|HABYTIME MINI LECTURE 19: HAIL
METEOROLOGIST JEFF HABY
Hail is a form of precipitation that falls from a convective storm. Hailstones are basically spherical and to be hail they need to have a
diameter of greater than 0.2 inches when they are observed. Precipitation that is ice and smaller than this size is either some form of
winter precipitation (snow, sleet, snow grain, etc.) or is an ice particle that is not large enough to be classified as hail. It could
be hail that melted to a small size.
Like a tree, hail commonly has growth rings. The temperature of the environment, how quickly the ice freezes on the hail stone, the type
of hail growth, etc. influence the shade and shininess of the hail. As the hail grows and falls, it can be at vastly different
elevations. The hail growth process can be different at different elevations.
Hail can grow by the dry process or the wet process. The dry process occurs when deposition occurs on the hailstone. Deposition is water
vapor going directly to the ice state as it deposits on the hail stone. Wet growth is liquid water or supercooled liquid water depositing
itself in the liquid state upon the hail stone and then freezing. Thus, wet growth is a similar process to freezing rain. These
processes look very different in appearance. Deposition will tend to look whiter and duller while freezing water tends to look
clearer and shinier. When water freezes on hail, the faster it freezes then the more likely it will trap tiny air bubbles. The
more air bubbles that are trapped then the milkier the hail will look. These are reasons why the layers of a hail stone can look
different even though all the layers are made of the same ice substance.
Hail can be tossed up and down an updraft several times before falling out of the storm to the surface. A strong updraft is required
for hail so that the updraft speed can support suspending or moving the hail higher aloft so that it can continue to grow. Once the hail
stone grows large enough that the updraft can no longer support it or once the hail stone moves out of the updraft, then the hail stone
will fall to the surface. Hail can melt before reaching the surface. When this happens the hail can turn to all heavy rain by the time
it reaches the surface. In fact, it is typical in strong storms for hail to be with heavy rain aloft even if only rain is reported
at the surface.