Theory 04/Meteorology & Severe Storms /Storm microphysics/3 min
Hail Size Is Residence Time
A stronger updraft should make bigger hail. The biggest stones often come from storms that are merely tall.
Maximum hail size is set by how long a stone remains in the growth region rather than by peak updraft speed, so hail size is controlled by storm depth and trajectory geometry rather than by the strongest vertical velocity.
01The evidence
- Hail generation and size depend on the interplay of updraft strength, residence time in the growth zone and melting on descent, with size limited by the time available before the stone falls out or melts.National Weather Service
02The tension
Forecasters use updraft speed as a proxy for hail size, and the correlation is real but loose. Storms with extreme updrafts sometimes produce small hail because the stone is ejected quickly, and moderate updrafts can produce large hail when a stone is held in the growth region. The proxy conflates two variables.
03The theory
Hail grows by accreting supercooled water, which requires time in the region where that water exists and where the stone is not falling. A faster updraft supports a larger stone but also shortens the time before the stone is carried above the growth zone and out.
The controlling variable is the residence time in the growth region, which depends on the updraft's vertical profile rather than its peak. A broad, deep updraft with a gentle profile holds stones longer than a narrow, violent one, and should produce larger hail.
This predicts that hail size should correlate with updraft depth and width more strongly than with peak velocity, and that storms with a 'clean' profile should produce the largest stones. It also explains why hail size is poorly correlated with tornado occurrence, since the two depend on different features of the same storm.
The applied benefit is a size forecast computed from the vertical velocity profile rather than from a single number, which the current models already resolve even if the output is not used that way.
04What this is not
Those proxies treat peak updraft as the determinant of size. This treats residence time as the determinant, which predicts that a deep, broad, moderate updraft can out-grow a narrow violent one.
05Predictions
- P1Maximum hail size should correlate with growth-region residence time better than with peak updraft speed.
- P2Storms with broad updraft profiles should produce larger hail than narrow storms at matched peak velocity.
- P3Hail size should be only weakly correlated with tornado occurrence, since the two depend on different profile features.
06What would refute it
Size tracking peak updraft across a large storm sample, with residence time adding nothing, would restore the proxy.
07What it would change
It would change how hail size is forecast and how storm seeding or nowcasting is targeted.
08Evidence base
- Factors contributing to hail generation and size in storms — NWShttps://www.weather.gov/media/lmk/soo/Hail_Size_Melting_Considerations_Web.pdf