How hail is formed?
The development of hail is associated with violent thunderstorms and very strong turbulence. Within thunderstorm clouds the water vapour condensates to water droplets or ice crystals, depending on the temperature regime. Through convection the particles are transported aloft in the centre and downwards at the edge of the cloud. Heavy particles like large water drops or hail stones can only be kept suspended or even lifted with very strong vertical winds. Therefore, hail develops in 10 % of the thunderstorms only.
During the up-lift super-cooled water accumulates at the small ice particles (coagulation) and freezes, thus adding a thin ice layer. Through multiple upward and downward movements an individual hail stone can grow to more than 10 cm diameter. The appearance of a hail stone depends on the growing conditions. Air bubbles give a opaque looking. If no air is embedded the ice is clear.
The hail falls down
Only when the weight of the hail stone exceeds the force of the updraft it falls on earth. During falling the hail stone is melting. If the diameter is more than 1 cm and the air temperatures are not too high the particle reaches the earth surface as ice. A hail stone of about 3 cm diameter gets a fall velocity of 90 km/h.