Connected to thunder clouds, hail is made up of practically spherical masses of ice. Each hailstone is made up of hundreds of ice crystals, in alternate transparent and translucent layers due to the presence of air bubbles. The transparent crystals form slowly in the inferior portion of a cumulonimbus, that is characterized by higher temperatures, while the opaque crystals are typical of the superior portion, where the lower temperatures cause the rapid formation of crystals that trap air bubbles as they grow. The alternate layers of each hail stone indicate that the strong vertical currents and the turbulence present within a cumulonimbus can transport the hailstones from one portion to the other of the cloud before they fall to the ground. In Italy, the dimensions of the hailstones generally do not exceed a couple of cm, but in tropical countries hailstones can reach sizes greater than 10 cm, up to a maximum of 20 cm!

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