Hail on radar, and the hail size chart

Hail on radar shows up as a core of very high reflectivity, usually above 50 dBZ, that reaches well above the freezing level, often with near-zero differential reflectivity (ZDR) and lowered correlation coefficient (CC) in the same place. No single one of those proves hail, and none of them measures the size of the stones that reach the ground.

The supercell, on radar introduces the storm that produces most giant hail and the three-body scatter spike. This page is the working reference for hail itself: the reflectivity clues, the dual-pol clues, the derived products forecasters lean on, and the size chart used for reports.

How hail forms, briefly

Per NSSL’s hail primer, raindrops carried by an updraft into freezing air grow as supercooled water freezes onto them, and a stone falls when the updraft can no longer hold it. Hail needs liquid water to grow, so it does not grow at the very top of the storm, and larger hail typically falls closer to the updraft than smaller hail.

The hail core in reflectivity

Hail is large, and wet hail is highly reflective, so the hail core is usually the strongest echo in the storm. NOAA’s JetStream guide puts 50–65 dBZ at heavy precipitation or some hail and above 65 dBZ at extremely heavy precipitation including water-coated hail; the weather radar colors guide covers that scale.

Where the core sits matters more than how bright it is. The NWS Warning Decision Training Division (WDTD) product “height of xx dBZ echo above x°C” measures the thickness between an echo top and a temperature level. Donavon and Jungbluth (2007, Weather and Forecasting) plotted the 50 dBZ echo height against the melting-level height for 183 severe hailstorms in the Upper Midwest and central Plains and found a strongly linear relationship (R² of 0.86). A warning criterion set at the 10th percentile of that regression gave a 90% probability of detection and a 22% false alarm rate. Two caveats from the same paper: severe hail then meant larger than 19 mm, and in strongly sheared supercell environments, weak echo regions and three-body scatter spikes often flagged the storm sooner.

Brightness is a poor size gauge. Matthew Kumjian’s review in the Journal of Operational Meteorology (2013, Part II) notes that reflectivity alone does not discriminate maximum hail size, and cites evidence that the highest values can come from many small-to-medium stones while the largest hail sits at more modest values along reflectivity gradients near the updraft.

The three-body scatter spike

Leslie Lemon’s 1998 Weather and Forecasting paper named the three-body scatter spike (TBSS): a region of echo generally 10–30 km long, aligned radially downrange from a core above 63 dBZ, found almost exclusively aloft, with low reflectivity and near-zero or weak inbound velocities. Its cause is non-Rayleigh scattering from large hydrometeors, most likely large, wet hail. The COMET training module built on that paper adds that the spike is typically weaker than 20 dBZ and sits the same distance beyond the core as the core is above the ground.

The spike is usually read as a sign of very large hail, but not only that. Lindley and Lemon (2007, Electronic Journal of Severe Storms Meteorology) reported weak spikes, as faint as −14 dBZ, with low-end severe hail of 1.9 to 2.5 cm from pulse storms sampled at close range. In CC the spike tends to stand out as a stripe of low values; the correlation coefficient guide covers why that stripe says nothing about the targets along it.

Hail in dual-pol data

Kumjian’s review (Part I) gives the physics. Hail that tumbles as it falls looks spherical on average, so its ZDR is close to zero, for stones of any size. That is the classic signature: a core of very high reflectivity with ZDR near 0 dB, where rain of the same reflectivity would give strongly positive ZDR. Three complications:

• Small melting hail looks like rain. It acquires a water coat that stabilizes it, and its ZDR can match or exceed that of large raindrops, above 3–4 dB. For smaller hail, Part II names high reflectivity with reduced CC as the better indicator.
• Very large hail can go negative. Oblate stones larger than about 5 cm can produce negative ZDR through resonance scattering.
• CC depends on wetness and shape, not size. Dry hail aloft can give CC as high as rain. Wet hail gives values below 0.95, and very large hail below 0.85, especially in the wet-growth region between about −10 and −20°C. Kumjian is explicit that reduced low-level CC and hail size have no direct relation.

The WSR-88D’s hydrometeor classification algorithm sorts these variables into ten categories. Kumjian notes that only rain/hail mixture represents a mixture, that non-uniform beam filling can cause false classifications, and that ground truth aloft is generally lacking. Treat it as a summary of the base data, not a hail detector.

ZDR columns mark the updraft, not the hail

A ZDR column is a vertical column of positive ZDR extending above the environmental 0°C level. Kumjian (Part II) places it within the updraft in ordinary storms and on its periphery in supercells, made of large drops and wet graupel and hail being lofted. It is an updraft indicator: he cites increases in column height or extent preceding a rise in low-level reflectivity by 10–15 minutes. Kuster and colleagues (2020, Weather and Forecasting), using rapid-update data on 45 Oklahoma storms, found that columns differed between severe and nonsevere storms and developed before upper-level reflectivity cores. The hail core itself is where you look for near-zero ZDR.

VIL density and MESH

Two derived products appear in hail warning work. Amburn and Wolf (1997) divided vertically integrated liquid (VIL) by the echo top to get VIL density, and found severe hail reports (then 19 mm and larger) increased substantially above 3.5 g/m³; above 4.0 g/m³ virtually every storm produced severe hail. WDTD warns that VIL density can run high in low-topped warm-season storms that rarely produce severe hail, and is underestimated for fast-moving or strongly tilted storms.

The WSR-88D hail detection algorithm of Witt and colleagues (1998) estimates the probability of hail, the probability of severe hail and a maximum expected hail size from a severe hail index, a thermally weighted vertical integration of reflectivity. Its authors reported substantial regional variation, best across the southern Plains and weaker farther east.

The hail size chart

Since 0001 UTC on 5 January 2010, per NWS Service Change Notice 09-52, the minimum hail size for a severe thunderstorm warning has been 1 inch (quarter), up from 3/4 inch (penny). The comparisons below are from the NWS La Crosse, Wisconsin spotter chart, which asks for reports to the nearest quarter inch.

Offices do not use identical objects: NWS Boise lists a softball as 4 inches. A measured diameter beats a comparison.

WeatherStory Radar decodes reflectivity, ZDR, correlation coefficient and KDP from raw Level II and shows the exact value and beam height of any gate you click.