Once hail grows beyond three-quarters of an inch in diameter, the National Weather Service deems it “severe.” And a grapefruit, if you haven’t hefted one in the produce aisle recently, is a sizable citrus. The Ruby Red hybrid, the first grapefruit ever patented, back in 1927, is capable of growing up to 6 inches in diameter. Hail is, in fact, capable of getting as large: A hailstone 8 inches across and tipping the scales at nearly 2 pounds holds the distinction of being the biggest ever to fall in the United States, according to the National Oceanic and Atmospheric Administration.
It’s likely the hail in Tuesday’s storm wasn’t quite as outsized — the National Weather Service, it turns out, has a rough guide for what it means when hail is the size of a certain common object. Pea-sized hail is a quarter inch in diameter, half-inch hailstones are marble-sized and penny-sized hail is when the stones become severe. (There’s a fondness for coins, probably because most people know what a quarter — equivalent to an inch of hail — looks like.) Hail the size of grapefruits, per the National Weather Service, is about 4 inches across. And hope you never hear that softball-sized stones — boasting 4.5-inch diameters, the largest in the agency’s estimation system — are falling.
Hail begins when so-called embryos of supercooled water form ice crystals. Typically, wind speeds within a storm dictate if this embryo will become hail and how large it can grow. Imagine that an ice crystal is the ball in a game of meteorologic ping-pong played vertically: Gravity spikes the ice crystal toward the ground. As it falls, the crystal will enlarge, collecting other droplets. Warm, stormy updrafts act as a receiving paddle of sorts, buffeting the growing hailstone skyward once again.
A piece of hail can go through several of these cycles, floating up and down while collecting layers of water and ice, until the stone is too heavy for the wind to hold aloft. For hail to grow to softball sizes, wind speeds need to be near the 100-mph mark; Tuesday’s storm, in comparison, had winds of about 70 mph.
To predict when and where severe hail will fail, meteorologic researchers like those at the University of Oklahoma have developed complex computer models. The Oklahoma model is particularly intensive, meshing the effects of sun radiation and rainfall physics with complex fluid dynamics equations over a geographic grid of a million points. Though their program currently requires one of the most powerful supercomputers in the country to run, the scientists hope — in a few years’ time, anyway — that such a model could give hours of warning before grapefruit-sized hail starts to fall.