Hail is a form of solid precipitation, which is formed in thunderstorms clouds (Cumulonimbus).

 

Thunderstorms clouds consists of droplets of liquid water (at temperatures lower than 0°С, the droplets can be in a thermodynamically instable supercooled condition) and ice crystals. These clouds can reach up to 16-17 km in height in the mid-latitude of the Earth. Three layers are differentiated in their vertical structure: warm – a layer at a temperature of over 0°C, where there are droplets of liquid water only; supercooled, mixed – a layer at a temperature lower than 0°С, where there are droplets of liquid water and ice crystals, while the droplets content vastly decreases at the expense of crystals at temperatures lower than -20°С; cloud anvil – a layer at a temperature lower than -40°С , where there are ice crystals only.

For more information:
• Severe Storms: online meteorology guide
• Thunderstorms

For the hail precipitation formation, the following conditions are necessary:

• High supercooled liquid water content in the cloud
• Hail embryos: small frozen raindrops or soft ice particles known as grauple, which are hardened conglomerates of snow crystals.
• Strong updrafts.

The formation and the growth of hailstones takes place in the supecooled part of the cloud. As the equilibrium vapor pressure over the water is greater than the saturation vapor pressure over the ice, at the same temperature, the liquid water evaporates and the vapor diffuses onto the ice crystals and they grow more rapidly than droplets (Bergeon-Findeisen process). The further growth of ice crystals to hailstones is as a result of the collision and coalescence of ice particles and water droplets. The strong updrafts ensure long lifetime of the hailstones in the cloud, which reach to large sizes. When the hailstones become too large, they overcome the updrafts and fall to the ground. The hailstones begin to melt in the layer with a temperature higher than 0°С, but they fail to melt fully and reach to ground.

For more information:
• Hail Basics
• Hail - wikipedia
• Hail Formation