Snow in a snowfall is soft but ice particles in a hailstorm are hard

Both the ice of a hailstone and the ice in snow flakes are the crystalline form of solid water. However, they are formed in different conditions and through different processes leading to differences in their internal structure and hence different physical properties.

Hailstones basically form in the upper atmosphere by direct condensation of water molecules in highly dense clouds consisting of water vapour.

The other way it is formed is by sudden cooling of large water drops falling in relatively dry air. In the hot summer afternoons towards the end of the season, the atmosphere is laden with a lot of water vapour and local super-saturation leads to condensation into water drops which cannot keep themselves afloat; these drops fall through warmer air and so evaporation takes place. This results in cooling and freezing into ice.

Hailstones are often so cold that they attract more atmospheric water vapour and form fresh layers of ice crystals, but with different orientation and packing density.

The repeated evaporation and condensation at the surface of the hailstones, impart to it the layered and often milky appearance. The hailstones are 5mm in size and are often much larger. And because a hailstone is a sizable mass of solid frozen water, it is hard.

However, the snow particles form when the water molecules in the atmosphere near the surface of the earth condense into micro-crystals of ice, less than 1 thousandth of a millimetre in size.

These crystals are tiny hexagonal flakes, which, in turn, acquire water molecules diffusing around in the atmosphere. This happens in very cold climatic conditions and in the atmosphere not so rich in water vapour.

The water molecules have to diffuse through the air to reach the crystal, and this diffusion slows their growth. The water molecules so encountered by the snow flake through random chances, annex the flake slowly.

The actual growth of the snow flakes is more complex than the simple process presented here to highlight the salient features and main difference between the growth of hailstones and snow flakes. The growth of snow flakes is not only slow, but also the fresh annexation proceeds in forming a dendrite shaped aggregate.

During growth, the flakes form small bumps locally, which develop branches with further ramifications. The balance between faceting and branching in snow is decided by several atmospheric parameters.

However, in all cases, the connectivity of tiny ice blocks is rather fragile in the snow flakes. This makes the snow flakes feathery and soft while the hailstones are hard.