Interactive hailstone dynamics calculator — real-time computation of mass, terminal velocity, kinetic energy, and damage potential.
Click any row to set the diameter slider to that size.
The equations driving the AMHIS physics engine.
| Diameter Range | Cd | Behavior |
|---|---|---|
| < 10 mm | 0.45 | Near-spherical, laminar wake |
| 10 – 30 mm | 0.55 | Surface irregularities increase drag |
| 30 – 50 mm | 0.60 | Lobed shapes, tumbling onset |
| > 50 mm | 0.80 | Highly irregular, full tumbling |
Side-by-side comparison of the legacy empirical model vs the AMHIS dynamic drag approach.
Three core innovations that set StormCast apart from legacy hail prediction models.
Legacy models use a fixed asymptotic velocity formula that caps terminal velocity at ~9.65 m/s regardless of hail size. AMHIS introduces size-dependent drag coefficients (Cd = 0.45 to 0.80) that correctly model how large, irregular hailstones tumble and experience greater air resistance. The result: accurate velocity predictions across the full diameter spectrum, especially for giant hail that causes catastrophic damage.
Real hailstones do not simply fall through still air. AMHIS models the full vertical trajectory including updraft recycling, downdraft acceleration, and melting layer transit. Updrafts exceeding 30 m/s can suspend and grow hailstones through multiple cycles, while the downdraft phase adds energy on descent. This produces accurate impact conditions that no empirical formula can capture.
Running full physics on every grid cell is computationally wasteful. AMHIS identifies convectively active cells using radar reflectivity and CAPE thresholds, then applies the expensive hail trajectory model only where storms exist. Non-convective cells get a fast-path evaluation. This delivers 10-50x speedup with zero accuracy loss on active storm cells, enabling real-time operation at 1km resolution.