Hail Explained: Sizes, Radar Clues & Protection

Hail is one of the costliest weather hazards in the United States, yet it gets far less attention than tornadoes. A single hailstorm crossing a metro area can damage tens of thousands of roofs and cars in under half an hour. Understanding how hail forms explains why some storms produce golf balls and others only rain, and why the radar picture can warn you before the first stone falls.

How hail forms

Every thunderstorm is built around an updraft: a column of warm, humid air rising rapidly through the storm. As that air climbs past the freezing level, water droplets are carried into air well below 32 °F but don't immediately freeze. These supercooled droplets stay liquid until they touch something to freeze onto.

A hailstone starts as a small seed — a frozen raindrop or a pellet of soft ice called graupel. As the updraft holds it aloft, supercooled droplets freeze onto its surface layer by layer. When the stone grows heavy enough that the updraft can no longer support it, it falls. If you cut a large hailstone in half you can often see rings of clear and cloudy ice, like tree rings, recording how fast droplets were freezing on as it passed through different parts of the cloud.

The key point is that hail size is controlled by updraft strength. A typical summer thunderstorm with a modest updraft can only suspend small pellets, which often melt into rain before reaching the ground. A supercell with a rotating updraft of 100 mph or more can hold stones aloft long enough for them to grow to baseball size or larger. That's why the largest hail is so strongly tied to supercells, and why the Plains, where supercells are most common, see the biggest stones (see the Tornado Alley Weather Guide).

The hail size chart

Hail reports are standardized against everyday objects so that spotters and the public can estimate size quickly without a ruler. These are the reference sizes used by the National Weather Service:

The United States record is a hailstone that fell near Vivian, South Dakota, on July 23, 2010: 8 inches across and nearly 2 pounds. Stones like that are rare, but golf-ball hail happens somewhere in the country on most spring and early-summer days.

Falling speed matters as much as size. Small hail drifts down at under 25 mph, but stones of two inches and larger can hit the ground at well over 40 mph, and the largest exceed 70 mph. Strong winds add a sideways component, which is how wind-driven hail breaks windows on the side of a house facing the storm while the other side escapes untouched.

Where hail falls in a storm

Hail doesn't fall evenly under a storm. It concentrates in a hail core next to the updraft, where the stones finally fall out once they're too heavy to support. In a typical storm moving northeast across the central U.S., the heaviest hail tends to fall on the north and northeast side of the storm's main precipitation area, often just ahead of the heaviest rain. In a supercell, the largest stones frequently fall close to the updraft region near the storm's rear flank — not far from where a tornado could form.

Hail also falls in swaths. A storm that keeps producing hail as it moves leaves a streak of damage that can be a few miles wide and tens of miles long, with neighborhoods on either side completely unaffected. This is why one side of town can lose every roof while the other side only sees rain.

Spotting hail on radar

Standard reflectivity radar (the colors described in How to Read a Weather Radar) can't confirm hail by itself, but it gives strong clues:

• Very high reflectivity. Hailstones are large and reflect a lot of energy, so hail cores show up in the deepest reds, magentas, and whites on the scale — roughly 60 dBZ and above. Heavy rain alone rarely gets that high. When a small part of a storm shows pink or white, hail is the most likely explanation.
• A small, intense core inside a storm that's growing in the loop. A storm whose highest-reflectivity core is expanding and holding steady across several frames has a strong updraft.
• A "hail spike." Very large hail can scatter the radar beam off the ground and back, creating a thin, false streak of weak echo extending straight away from the radar behind the core. Meteorologists call it a three-body scatter spike; if you see one, the storm almost certainly contains large hail.
• Supercell structure. A storm that moves to the right of the other storms around it, has a hook or notch on its trailing side, and stays isolated is the type most capable of large hail.

Forecasters have more tools than the public map shows. Dual-polarization radar measures the shape and uniformity of what it's sampling, and tumbling, irregular hailstones look very different from flattened raindrops. Combined with spotter reports, that's how warnings can state an expected hail size such as "up to golf ball size."

Protecting yourself and your property

People first. Golf ball hail can cause serious head injuries, and larger stones can be deadly. Get indoors before the core arrives, and stay away from skylights and windows on the side facing the storm. If you're caught driving, don't stop under an overpass, where you block traffic and can be exposed to dangerous wind. Pull into a parking garage or a gas station canopy if one is immediately available. Otherwise stop safely, keep your seatbelt on, and turn away from the windows, covering your head with a coat or blanket in case the glass breaks.

Vehicles. A garage or covered parking is the only real protection. Padded hail car covers help with smaller stones but are no match for baseball-size hail. When a Severe Thunderstorm Warning mentions hail in the path of a storm still 20 to 30 minutes away, that's often enough time to move a car under cover.

Homes. Impact-resistant roofing, rated by the UL 2218 standard (Class 4 is the highest), dramatically reduces hail claims and can lower insurance premiums in hail-prone states. After a storm, inspect gutters, siding, and window screens. Small dents in gutters and downspouts are a good indicator of the size of stones that fell on the roof, which is harder to see safely. Photograph damage before cleanup, and be wary of door-to-door "storm chaser" contractors who appear right after a major hail event.

Using RadarBug when hail is possible

• Turn on NWS Alerts & Warnings. Severe Thunderstorm Warnings state the expected hail size and the storm's path. A warning with the "destructive" tag (baseball-size hail or 80 mph winds) also sends a Wireless Emergency Alert to phones in the area.
• Use the radar loop with a 20- to 30-minute look-back to judge which way a storm's pink or white core is moving and how quickly. Measure distance and speed across frames to estimate arrival time.
• Remember that radar shows what's high in the storm. Hail seen on radar may melt to rain before reaching the ground in summer, or arrive at the ground a few minutes after the core passes overhead aloft.