Hail consists of balls or irregular pieces of ice (≥5 mm diameter) that fall from cumulonimbus clouds. Unlike sleet or ice pellets, hailstones grow within the cloud through accretion: supercooled water droplets freeze onto an embryo (ice crystal or frozen droplet) as it is carried repeatedly through zones of liquid water by powerful updrafts. Each trip adds a layer of ice — which is why cut hailstones reveal concentric rings like an onion.
Hailstone size depends on updraft strength: stronger updrafts support larger stones for longer. Updrafts in severe supercells can exceed 50 m/s (180 km/h), sustaining stones of 5–10 cm or more. The largest authenticated hailstone — found in Vivian, South Dakota (2010) — measured 20 cm in diameter and weighed 879 g. Hail falls at terminal velocities of 30–70 km/h for small stones to over 150 km/h for the largest, which is why large hail is extremely destructive.
Hail causes billions of euros in damage worldwide annually, primarily to agriculture (destroyed crops), vehicles, and buildings (shattered skylights, dented roofs and bodywork). In Spain, hail is a significant hazard in the Ebro valley, the Mediterranean coast, and agricultural regions of Castilla-La Mancha and Andalusia — grape and fruit harvests are particularly vulnerable. Hail suppression programs using cloud seeding (silver iodide rockets or aircraft) are employed in some agricultural regions, though their effectiveness remains debated. Dual-polarisation radar can identify hail signatures in real time, enabling warnings with lead times of 15–30 minutes.