• Hail consists of pieces of ice that can be transparent or partially opaque and can range in size from as small as peas to as large as grapefruits.

Hail forms inside of cumulonimbus clouds (cumulonimbus clouds are anvil shaped and usually thunderstorm-producing clouds) when there is a strong updraft to carry graupel pellets back up into the cloud.  [Graupel is simply frozen raindrops, similar to sleet].  The strong updraft carries raindrops and ice crystals alike back up into the cloud where temperatures are below freezing and raindrops will freeze into sleet or graupel if there is a freezing nucleus available.  The graupel is then carried up through the cloud where millions of supercooled water droplets collide with the ice surface and are instantly frozen on causing the graupel to become larger.  [This process is called accretion].  When the now larger graupel or hail stone reaches the top of the cloud, it begins to fall back downward on the outer edge of the cloud where the updraft is weaker.  The hail continues its descent until it falls back down into an area where the updraft is stronger and this cycle begins again with the hail stone growing another ring of ice.  This cycle will continue and the hail stone will become larger until finally it becomes too heavy for the updraft to carry upward.  At this point it falls out of the bottom of the cloud, sometimes causing damage to whatever it lands on.

 

The diagram above was made based on fig. 8.22 of Ahrens 1994.

The layers of a hailstone sometimes differ in color from transparent to opaque, this is due to the temperature and the amount of supercooled water droplets within the cloud.  If there is a very high number of supercooled water droplets, they may not all have time to freeze onto the stone before it enters a warmer layer and then back up into the sub-freezing layer.  When the stone re-enters the sub-freezing layer, the liquid water that is covering it freezes as more of a clear glaze of ice.

 

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