Graupel and Hail Formation Through Accretion

Core Idea

Graupel forms when ice crystals or snow particles fall through supercooled liquid water regions and rapidly accumulate ice through accretion (riming). Hail results from graupel being lofted by strong updrafts into subfreezing regions, growing additional ice layers before eventually falling. Severe weather often correlates with high graupel production and large hail, which indicates vigorous mixed-phase microphysics.

How It's Best Learned

Examine radar signatures showing the presence of graupel (high reflectivities); relate graupel production to updraft strength and moisture availability; study vertical profiles in severe thunderstorms.

Common Misconceptions

• Confusing graupel with hail (graupel is soft and opaque; hail is hard with concentric layers and forms in strong updrafts).
• Thinking large hail requires cold upper atmosphere (it requires strong updrafts in warm, moist environments).

Explainer

You already know from the Bergeron process that ice crystals grow efficiently in mixed-phase clouds by consuming supercooled liquid water through a vapor pressure difference. You also know from latent heating that phase changes release energy that can strengthen updrafts. Graupel and hail formation takes these ideas a step further: instead of ice crystals growing delicately through vapor deposition, they grow violently through direct collision with supercooled droplets — a process called accretion or riming.

Picture a small ice crystal or snowflake falling through a cloud region thick with supercooled liquid droplets — water that remains liquid despite being well below 0°C. As the ice particle collides with these droplets, they freeze almost instantly on contact, coating the particle with a rough, opaque shell of ice. This is graupel: a rounded, soft pellet typically 2–5 mm across, looking somewhat like a small Styrofoam ball. Graupel forms rapidly because the collision-freezing mechanism is much faster than vapor deposition. If you've ever seen small white pellets bouncing off the ground during a spring thunderstorm, you've seen graupel.

Hail begins where graupel leaves off, but requires a crucial ingredient: a powerful updraft. In ordinary clouds, graupel simply falls to the ground once it grows heavy enough. In severe thunderstorms with updrafts exceeding 30 m/s (about 70 mph), graupel gets lofted back upward into the subfreezing zone. Each pass through the supercooled liquid water layer adds another coat of ice. If the liquid water concentration is high and the droplets freeze slowly, the ice grows clear and dense (wet growth). If the droplets freeze instantly, the layer is opaque and bubbly (dry growth). This is why cutting a hailstone in half often reveals alternating clear and opaque rings — each ring records one trip through the updraft cycle. The stone grows until it becomes too heavy for even the strongest updraft to support, then plummets to the surface.

The size of hail is therefore a direct indicator of updraft strength, not simply cold temperatures aloft. A supercell thunderstorm in the warm, humid Great Plains can produce baseball-sized hail because its updraft is sustained and intense, fed by enormous amounts of latent heat released as supercooled water freezes during accretion. This connection between microphysics and storm dynamics is why radar operators watch for high reflectivity cores aloft — they signal vigorous riming and potential hail, making graupel production a key diagnostic for severe weather warnings.