Hail is typically associated with cumulonimbus clouds. The formation of hail is caused by the supercooling of uplifted liquid below freezing temperatures during severe thunderstorms.
Clouds are generally classified according to their altitudes. Cirrus clouds have high elevations and may form as high as 20,000 feet from the ground. Low-level clouds known as stratus clouds have average heights of around 6,500 feet. Mid-level clouds, which are prefixed with "alto," are located between cirrus and stratus clouds. Cumulus clouds commonly form at low altitudes, but they can rapidly rise to great heights. The presence of cumulus clouds in the sky usually indicate fair and sunny weather. When cumulus clouds gather enough water vapor to become dense and heavy, they turn into cumulonimbus clouds, otherwise known as "thunderheads."
Cumulonimbus clouds are formed from rising columns of warm air, which undergoes cooling in the atmosphere. The accumulated moisture in the air starts to condense to produce cumulus clouds, which later take the shape of thunderheads when the increased volume of moisture reaches saturation point. During thunderstorms, the supercooled liquid that form in the updraft of the cumulonimbus clouds turn into varied sizes of hailstones, depending on how fast the upward currents of air are. Hail eventually falls to the earth, which is often accompanied by lightning, heavy rains, strong winds and even tornadoes.