Hail on Radar

In short: hail grows in strong updrafts, in the part of the storm between about -10 and -30 °C, where frozen pellets collect supercooled water and freeze it on in layers. The stronger and wider the updraft, the bigger the stones it can hold up. On radar, big hail shows as very strong reflectivity high in the storm, with dual-pol and a few distinctive artifacts confirming it. MESH turns all of that into an estimated size, which is useful and easy to over-read.

How hail grows

A hailstone starts as a small frozen seed, a pellet of soft ice or a frozen raindrop. Inside the updraft it meets supercooled water, droplets still liquid well below freezing. They freeze onto the seed on contact. Each trip through a different part of the updraft adds a layer, which is why a cut hailstone shows rings.

Three things set how big hail can get:

• Updraft strength. A stone falls once its weight beats the updraft holding it up. Very large hail needs a very strong updraft, which is why supercells produce most of it.
• Time in the growth zone. SPC’s hail page names the layer from -10 to -30 °C as the one where large CAPE favors rapid growth. A wide updraft keeps stones in that layer longer.
• Melting on the way down. A lower freezing level means less distance to melt through. Melting matters most for small hail. Very large stones lose little before they land.

In MesoRadar: the Skew-T tool's Hail group, under More, gives Hail growth zone CAPE (CAPE between -10 and -30 °C) and SHIP, the SPC's significant hail parameter (Pro). The National tab's Freezing Level (0°C Height) maps the melting level (Pro).

What size counts

The NWS warns for hail 1 inch and larger, quarter size. Forecasters call 2 inches and larger significant. Warnings add a damage tag: considerable for hail 1.75 inches (golf ball) or 70 mph wind, and destructive for hail 2.75 inches (baseball) or 80 mph wind. Only destructive triggers phone emergency alerts. A Severe Thunderstorm Warning can also carry the tag TORNADO…POSSIBLE when a brief tornado could form with little notice.

The reflectivity clues

Hail is large, so it returns a lot of energy to the radar. That makes reflectivity the first clue, and also the first trap.

• Very high values. Cores of 60 dBZ and up suggest hail. Wet hail, coated in melting water, reflects most strongly of all, and can push readings past 65 to 70 dBZ.
• Height matters more than the number. A strong core near the ground can be heavy rain. A strong core well above the freezing level, where large particles are almost all ice, is the hail signal. Step up the tilts, or use a cross-section, and see how high the 50 dBZ echo reaches.
• The hail is not where the rain is. Big hail usually falls near the updraft, on the edge of the core closest to the inflow, not in the middle of the heaviest rain. That is also why driving into a storm’s core is dangerous: the largest stones often fall at the edge nearest the updraft, so a car driving in from clear air meets the biggest stones first, and hail of golf-ball size and up can break car windows.

In MesoRadar: Reflectivity on the Local tab, and Slice (Pro, Level 2) for the vertical cut. How to see the storm from the side.

The dual-pol clues

The dual-pol products see shape, and shape is where hail and rain part ways.

• ZDR near zero inside very high reflectivity: large raindrops are flattened and read high ZDR. Big hail tumbles as it falls and looks round on average, so ZDR drops toward zero. A strong core with ZDR near zero is one of the best hail signals there is. Melting hail with a water coat can read higher, so a small, wet stone can hide.
• CC dips. Rain is uniform, so CC sits near 1. Mixed rain and hail drop it, roughly 0.85 to 0.95, down to about 0.75 for very large hail. Because hail lowers CC too, a low-CC patch counts as a debris signature only when it sits on a velocity couplet.
• KDP stays low. KDP responds to liquid water and largely ignores hail. High reflectivity with low KDP points to ice, not rain.
• HCA, the radar’s hydrometeor classification, combines all of these and labels each pixel with a best guess, including hail. Treat it as a summary of the clues above, not a measurement.

In MesoRadar: Differential Reflectivity (ZDR), Correlation Coefficient, Specific Differential Phase and Hydrometeor Classification (HCA), in the Local tab's Radar Product picker.

The hail spike

The three-body scatter spike is an artifact that only big hail causes. Part of the beam hits the hail, bounces down to the ground, back up to the hail, and then back to the radar. That extra trip takes longer, so the radar plots it as weak echo farther away, a thin spike pointing straight away from the radar behind the core.

• It points along the beam, away from the radar, always.
• It appears on tilts that cut through the hail core aloft, usually not the lowest one.
• Its weak values sit where there may be no storm at all. It is not weather; it is the radar’s fingerprint of large hail.

Seeing a spike means large hail aloft, usually 1 inch or larger. Not seeing one proves nothing.

MESH and hail swaths

MESH, the Maximum Estimated Size of Hail, comes from MRMS. It estimates the biggest hail a storm is producing from how much strong reflectivity sits above the freezing level, weighted toward the colder levels where hail grows. Mapped as the maximum over an hour or a day, it draws hail swaths along each storm’s path.

How to read it honestly:

• It is an estimate, and by design a high one. Witt and colleagues fit MESH so that about three quarters of observed maximum hailstones are smaller than the estimate. Comparisons with hail reports find every version least reliable as stones approach 4 inches (Wendt and Jirak 2021).
• It is about the air above the ground. Hail can melt, or be blown sideways, before it lands.
• Swaths are excellent for location. “Did the big core cross my neighborhood, and when?” is exactly what the 24-hour swath answers. Pair it with ground reports for size.

In MesoRadar: the National tab, Hail Size (MESH), with Now, 1 hr max, 6 hr max or 24 hr max (Pro). Hail Probability maps the chance of severe hail from the same method (Pro). Storms with hail detected by the radar's algorithm carry a green triangle, larger for severe (Pro). Hail reports from the ground show as green badges. Map symbols.

VIL

Vertically integrated liquid adds up the reflectivity through the whole column and converts it to an amount of water. Tall, intense cores, which often hold hail, give high VIL. It depends on how deep storms are, which changes with season and place, so compare a storm against others the same day rather than against a fixed number.

In MesoRadar: Vertically Integrated Liquid, on the Local tab (from the single radar) and the National tab (MRMS, Pro).