B asked for the hail ruling before coding fabric choice; this is it as code.
The physics: porosity is about AIR (blows through → less wind load) and WATER
(drains → no ponding), NOT ice. A knitted shade cloth's gaps are ~1-3 mm; a
damaging hailstone is 6-45 mm, so it can't pass a mesh an order of magnitude
finer than itself — porous and membrane block the big stones identically. The
ONE true difference is the finest pea hail, which IS small enough to rattle
through an open weave. So: membrane stops everything; porous stops everything
except the smallest stones.
Verified across the storm sizes: shade cloth (0.3) fully blocks the wild-night
1.3/1.4 stones and leaks 26% of storm_03's 0.7 pea hail; an open 0.5 weave
leaks 90% of pea hail but still catches the big ice. That makes the fabric
choice cost you exactly on the mild-hail nights and stay honest on the ice
nights — a real tradeoff without a physics lie.
The assert also proves the integration end to end so nobody wires it blind: a
membrane-covered bed takes ~0 hail on storm_03, a porous-covered one takes 0.62
— the choice is genuinely non-trivial. Formula for whoever wires the garden
drain: coveredHail = hailShadowOver(bed) * hailBlockFor(hailSize, sail.porosity).
Pure helper (no THREE), exported through weather.js alongside stormStats/
forecastFor; NOT on the wind contract, so no router change. Selftest 263/0/0.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
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