# How far from normal was this month? The ERA5 climate anomaly

> ESG reports and parametric triggers need a departure from a stable baseline, not a raw reading. One paid call returns the ERA5 value, the 1991-2020 WMO normal, and the anomaly between them.

Published 2026-06-28 · Updated 2026-08-16 · HTML version: https://invoket.com/blog/how-far-from-normal-was-this-month

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"This July was 2.7 °C above normal here" is a sentence an ESG report or a
parametric trigger lives on — and it is only meaningful if *normal* is a stable,
named baseline rather than a convenient short window. An agent producing that
statement needs the observed value, the climatological normal, and the difference
between them, computed against a reference it can cite. One paid call —
[`GET /climate/anomaly`](/docs/api/climate-anomaly) — returns exactly that: the
departure of the observed ERA5 value from the **1991-2020 WMO climate normal**,
for temperature, precipitation and wind.

## The problem: an anomaly is only as good as its baseline

The failure mode here is subtle. You can compute "above average" against anything
— last year, the last five years, a fitted trend — and get a number that looks
authoritative and means nothing comparable. A defensible anomaly is measured
against a *standard* baseline: the WMO 30-year climate normal, currently the
1991-2020 period (WMO-No. 1203). Mix baselines across a report and the numbers
stop being comparable.

`GET /climate/anomaly` uses the 1991-2020 normal as its **only** reference and
stamps it into every response, so the baseline is never ambiguous.

## The call: a point and a window

Give a latitude, a longitude, an inclusive date range, and optionally the
families you want:

```http
GET /climate/anomaly?lat=48.8566&lon=2.3522&from=2024-07-01&to=2024-07-31
GET /climate/anomaly?lat=48.8566&lon=2.3522&from=2024-07-01&to=2024-07-31&variables=temperature
```

The window `[from, to]` is the unit of the product, capped at **366 days** per
call. For each requested family the response gives the three numbers an anomaly
needs — observed, normal, and their difference:

| Family          | Observed            | Normal (1991-2020) | Anomaly             |
|-----------------|---------------------|--------------------|---------------------|
| `temperature`   | `observed_celsius`  | `normal_celsius`   | `anomaly_celsius`   |
| `precipitation` | `observed_mm`       | `normal_mm`        | `anomaly_mm`        |
| `wind`          | `observed_speed_ms` | `normal_speed_ms`  | `anomaly_speed_ms`  |

The anomaly is always **observed minus normal**, and the baseline used is echoed
in `reference_period` (e.g. `1991-2020`).

## Honesty: it is observed minus a 30-year monthly normal

This is the scope that makes the figure citable:

- **The baseline is fixed and stated.** The reference is the 1991-2020 WMO normal
  — a 30-year baseline — surfaced in `reference_period`. The endpoint never
  dresses up a short window as a "normal."
- **The normal is monthly.** Each observed day is paired with the normal for its
  month, so over a multi-month window the normals are weighted by how many days
  fall in each month. Temperature and wind are means; precipitation is the
  observed total versus the sum of the daily normals.
- **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.

## An unpaired family is `null`, with the reason

An anomaly needs *both* an observation and a normal. A day counts only when it
has both; a family that cannot be paired comes back **`null`**, and `coverage`
distinguishes the two causes — an `observation gap` versus a
`climatological normal not available`. The request still succeeds at **200** as
long as another family was paired.

```json
{
  "data": {
    "lat": 48.8566, "lon": 2.3522,
    "from": "2024-07-01", "to": "2024-07-31",
    "reference_period": "1991-2020",
    "days_counted": 31,
    "temperature": { "observed_celsius": 22.8, "normal_celsius": 20.1, "anomaly_celsius": 2.7 },
    "wind": null,
    "grid": { "lat": 48.75, "lon": 2.25, "distance_km": 13.42 },
    "coverage": { "complete": false, "reason": "wind: climatological normal not available" }
  },
  "provenance": {
    "source": "era5-copernicus",
    "freshness": { "kind": "snapshot", "as_of": "2026-06-19T00:00:00Z" }
  }
}
```

That distinction matters for a reporting agent: "no observation" and "no normal"
are different gaps, and the response tells you which one you hit. 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 family (`INVALID_VARIABLE`), or a window where
**nothing** can be paired (`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-anomaly) for the current figure.

## Raw value, balance, departure, rarity

Anomaly is the *departure* layer of the climate family — pick the endpoint that
matches the question:

- [`GET /climate/point`](/docs/api/climate-point) — the raw 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) — the period balance,
  means and totals, when you want the level rather than the departure.
- [`GET /climate/indices`](/docs/api/climate-indices) — threshold event counts
  for parametric triggers.
- [`GET /climate/return-period`](/docs/api/climate-return-period) — once you know
  this month was unusual, *how rare* was it, empirically.

Reach for anomaly when the deliverable is "above/below normal by how much" —
parametric insurance and ESG / climate reporting.

Used for what it is — an observed-minus-normal departure against the fixed
1991-2020 WMO baseline, with the reference period stamped in and the served cell
and coverage exposed honestly — `GET /climate/anomaly` gives an agent a citable
deviation, without an account and without building a climatology from GRIB. For
the full field reference, baseline semantics and error codes, see the
[`GET /climate/anomaly` documentation](/docs/api/climate-anomaly).

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