HAIL

Precipitation in the form of balls
or lumps of ice, usually called hailstones.

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Hail is pieces of ice falling from the sky. They form in very cold cumulonimbus rain clouds. Most hailstones melt before they reach the ground. Hailstorms are usually short and violent.

Hail is formed through a sort-of roller coaster ride through intense thunderstorms. Strong convection currents lift small ice pellets high into the middle and upper portions of a cumulonimbus cloud. This is where super-cooled water droplets collide and the ice pellet grows through a process called accretion. Once the pellet is too heavy for the updrafts to keep it within the cumulonimbus, it begins to fall, and if it does not melt completely before reaching ground level, it comes out as hail. Hail can be classified into three stages of development, Grauple, Small Hail, and Hailstones.

Sometimes hailstones can be very big, but usually they are very small.

There are two ideas about hail formation. In the past, the prevailing thought was that hailstones grow by colliding with supercooled water drops. Supercooled water will freeze on contact with ice crystals, frozen rain drops, dust or some other nuclei. Thunderstorms that have a strong updraft keep lifting the hailstones up to the top of the cloud where they encounter more supercooled water and continue to grow. The hail falls when the thunderstorm's updraft can no longer support the weight of the ice or the updraft weakens. The stronger the updraft the larger the hailstone can grow.

Recent studies suggest that supercooled water may accumulate on frozen particles near the back-side of the storm as they are pushed forward across and above the updraft by the prevailing winds near the top of the storm. Eventually, the hailstones encounter downdraft air and fall to the ground.

Hailstones grow two ways: by wet growth or dry growth processes. In wet growth, a tiny piece of ice is in an area where the air temperature is below freezing, but not super cold. When the tiny piece of ice collides with a supercooled drop, the water does not freeze on the ice immediately. Instead, liquid water spreads across tumbling hailstones and slowly freezes. Since the process is slow, air bubbles can escape resulting in a layer of clear ice.

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