Hail

Hail
Hail in a Thunderstorm
Hail may be present in temperatures well above zero Celsius, the occurrence at +20 Celsius is quite normal.

Hail
Precipitation consisting of small balls or pieces of ice with a diameter ranging from 5 mm to greater than 50 mm falling either separately or agglomerated.
High (Pressure System) ...

Hail. Precipitation in the form of hard pellets of ice which fall from cumulo-nimbus clouds and are often associated with thunderstorms.
Hand Starter. A mechanism for the rotation by hand of an aero-engine's crankshaft for starting.

Hail dimples! That's why the airplane is a solid $4,000 less than most of its brethren. But at ten-grand, who cares?

Hail can cause considerable damage to aircraft and is usually encountered between 10,000 and 30,000 feet. At times it can also be found in clear air near thunderstorms.
Icing ...

The collector, which is heated to melt any frozen precipitation such as snow or hail, funnels water into a two-chamber, pivoting container called a bucket. Precipitation flows through the funnel into one compartment of the bucket until 0.01-inch (0.

PRECIPITATION- Any or all forms of water particles (rain, sleet, hail, or snow) that fall from the atmosphere and reach the surface.
PRECISION APPROACH-
(See PRECISION APPROACH PROCEDURE.) ...

SIGMET - warning of severe weather conditions (active thunderstorms, hail, severe turbulence, icing etc.) issued my Met offices.
sl - sea level.

Convective SIGMET. Weather advisory concerning convective weather significant to the safety of all aircraft, including thunderstorms, hail, and tornadoes.
Related Definitions from Aviation Glossary ...

strong surface wind not associated with convection (usually 35 knots or stronger)

hail (usually an inch or larger)
significant wind shifts
thunderstorms expected to produce significant wind (usually 35 knots or more) ...

See also: Aircraft, Direct, Flight, Aviation, Pilot