Hail is solid precipitation of ice, formed inside strong thunderstorm updrafts when supercooled water droplets freeze onto a growing ice embryo. Stones must be at least about a quarter inch in diameter to be classified as hail rather than smaller frozen precipitation.
How hail grows
A hailstone starts as a small ice particle, usually a graupel pellet or a frozen droplet, lofted above the environmental freezing level by an updraft. In the mixed-phase layer between roughly freezing and about minus 40 degrees Celsius, supercooled liquid water is still present. When that water hits the ice embryo, it freezes on contact and adds to the stone.
Two growth regimes matter. In dry growth, the embryo is cold and liquid water content is low, so droplets freeze on impact and trap air bubbles into an opaque, milky layer. In wet growth, the surface temperature stays near 0 degrees Celsius, water spreads across the stone before freezing, and the ice is clear. Repeated trips through these regimes produce the onion-like layering seen when a large stone is cut open.
Stone size is set by the balance between updraft strength, which decides how long a stone stays in supercooled water, and the amount of supercooled liquid available. NSSL emphasizes that stronger, more sustained updrafts grow bigger stones, which is why the largest hail is associated with supercells.
Environments that favor large hail
Forecasters weigh several ingredients together. Hail specifically responds to CAPE in the mixed-phase layer where growth happens, not just to total CAPE. Steep midlevel lapse rates keep parcels buoyant through that critical zone. A wet bulb zero height in a productive range gives large stones time to fall through subfreezing air without melting away.
Deep-layer shear supports supercell structure, which stretches a single updraft into a long, tilted growth channel. A wide, rotating updraft, indicated by a broad BWER and a healthy ZDR column, is the kinematic signature of an efficient hail factory.
How radar sees hail
High reflectivity in a deep, elevated core is the first clue: values above about 55 dBZ at midlevels are consistent with hail. Vertically integrated liquid (VIL) captures the mass of that column and is a long-standing operational shorthand for hail potential. MRMS Maximum Estimated Size of Hail (MESH) refines this using both VIL and a temperature-based scaling.
Dual-polarization variables add microphysical detail. Low ZDR near zero in a high-reflectivity core suggests tumbling, roughly spherical stones. A ZDR column extending well above the environmental freezing level marks the updraft where large drops are being lofted into growth zones. Reduced correlation coefficient in a reflectivity core is another useful hail cue, especially in mixtures of rain and hail.
How chasers and spotters report it
Reports use size categories keyed to familiar objects: pea, dime, quarter, golf ball, tennis ball, baseball, softball. A stone one inch or larger meets the severe criterion in the United States. Chasers typically avoid the highest-reflectivity forward-flank core in a supercell because that is where the largest stones fall, often at fifty to more than one hundred miles per hour.
Ground reports anchor the radar algorithms. When a spotter reports a maximum stone size, that ground truth feeds back into how MESH and similar products are calibrated. NSSL's mPING project has expanded this feedback loop by collecting citizen reports of hail and other precipitation types.
Where hail forecasting misleads
Radar-based hail size is an estimate. Two supercells with similar VIL and MESH can produce very different stone distributions depending on updraft structure, storm motion, and how much melting occurs on the way down. Values near thresholds should be read as probabilities, not switches.
Big hail can also fall from storms that never met a classical severe hail environment when a compact updraft delivers a brief but very intense growth episode. The reverse is equally true: environments that look ideal on paper sometimes deliver mostly small stones because storms never organize into supercells or the updraft base sits above the deepest supercooled layer.