Like other precipitation in cumulonimbus clouds, hail begins as water droplets.
As the droplets rise and the temperature goes below freezing, they become
supercooled water and will freeze on contact with condensation nuclei. A
cross-section through a large hailstone shows an onion-like structure.
This means the hailstone is made of thick and translucent layers,
alternating with layers that are thin, white and opaque. Former theory
suggested that hailstones were subjected to multiple descents and ascents,
falling into a zone of humidity and refreezing as they were uplifted.
This up and down motion was thought to be responsible for the successive
layers of the hailstone. New research, based on theory as well as field study,
has shown this is not necessarily true.
The storm's updraft, with upwardly directed wind speeds as high as 110 miles
per hour (180 km/h), blows the forming hailstones up the cloud. As the
hailstone ascends it passes into areas of the cloud where the concentration
of humidity and supercooled water droplets varies. The hailstone’s growth
rate changes depending on the variation in humidity and supercooled water
droplets that it encounters. The accretion rate of these water droplets is
another factor in the hailstone’s growth. When the hailstone moves into an
area with a high concentration of water droplets, it captures the latter
and acquires a translucent layer. Should the hailstone move into an area
where mostly water vapour is available, it acquires a layer of opaque white ice.
Furthermore, the hailstone’s speed depends on its position in the cloud’s
updraft and its mass. This determines the varying thicknesses of the layers
of the hailstone. The accretion rate of supercooled water droplets onto the
hailstone depends on the relative velocities between these water droplets and
the hailstone itself. This means that generally the larger hailstones will
form some distance from the stronger updraft where they can pass more time
growing.As the hailstone grows it releases latent heat, which keeps its
exterior in a liquid phase. Because it undergoes 'wet growth', the outer
layer is sticky (i.e. more adhesive), so a single hailstone may grow by
collision with other smaller hailstones, forming a larger entity with an irregular shape.
The hailstone will keep rising in the thunderstorm until its mass can no longer
be supported by the updraft. This may take at least 30 minutes based on the
force of the updrafts in the hail-producing thunderstorm, whose top is usually
greater than 10 km high. It then falls toward the ground while continuing to grow,
based on the same processes, until it leaves the cloud. It will later begin to melt
as it passes into air above freezing temperature.
Thus, a unique trajectory in the thunderstorm is sufficient to explain the layer-like
structure of the hailstone. The only case in which multiple trajectories can be
discussed is in a multicellular thunderstorm, where the hailstone may be ejected
from the top of the "mother" cell and captured in the updraft of a more intense
"daughter" cell. This, however, is an exceptional case.