Browse the full Resources Index
Hail is often reported to the nearest common object, a pea, a nickel, a golf ball, a baseball, and those comparisons are not just folk shorthand. The National Weather Service uses a standardized size chart tying hail diameter to familiar objects precisely because size correlates closely with the kinetic energy a hailstone carries and, in turn, with the kind of damage it is capable of producing on impact.
Hail forms when updrafts within a thunderstorm carry ice particles repeatedly through layers of supercooled water, each pass adding another layer of ice, similar to the way an onion builds layers. Stronger, more sustained updrafts allow a stone to stay aloft longer and grow larger before gravity and the weight of the stone overcome the updraft and it falls. Cutting a hailstone in half and examining the concentric rings under light, a technique sometimes used by researchers and occasionally referenced in forensic hail analysis, can reveal how many growth cycles a particular stone went through.
Small hail, generally under half an inch, rarely causes meaningful damage to asphalt shingles or metal panels, though it can still mark soft aluminum on gutters, vents, and downspouts if wind-driven at an angle. Hail in the three-quarter inch to one inch range, roughly penny to quarter size, is the threshold where granule displacement and mat fracturing on asphalt shingles become more common, particularly on older or more brittle roofing. Stones an inch and a quarter or larger, golf ball size and above, carry enough mass and velocity to fracture fiberglass mat, crack impact-resistant shingles rated under UL 2218, dent metal roofing and siding, and break window glass and skylights outright.
Velocity matters as much as diameter. A stone falling with strong wind assistance strikes at an angle rather than straight down, which changes both the shape of the impact mark and which slopes of a roof or which elevations of siding take the brunt of a storm. This is one reason hail damage on a structure is rarely uniform from one side to another, and why an inspection typically documents damage by roof slope or by wall elevation rather than treating a building as a single undifferentiated surface. Hail density also plays a role independent of diameter; a stone with air pockets and a softer, more granular structure behaves differently on impact than a dense, glassy stone of the same size, which is part of why two storms reporting the same maximum hail size can leave noticeably different damage behind.
Weather data sources used to reconstruct a hail event include National Weather Service storm reports, dual-polarization radar products that estimate probable hail size aloft, and in some cases hail pad networks or crowd-sourced reports collected by services that specialize in storm verification. None of this data determines what is or is not covered under a specific policy, which remains a matter between the policyholder and the carrier, but it does provide an objective, third-party reference point for whether a hail event of a given intensity actually passed over a specific address on a specific date, which is useful groundwork before anyone starts discussing the physical condition of a roof.
This article is general education about how the appraisal process commonly works. It is not legal advice, and specific procedures can vary by state and policy.
Russ Lis is a working property insurance appraiser and umpire based in Minnesota, serving clients nationwide. Contact Appraisal Resolution.