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Hailstones are balls (or spikes, or flattish pancakes) of frozen precipitation that measure at least 0.2 inches across, according to the National Oceanic and Atmospheric Administration’s Severe Storms Laboratory. Several other types of smaller frozen precipitation are known as “ice pellets,” reports the National Snow & Ice Data Center, and may take the form of graupel (soft balls of water droplets clinging to a snow crystal and looking like Styrofoam) or sleet (essentially icy raindrops). In the sky, either of these can serve as an “embryo,” the little nucleus around which a hailstone can grow. The longer a fledgling hailstone stays lofted in a thunderstorm’s fierce updraft, the bigger it gets. Beyond that minimum 0.2-inch threshold, there are a few finer distinctions between hailstones, thrown around by researchers and sometimes forecasters at the National Weather Service. “Severe” hail has a maximum dimension of one inch or more, “significantly severe” stones are larger than two inches, and “giant” hail is bigger than four inches.

“Giant” sounds pretty big, but this crop of researchers didn’t think it seemed quite big enough. A hailstone of more than four inches is “certainly very large,” says Matthew Kumjian, a meteorologist at Penn State University and lead author of the paper. But, he adds, while stones of that size are rare, “they are not exceptional.” Hailstones bigger than four inches are reported 30 to 40 times a year in the United States alone, he says. Stones larger than six inches, though, are few and far between. Kumjian’s co-author, graduate student Rachel Gutierrez, combed through reports and found about 10 confirmed instances in the last 10 or 15 years, mostly in the U.S. (There were a handful of unconfirmed reports in Australia, Africa, and Asia, but photos or official measurements were missing.)

The researchers suspect that there are probably more of these spectacularly sized hailstones dropping down across the country, but they’re likely going unnoticed. When measuring hail, time is of the essence: Hailstones vanish fairly quickly, especially in hot or humid conditions, or if they shatter on impact; even large ones with cushioned falls might be overlooked. The most severe hailstorms in the United States are in the Great Plains, Kumjian says, where people are spread fairly far apart…

They’re huge; they’re rare; and they’re melting all the time: “The Slippery Problem of Measuring Enormous Hunks of Hail.”

* Matthew Kumjian, a meteorologist at Penn State University, on measuring hailstones

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As we check the weather, we might recall that it was on this date in 1883 that the volcano on the Indonesian island of Krakatoa began to release huge plumes of steam and ash. Roughly three months later, on August 27, it erupted in earnest– with a sound so loud that it circled the earth four times.  (As big as the explosion was, it was not the biggest in history: experts suggest that Santorini’s eruption in 1628 BCE was three times as powerful.)

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