NOAA National Severe Storms Laboratory
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HAIL FAQs

Hail can be predicted using radar. On Doppler radar, hail generally sends a return signal that looks like extremely heavy rainfall. We are also studying the use of polarimetric radar (radar that sends horizontal and vertical pulses) because it can detect actual ice, and determine the location of severe hail.

Forecasting Hail

Can hail be forecast or predicted?

To forecast hail, deep moist convection is required, in addition to these three basic ingredients:

•  Adequate updraft to keep the hailstone aloft for an appropriate amount of time.

•  Sufficient supercooled water near the hailstone to enable growth as it travels through an updraft.

•  A piece of ice or snow for it to grow upon

What kinds of thunderstorms produce hail?

There is no clear distinction between storms that do and do not produce hailstones. Nearly all severe thunderstorms probably produce hail aloft, though it may melt before reaching the ground. Multi-cell thunderstorms produce many hailstones, but not usually the largest hailstones. In the life cycle of the multi-cell thunderstorm, the mature stage is relatively short so there is not much time for growth of the hailstone. Supercell thunderstorms have sustained updrafts that support large hail formation by repeatedly lifting the hailstones into the very cold air at the top of the thunderstorm cloud. In general, hail 2 inches (5 cm) or larger in diameter is associated with supercells (a little larger than golf ball size which the NWS considers to be 1.75 inch.) Non-supercell storms are capable of producing golf ball size hail.

In all cases, the hail falls when the thunderstorm's updraft can no longer support the weight of the ice. The stronger the updraft the larger the hailstone can grow.

HOW DOES NSSL CONTRIBUTE?

Polarimetric radar – NSSL is developing techniques using mathematical functions to weigh the relative importance of the polarimetric variables as they relate to identifying each cloud (cloud water and cloud ice) and precipitation (snow, ice pellets, hail, and rain) particle type. This information can be used to improve predictions from short-term computer forecast models.

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