Hailstorm Climatology
This layer maps how often damaging, size-resolved hail occurs around the world.
Source: Prein & Holland (2018) global backbone · GridRad-Severe v4.2 (CONUS radar) · MeteoSwiss CHHC (footprint law) Year: 1979–2023 Category: Physical risks · Mitigating services · Climate Acute Risks Coverage: Global Format: Raster grid (~5 km, 0.05°) Used in risk analysis: Yes — gates Mitigating services
What it shows
This layer maps how often damaging hail occurs around the world, resolved by hailstone size. For the first time it carries the annual rate at which hail exceeds 25, 40 and 60 mm in diameter, rather than an undifferentiated hail-frequency index. Size is what drives damage: the loss a hailstone causes scales roughly with the cube of its diameter, so a layer that distinguishes a 25 mm stone from a 60 mm stone is far more informative for asset risk (roofing, vehicles, solar panels, façades) and crop risk than a plain frequency map.
It replaces the previous global hail layer (NASA PMHC, a ~200 km passive-microwave hail-frequency index with no size axis) and is roughly 40× finer.
How it is built
The layer fuses three open, commercially redistributable sources onto a single 0.05° global grid:
- Global backbone — Prein & Holland (2018). A physically-based estimate of hail hazard derived from the large-scale storm environment, providing the annual rate of ≥25 mm hail everywhere on land, including Europe.
- United States — GridRad-Severe v4.2. Where high-quality radar exists (the contiguous US), observed radar hail sizes are inserted directly and the surrounding backbone is smoothly matched to them at the seam.
- Footprint scaling — MeteoSwiss. Every hail grid reports the largest stone anywhere in a cell (a ~1–2 km² maximum). Buildings are much smaller, so a size-vs-area law fitted on the Swiss hail climatology converts the cell-maximum size to the size expected on a 100 m² building footprint — the scale a damage function should actually use.
The size bands outside the US are extrapolated from the ≥25 mm rate with a size-shape model calibrated on the US radar. Two physical corrections are applied to the environment-based backbone: its rate is capped at the maximum ever observed by radar, and a tropical melting correction damps the deep tropics — where a high, stable freezing level melts most stones before they reach the ground — so the equator no longer appears (incorrectly) to out-produce the mid-latitudes in large hail.
How to read it
The displayed field is the annual number of days with cell-maximum hail ≥ 25 mm. Higher values mean damaging hail is more frequent; the true hot-spots are the mid-latitude continental hail belts (the US Great Plains, northern Argentina, the Alpine foreland, northern India). Because the backbone is a smoothed, environment-based estimate away from the US, read a single cell as a regional signal rather than a precise site measurement. The finer footprint size bands (≥25/40/60 mm on a building footprint) are used internally by the damage model; they are not the symbolised field.
Class thresholds
The underlying quantity is a physical hazard measure: the annual rate of damaging hail (cell-maximum diameter ≥ 25 mm), in hail-days per year. Higher means more frequent damaging hail. The five classes map to the following thresholds on that rate:
| Class | Hail-days ≥ 25 mm per year |
|---|---|
| Very low | < 0.1 |
| Low | 0.1 – 0.5 |
| Moderate | 0.5 – 1.5 |
| High | 1.5 – 4.0 |
| Very high | ≥ 4.0 |
A site is flagged on the Mitigating services dimension when the layer value reaches 1.5 hail-days ≥ 25 mm per year or above (the High class).
How these thresholds were set. The rate values come from the fused hail layer described above. The five-class split (0.1 / 0.5 / 1.5 / 4.0) is a Darwin in-house classification of that surface — graded bins chosen to separate damaging-hail-prone from hail-rare regions — not a published external threshold set. They replace the earlier PMHC-density bins now that the layer carries an absolute, size-resolved rate.
References: Prein & Holland (2018), Global estimates of damaging hail hazard; Murphy & Homeyer, GridRad-Severe v4.2; MeteoSwiss Swiss Hail Climatology; Schmid et al. (2024) hail damage function.
Source
- Prein, A. F., & Holland, G. J. (2018). Global estimates of damaging hail hazard. PANGAEA, doi:10.1594/PANGAEA.893160 (CC BY 3.0) — global backbone, including Europe.
- GridRad-Severe v4.2 (Murphy & Homeyer). Zenodo 13887225 (CC BY 4.0) — contiguous-US radar hail sizes.
- MeteoSwiss Swiss Hail Climatology (CHHC) (CC BY 4.0, "Source: MeteoSwiss") — the cell-maximum → building-footprint size law.
Comparison with the WWF Risk Filter Suite
There is no equivalent indicator in the WWF Risk Filter Suite. Hail is a geophysical / acute-weather hazard rather than a biodiversity or water risk, so it sits outside the WWF methodology. This layer is a Darwin extension that broadens physical-hazard screening.
Risk analysis
A site is flagged on a dimension by combining a proximity trigger (this layer) with an activity trigger (the entity's ENCORE pressure/service). Proximity only → Potentially material; proximity and the matching ENCORE pressure/service is material → Very material; neither → Not material.
| Dimension | ENCORE service / pressure | Proximity trigger (this layer) | Activity trigger (entity) |
|---|---|---|---|
| Mitigating services | Hailstorm | Layer value at or above 1.5 hail-days ≥ 25 mm / yr | — |
Legend
Symbolised field: Damaging hail frequency (≥ 25 mm, days / year)
| Value (Damaging hail frequency ≥ 25 mm, days/yr) | Label | Colour |
|---|---|---|
| 0 | 1 - Very low (<0.1) | #ffffcc |
| 0.1 | 2 - Low (0.1–0.5) | #c7e9b4 |
| 0.5 | 3 - Moderate (0.5–1.5) | #7fcdbb |
| 1.5 | 4 - High (1.5–4.0) | #fd8d3c |
| 4.0 | 5 - Very high (>4.0) | #e31a1c |
Generated from the datascience hail layer build (risks/hail/fuse_hail_global.py, HAIL_LAYER_V2.md); darwin layer registration pending (darwin#7400).