Graupel becomes hail when the balance of microphysical growth processes, ambient thermodynamics, and storm dynamics allows rimed ice to survive multiple circulation cycles and accrete enough mass. Observations and theory describe a progression from small, porous graupel particles formed by riming to denser, layered hailstones produced by alternating phases of accretion, melting, and freezing. Gerald J. Heymsfield NASA Goddard Space Flight Center has written extensively on how these microphysical pathways determine hail density and size.
Microphysical drivers
The core processes are riming, wet growth, dry growth, and secondary ice production. Riming is accretion of supercooled liquid droplets onto an ice embryo; when riming is rapid and droplets freeze on contact, growth is largely dry, producing irregular, low-density graupel. When collisions deposit enough heat that the surface partially melts, wet growth forms a smooth, higher-density shell because surface water redistributes and refreezes into concentric layers. The transition depends sensitively on droplet size and liquid water content because small differences change whether impacts freeze instantaneously or produce a film of water. Secondary ice multiplication during intense riming can increase the number of embryos and thereby alter competition for supercooled water; this process was first characterized by Hallett and Mossop and remains central to explaining sudden bursts of ice particle number.
Environmental controls and consequences
Updraft strength and cloud-layer thermodynamic structure set the residence time and temperature history of particles. Strong, sustained updrafts keep embryos aloft long enough for repeated riming and allow hailstones to cycle through regions of differing supercooled liquid water content, promoting layered accretion and large sizes. The vertical distribution of supercooled liquid water and ambient temperatures controls whether growth is dominated by wet or dry modes. Storm electrification and fragmentation during hailstone collisions can create new embryos, feeding further growth. Regional factors matter: deep, moisture-rich convective clouds over continental plains tend to favor large hail, which has important agricultural and infrastructure consequences for those territories.
Understanding these microphysical mechanisms guides interpretation of radar polarimetry, in situ aircraft probe measurements, and laboratory experiments. Institutions such as the National Oceanic and Atmospheric Administration and NASA provide observational frameworks used by researchers to relate process-level physics to forecasts and risk assessments. Improved knowledge of the graupel-to-hail transition helps quantify hail frequency and severity, informing mitigation strategies for vulnerable communities and sectors.