# ETCCDI climate indices for parametric-insurance triggers

> A parametric policy pays on a measured threshold — frost days, a one-day rainfall max, a wind gust. One paid call returns standard ETCCDI climate indices from ERA5, with the thresholds it applied.

Published 2026-06-28 · Updated 2026-08-16 · HTML version: https://invoket.com/blog/parametric-insurance-triggers-for-any-location

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A parametric insurance policy does not pay on a loss adjuster's visit — it pays
on a *measured index* crossing a threshold: so many frost days, a one-day
rainfall above X mm, a wind gust above Y m/s. An agent pricing or settling such a
trigger needs that index computed the standard way, on the right source variable,
with the threshold stated. One paid call —
[`GET /climate/indices`](/docs/api/climate-indices) — returns event indices for a
GPS point and a window, using the standard **ETCCDI / Climdex** definitions and
echoing back the thresholds it applied.

## The problem: a trigger is only as good as its definition

"Number of frost days" sounds unambiguous and is not, unless you pin three
things: the *source variable* (true daily minimum temperature, not a daily-mean
approximation), the *threshold* (below 0 °C), and what happens when the data
needed is missing. A policy whose index is computed on the wrong basis is a
dispute waiting to happen.

`GET /climate/indices` computes the temperature indices on the **true daily
Tmin/Tmax**, follows the ETCCDI conventions, and reports the threshold values it
used — so the trigger an agent reasons over matches the trigger written into the
contract.

## The call: a point, a window, a set of indices

Give a latitude, a longitude, an inclusive date range, and optionally the indices
and configurable thresholds you want:

```http
GET /climate/indices?lat=48.8566&lon=2.3522&from=2024-01-01&to=2024-12-31&indices=frost_days,summer_days
```

The window `[from, to]` is the unit of the product, capped at **366 days** per
call. The available selectors and their ETCCDI definitions:

| Selector          | Unit | Definition                                                       |
|-------------------|------|------------------------------------------------------------------|
| `frost_days`      | days | daily **minimum** temperature below 0 °C (fixed threshold)       |
| `summer_days`     | days | daily **maximum** above `hot_threshold` (default 25 °C)          |
| `tropical_nights` | days | daily **minimum** above 20 °C (fixed threshold)                  |
| `max_dry_spell`   | days | longest run of days with precipitation below `dry_mm` (default 1)|
| `heavy_rain_days` | days | days with precipitation at or above `heavy_mm` (default 10)      |
| `rx1day`          | mm   | maximum single-day precipitation in the window                   |
| `prcptot`         | mm   | total precipitation over the window                              |
| `max_gust`        | m/s  | maximum daily wind gust in the window                            |

## Honesty: which thresholds are fixed, which you set

This is the scope point that makes the index defensible. The frost (0 °C) and
tropical-night (20 °C) thresholds are **fixed** by the ETCCDI definitions and
cannot be changed. Only three are configurable — the summer-day temperature
(`hot_threshold`), the dry-day ceiling (`dry_mm`) and the heavy-rain floor
(`heavy_mm`) — and whatever values were actually applied come back in
`data.thresholds`. There is no guessing about what the number means.

Two further honesty notes:

- **Temperature indices use true Tmin/Tmax**, never a daily-mean stand-in:
  `frost_days` and `tropical_nights` on the daily minimum, `summer_days` on the
  daily maximum.
- **It is a grid value, not a station.** ERA5 is a gridded reanalysis on a mesh
  of about 0.25 degrees; `grid.distance_km` exposes how far the served cell sits
  from the requested point.

## A missing source variable is `null`, never invented

The cleanest honesty rule for a trigger: if the store does not carry the source
variable an index needs, that index comes back **`null`** with
`coverage.complete: false` and a `reason` naming the field and its underlying
variable — the rest of the answer is still served, at **200**.

```json
{
  "data": {
    "lat": 48.8566, "lon": 2.3522,
    "from": "2024-01-01", "to": "2024-12-31",
    "days_counted": 366,
    "indices": { "heavy_rain_days": 9, "rx1day_mm": 38.4, "max_gust_ms": null },
    "thresholds": { "hot_celsius": 25, "dry_mm": 1, "heavy_mm": 10 },
    "grid": { "lat": 48.75, "lon": 2.25, "distance_km": 13.42 },
    "coverage": { "complete": false, "reason": "no source data for: max_gust_ms (gust)" }
  },
  "provenance": {
    "source": "era5-copernicus",
    "freshness": { "kind": "snapshot", "as_of": "2026-06-19T00:00:00Z" }
  }
}
```

This matters for a settlement agent: a `null` index is "we cannot compute this
trigger," not "the trigger did not fire." Read the index value *and* `coverage`
before acting. Per the x402 golden rule, only requests the service genuinely
cannot answer leave the 200 range: invalid coordinates (`INVALID_COORDS`), a bad
or over-long period (`INVALID_PERIOD`), an unknown index (`INVALID_INDEX`), an
out-of-range threshold (`INVALID_THRESHOLD`), or a window with **no** covered day
at all (`OUT_OF_RANGE`).

## Where it fits in the x402 loop

Each paid call follows the same pattern as every Invoket endpoint. The
[Quickstart](/docs/quickstart) walks the discover → `402` → pay → replay cycle
with runnable snippets, and [For agents](/docs/for-agents) covers the discovery
surfaces and the live `/catalog`. Price and accepted rails are not pinned in this
article — they are served live by the
[catalog](https://api.invoket.com/catalog); see the
[endpoint reference](/docs/api/climate-indices) for the current figure.

## From a raw value to a trigger to a probability

Indices are the trigger layer. They sit between the raw series and the
probabilistic questions a risk model asks:

- [`GET /climate/point`](/docs/api/climate-point) — the underlying daily values
  for a single date (see
  [Historical weather for any GPS point and date, from ERA5](/blog/historical-weather-for-any-gps-point)).
- [`GET /climate/aggregate`](/docs/api/climate-aggregate) — period means, totals
  and daily-mean extremes when you need a balance rather than an event count.
- [`GET /climate/anomaly`](/docs/api/climate-anomaly) — how far a value sat from
  the 1991–2020 normal.
- [`GET /climate/return-period`](/docs/api/climate-return-period) — how rare a
  season's value was, empirically — the natural next question after a trigger.

Reach for indices specifically when the deliverable is a **threshold event
count**: parametric insurance triggers, agriculture and construction planning.

Used for what it is — standard ETCCDI indices from ERA5, computed on the true
source variables, with the applied thresholds and the coverage exposed honestly —
`GET /climate/indices` gives a trigger agent a number it can defend, without an
account and without computing indices from GRIB. For the full selector list,
definitions and error codes, see the
[`GET /climate/indices` documentation](/docs/api/climate-indices).

ERA5 values are derived from Copernicus Climate Change Service (C3S) information.