What Causes Hail?
QUICK ANSWER
Hail is caused by strong updrafts in thunderstorms that repeatedly carry ice particles up through subfreezing levels of the atmosphere. As ice particles encounter supercooled water droplets, they grow by accretion. Strong updrafts can keep hailstones aloft for many minutes, allowing them to grow large before falling. Supercell storms produce the largest hail.
Hail is one of the most distinctive products of severe thunderstorms, requiring specific atmospheric conditions to form. Unlike snow or sleet, hail forms when ice grows in successive layers as it moves through different parts of a storm. Understanding what causes hail explains why certain storms produce large hailstones while others produce none, and why hail occurs in some regions far more than others.
What conditions produce hail?
Hail requires specific atmospheric conditions to form. The thunderstorm must extend well above the freezing level (where temperatures drop below 32°F), allowing ice to form aloft. Strong updrafts are needed to lift ice particles back into the cold upper portions of the cloud, where they can grow. The cloud must contain supercooled water droplets (liquid water at temperatures below freezing) for ice to accumulate onto growing hailstones. Strong wind shear helps tilt the storm so updrafts and downdrafts are separated, allowing the updraft to support large hailstones longer.
Why do supercells produce the most hail?
Supercell thunderstorms produce the largest and most damaging hail because their persistent rotating updrafts can support hailstones longer than ordinary thunderstorms. While typical thunderstorms have updrafts up to 50 mph, supercells can have updrafts of 100+ mph. These strong updrafts keep large hailstones suspended for many minutes, allowing them to grow significantly. The tilted storm structure of supercells also lets hail fall away from the strongest updraft, so the updraft can continue to feed the storm even while producing large hail. Most hail larger than 2 inches comes from supercells.
What determines hail size?
Hail size depends primarily on updraft strength. Stronger updrafts can support heavier hailstones longer, allowing more growth. The amount of supercooled water available also matters; more available water means faster growth. The temperature profile of the storm affects growth rate. The time spent in growth regions of the cloud (which depends on how the updraft circulates ice particles) determines final size. The largest hailstones (over 4 inches) require updrafts of 100+ mph, common only in the strongest supercell thunderstorms. Most hail is small (less than 1 inch), with larger sizes increasingly rare.
Where is hail most common?
Hail is most common in regions where severe thunderstorms regularly occur. The US Great Plains (especially eastern Colorado, southeastern Wyoming, Nebraska, Kansas, and Oklahoma) form 'hail alley' with the highest US hail frequency. Other hail-prone regions include parts of Argentina, northern India, central China, and parts of Europe (northern Italy, southern Germany). High-altitude regions tend to get more hail because the freezing level is closer to the ground. Tropical regions get less hail than mid-latitudes because their thunderstorms typically don't extend as high relative to the freezing level.
What conditions produce hail?
Hail requires specific atmospheric conditions to form. The thunderstorm must extend well above the freezing level (where temperatures drop below 32°F), allowing ice to form aloft. Strong updrafts are needed to lift ice particles back into the cold upper portions of the cloud, where they can grow. The cloud must contain supercooled water droplets (liquid water at temperatures below freezing) for ice to accumulate onto growing hailstones. Strong wind shear helps tilt the storm so updrafts and downdrafts are separated, allowing the updraft to support large hailstones longer.
Why do supercells produce the most hail?
Supercell thunderstorms produce the largest and most damaging hail because their persistent rotating updrafts can support hailstones longer than ordinary thunderstorms. While typical thunderstorms have updrafts up to 50 mph, supercells can have updrafts of 100+ mph. These strong updrafts keep large hailstones suspended for many minutes, allowing them to grow significantly. The tilted storm structure of supercells also lets hail fall away from the strongest updraft, so the updraft can continue to feed the storm even while producing large hail. Most hail larger than 2 inches comes from supercells.
What determines hail size?
Hail size depends primarily on updraft strength. Stronger updrafts can support heavier hailstones longer, allowing more growth. The amount of supercooled water available also matters; more available water means faster growth. The temperature profile of the storm affects growth rate. The time spent in growth regions of the cloud (which depends on how the updraft circulates ice particles) determines final size. The largest hailstones (over 4 inches) require updrafts of 100+ mph, common only in the strongest supercell thunderstorms. Most hail is small (less than 1 inch), with larger sizes increasingly rare.
Where is hail most common?
Hail is most common in regions where severe thunderstorms regularly occur. The US Great Plains (especially eastern Colorado, southeastern Wyoming, Nebraska, Kansas, and Oklahoma) form 'hail alley' with the highest US hail frequency. Other hail-prone regions include parts of Argentina, northern India, central China, and parts of Europe (northern Italy, southern Germany). High-altitude regions tend to get more hail because the freezing level is closer to the ground. Tropical regions get less hail than mid-latitudes because their thunderstorms typically don't extend as high relative to the freezing level.
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