Grote hagelstenen ontstaan in gigantische onweerswolken die soms tot meer dan 10 kilometer hoogte uitgroeien en waarin krachtige omhoog en omlaag gerichte luchtstromingen voorkomen.
Bron:
http://nl.wikipedia.org/wiki/Hagel_(neerslag)
Hail has a diameter of 5 millimetres (0.20 in) or more. Stones just larger than golf ball-sized are one of the most frequently reported hail sizes. Hailstones can grow to 15 centimetres (6 in) and weigh more than 0.5 kilograms (1.1 lb). In large hailstones, latent heat released by further freezing may melt the outer shell of the hailstone. The hailstone then may undergo 'wet growth', where the liquid outer shell collects other smaller hailstones. The hailstone gains an ice layer and grows increasingly larger with each ascent.
Bron:
http://en.wikipedia.org/wiki/Hail
Voor specifiekere informatie over hagelvorming, klik op de links.
EDIT: Misschien een iets duidelijkere uitleg:
Hail forms in the updraft region of a thunderstorm. A hail producing thunderstorm will need to have a strong updraft. Extremely large hail requires an extreme updraft. Hail forms as supercooled droplets, ice crystals and water freeze upon an embryo. Once hail forms, gravity will apply a force on the hail to bring it to the earth's surface. If the updraft is stronger than the influence from gravity, the hail stone will continue to grow. Once the hail reaches a size that the updraft can no longer keep aloft, the hail stone will make its way to the surface.
Bron:
http://www.theweatherprediction.com/severe/gianthail/