# Petrel Aridity — Layers — release notes

Rolling version history for the Petrel Aridity **Layer** — the UNEP-style
aridity index (P/PET) at native ~28 km (0.25°) globally, with its aridity-class
map and a bootstrap confidence-interval companion. Newest release first.

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## v0.1 · 2026
> **Status: not yet live.** Initial release under preparation; publishes once the
> global build and previews are signed off.

- **Initial release.** The long-term aridity index — mean precipitation divided
  by mean potential evapotranspiration (P/PET) — computed from ERA5 1940–2024,
  with PET from Thornthwaite (temperature-only, so it spans the full 85-year
  record). Ships three layers at **native ~28 km (0.25°)**: the aridity index
  itself, the five-class UNEP classification (hyper-arid / arid / semi-arid /
  dry-subhumid / humid), and a 90% block-bootstrap confidence-interval
  half-width so users can see where the index is well constrained.
- **Native resolution, honestly named.** Ships at the ERA5 grid rather than an
  interpolated 90 m — the climatology doesn't support finer output, so the
  filenames carry the true `_0p25deg` resolution.
- **Free, non-commercial Layer.** Distributed under CC-BY-NC-4.0; a derived
  input data product rather than a paid Hazard. Commercial use is licensed
  separately.