GET /climate/anomaly

Returns the climate anomaly for one GPS point over a date range: the departure of the observed ERA5 value from the 1991-2020 WMO climate normal, for temperature, precipitation and wind. For each family you get the observed value, the normal and their difference, so an agent can answer “this month was X degrees above the normal here” in a single call. The lookup is local, so a covered window resolves in milliseconds without an account or API key.

This is the departure signal: parametric insurance and ESG / climate reporting need the deviation from a stable baseline, not just a raw reading. The baseline is the standard 30-year WMO normal, and it is the only reference the endpoint uses. See the live /catalog for the authoritative endpoint listing and price.

x402 golden rule: the agent pays for the answer to its question. A well-formed request where the observed value and the normal are both available is a successful answer -> 200, even when one family is null because its observation or its normal is missing: that is reported through coverage. Requests the service cannot answer - invalid coordinates, an invalid period, an unknown variable, or a window where nothing can be paired - leave the 200 range.

Parameters

Locate the point either with lat/lon coordinates or with a place name via city (alias location) - never both. Place names resolve against an embedded GeoNames cities500 gazetteer (exact match, case- and accent-insensitive, no fuzzy matching; ambiguity resolved by population, narrowed by country=), and the resolved place is echoed back in data.location - see GET /climate/point for the full contract.

The window [from, to] is the unit of the product and is capped at 366 days per call.

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

GET /climate/anomaly?city=Zurich&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

200 response - UnifiedResponse

{
  "data": { ... },
  "provenance": {
    "source": "era5-copernicus",
    "fetched_at": "2026-06-20T12:00:00Z",
    "freshness": { "kind": "snapshot", "as_of": "2026-06-19T00:00:00Z" }
  }
}

• provenance.source: stable identifier of the served store source, typically era5-copernicus.
• freshness.kind: snapshot for ERA5 reanalysis; as_of is the store production date that backed the answer.
• ERA5 values are derived from Copernicus Climate Change Service information.

Fields of data

A family that was not requested via variables is omitted. A family that was requested but cannot be paired - no observation, or no normal - is serialized as null and explained through coverage.

temperature

precipitation

wind

Wind speeds are magnitudes of the daily-mean wind vector (lower than the mean wind speed when direction varies during the day); observed and normal are compared on the same basis, so the anomaly is consistent.

coverage

Baseline honesty

The reference is always the 1991-2020 WMO climate normal (a 30-year baseline), made explicit in reference_period. The endpoint never invents an anomaly against a short window dressed up as a normal: a value is only an anomaly when it is measured against this 30-year reference.

• The normal is a monthly climatology. Each observed day is paired with the normal for its month; over a multi-month window the normals are therefore weighted by the number of days in each month. Temperature and wind are means of the paired observed and normal values; precipitation is the observed total versus the sum of the daily normals.
• A day counts only if it carries both an observation and a normal. A family with no observation returns null with an observation gap reason; a family with an observation but no normal returns null with a climatological normal not available reason.
• The observed side has a finite window that differs by family: temperature and precipitation observations cover a rolling 30-year window (the WMO-standard climatological period), wind a rolling ~5-year window - a deep window can therefore pair temperature and precipitation while wind comes back null with an observation gap reason.
• ERA5 is a gridded reanalysis with a typical global mesh of about 0.25 degrees, not a weather-station reading. A window where nothing can be paired - no observed day, or no normal for any family - returns 400 OUT_OF_RANGE.

Example - a warm month

{
  "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
    },
    "precipitation": {
      "observed_mm": 18.4,
      "normal_mm": 52.6,
      "anomaly_mm": -34.2
    },
    "grid": { "lat": 48.75, "lon": 2.25, "distance_km": 13.42 },
    "coverage": { "complete": true }
  },
  "provenance": {
    "source": "era5-copernicus",
    "fetched_at": "2026-06-20T12:00:00Z",
    "freshness": { "kind": "snapshot", "as_of": "2026-06-19T00:00:00Z" }
  }
}

Example - a family with no normal available

When a requested family has an observation but no climate normal at the grid cell, the anomaly cannot be formed: the family is null and coverage says why. The request still succeeds with 200 as long as another family was paired.

{
  "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",
    "fetched_at": "2026-06-20T12:00:00Z",
    "freshness": { "kind": "snapshot", "as_of": "2026-06-19T00:00:00Z" }
  }
}

Errors

Only requests the service cannot answer leave the 200 range.

{ "error": "invalid from '01-07-2024', expected YYYY-MM-DD", "code": "INVALID_PERIOD" }

{ "error": "requested window has no observed day or no normal within the store", "code": "OUT_OF_RANGE" }

See also

• GET /climate/indices - threshold-based climate event indices over a date range.
• GET /climate/return-period - empirical return period of an observed value against the record.
• For agents - discovery surfaces, the live /catalog and how settlement works.