From AMS Glossary
(term edited 6 Oct 2015)
A single unit of hail, ranging in size from that of a pea to, on rare occasions, exceeding that of a grapefruit (i.e., from 5 mm to more than 15 cm in diameter).
Hailstones may be spheroidal, conical, or generally irregular in shape. The spheroidal stones often exhibit a layered internal structure, with layers of ice containing many air bubbles alternating with layers of relatively clear ice. These probably correspond to dry growth and wet growth 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 accretion of supercooled water drops and sometimes also by accretion of minor amounts of small ice particles. Large hail may contain liquid water and be spongy (an intimate mixture of ice and water) in some regions; it is usually solid ice with density greater than 0.8 g cm–3. Small hail may be indistinguishable from large graupel (snow pellets) except for the convention that hail must be larger than 5 mm in diameter. The density of small hail 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 hailstorm in Vivian, South Dakota, on 23 July 2010. The stone from this event weighed 870 g, with a diameter in excess of 20 cm.