Why the hail record is wrong

Everything snaps to a coin

Hail diameters are continuous. Reports are not. A stone that is genuinely 1.15 inches gets reported as a quarter, and one at 1.6 inches becomes a golf ball, because the reference objects are what people have to hand. The result is a record that clusters on a dozen values and is empty between them.

This is not a criticism of spotters; it is what the method does. It is also fixable in one move, which lesson 2 already gave you: measure it and say you measured it. A measured 1.15 inches is worth more than an estimated quarter precisely because it does not land on the grid.

And everything lands low

Blair and colleagues went and checked. Their field campaign sampled 73 severe thunderstorms between 2011 and 2015, measuring what actually fell, and compared it to the record.

Companion work put the underestimate of a storm's maximum size at close to a factor of two. Supercells produced the largest stones by a wide margin, which is lesson 1's physics showing up in the archive. The record is a floor, not a measurement.

Nobody is outside at three in the morning

Two biases decide whether hail enters the record at all, and neither is about the hail. Reports need a person, so they thin out at night and away from towns, in exactly the way module 1 lesson 7 described for tornadoes.

Wendt and Jirak quantified the night half by comparing human reports against MESHMaximum Estimated Size of HailA radar-derived estimate of the largest hail a storm is producing. It describes hail in the cloud, so it can disagree with what actually reaches the ground., the radar hail estimate. About 30% of significant hail reports occurred at night, against about 40% of MESH. Radar, which does not sleep, finds a larger share of its hail after dark than people do. Their plain-language version: "Human observers are increasingly less likely to be available to observe severe hail after sunset." Across the whole record MESH diagnoses severe hail hours two to four times more often than Storm Data has reports.

MESH is not a measurement

MESH comes from SHISevere Hail IndexA thermally weighted integration of the radar reflectivity profile above the freezing level. MESH is derived from it, and so is the probability that a storm is producing severe hail., a thermally weighted integration of the reflectivityHow much of the radar pulse bounced back from whatever the beam was passing through on that tilt. It is the familiar green-to-red precipitation picture, and it describes what is up in the beam, not what is reaching the ground. profile above the freezing levelThe height at which the air reaches 0 °C. A hailstone has to survive the fall from there to the ground, so a high freezing level means more melting and smaller hail at the surface., with POSHProbability of Severe HailThe chance a storm is producing hail of an inch or more, derived from SHI against a threshold that scales with the height of the wet-bulb zero. derived against a threshold that scales with the wet-bulb-zero height. What matters for a spotter is not the machinery but the design intention, which Witt and colleagues stated when they built it.

So MESH is roughly the diameter that three quarters of stones fall short of. A report coming in under MESH is the expected case, not a contradiction. And the skill numbers are sobering:

One more seam in the record

The severe threshold moved from 0.75 to 1.00 inch on 5 January 2010, and the NWS estimated around 40% of severe thunderstorm warnings had previously been issued for hail below the new mark. That makes the date a genuine discontinuity: hail climatology before 2010 is not directly comparable to after it, and any trend drawn straight through it is measuring a paperwork change.