downward without much tumbling and are often smaller and not as layered. Irregular hailstones often have a lobate structure and are not composed of smaller hailstones frozen together. Hailstones grow by of s and sometimes also by of minor amounts of small ice . Large may contain liquid and be spongy (an intimate mixture of ice and ) in some regions; it is usually solid ice with greater than 0.8 g cm–3. Small may be indistinguishable from large ( ) except for the convention that must be larger than 5 mm in diameter. The of can be much less than 0.8 g cm–3 if they are dry; if partly melted, such hailstones become spongy. The largest recorded hailstone by weight to fall in the United States occurred in a in Vivian, South Dakota, on 23 July 2010. The stone from this event weighed 870 g, with a diameter in excess of 20 cm.