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Open Data Index›Worked-out question
Worked out · 2 sources · 3 steps

How much warmer has it become?

De Bilt has daily temperatures from 1 January 1901, so one request to KNMI answers this question. Averaged over 1996–2025 it was 10.8 °C there, against 8.9 °C in 1901–1930: 1.8 degrees warmer. The warmest years were 2024 (11.80 °C) and 2023 (11.79 °C). This is the raw series of a single station, not corrected for changes in how and where it measures. For a place without a weather station you need ERA5, which requires an account.

Short answerIn De Bilt, 1.84 degrees: from an average of 8.93 °C in 1901–1930 to 10.77 °C in 1996–2025. Most of it came after 1990; 1961–1990 still averaged 9.40 °C.

De Bilt is 1.8 degrees warmer than a century ago: an average of 10.8 °C in 1996–2025, against 8.9 °C in 1901–1930

Annual mean temperature in De Bilt, 1901 – 2025, in °C. The highest point was 2024 (11.8).

Source: KNMI, daggegevens van het weer (station De Bilt) · opgehaald 18 oktober 2026 · selection: station 260 (De Bilt); TG: etmaalgemiddelde temperatuur, alleen volledige jarenOpen the table Single block

Step by step

  1. 1

    Fetch daily temperatures for De Bilt since 1901.

  2. 2

    Average per year and compare thirty-year periods.

  3. 3

    For places without a station: use ERA5.

Join on stationsnummer + datum

Start with AI

Paste this prompt into Claude or ChatGPT. It names the real endpoints, the join key and the pitfalls, so the assistant writes usable code straight away. Always check the result against the source itself.

Make a web page with 'warming stripes' for a KNMI weather station: one vertical stripe per year, coloured by the difference from the 1901–1930 average (blue colder, red warmer), with the annual mean as a line below.

Data: KNMI daily data, no key. One POST request; CORS is open, so it works from the browser:
curl -d "stns=260&vars=TG&start=19010101&end=20251231&fmt=json" https://www.daggegevens.knmi.nl/klimatologie/daggegevens

What you need to know:
- The response is a list of {station_code, date, TG}, about 45,000 days for De Bilt.
- TG is the daily mean temperature in tenths of a degree: TG 115 is 11.5 °C. Also available: TX (maximum), TN (minimum), RH (rainfall in 0.1 mm; -1 means less than 0.05 mm).
- Station 260 is De Bilt, from 1901. Other stations start later: pick your own reference period and show it in the chart.
- Only use years with at least 365 days with a value. Leave out the current year.
- This is the raw series of one station, not corrected for changes in how it measures. Say so under the chart.

Output: the stripes, a tooltip with year and temperature, and a sentence computed from the data, like 'Averaged over 1996–2025 it was 1.8 degrees warmer than in 1901–1930'. Plus a dropdown for other stations (the same call with a different stns number).
Open in ClaudeOpen in ChatGPT

Try it yourself

A small script that answers the core of the question. Needs: Python 3.10+ en requests. Tested on 2026-10-15; below is the real output of that run.

# Hoeveel warmer is het geworden? Jaargemiddelde temperatuur in De Bilt (station 260) sinds 1901.
# Bron: KNMI daggegevens. Geen sleutel nodig. Waarden in tienden van een graad.
import requests

r = requests.post("https://www.daggegevens.knmi.nl/klimatologie/daggegevens",
                  data={"stns": "260", "vars": "TG", "start": "19010101", "end": "20251231", "fmt": "json"},
                  headers={"User-Agent": "opendataindex-voorbeeld/0.2 (+https://opendataindex.nl)"}, timeout=120)
dagen = r.json()   # [{"station_code": 260, "date": "1901-01-01T00:00:00.000Z", "TG": -49}, ...]

per_jaar = {}
for d in dagen:
    if d["TG"] is not None:
        per_jaar.setdefault(int(d["date"][:4]), []).append(d["TG"] / 10)   # tienden -> °C

# Alleen volledige jaren (minstens 365 dagen met een meting).
jaar = {j: sum(v) / len(v) for j, v in per_jaar.items() if len(v) >= 365}

def gemiddeld(van, tot):
    w = [jaar[j] for j in range(van, tot + 1) if j in jaar]
    return sum(w) / len(w), len(w)

print(f"{len(dagen):,} dagen opgehaald, {len(jaar)} volledige jaren ({min(jaar)}-{max(jaar)})\n")
for van, tot in [(1901, 1930), (1931, 1960), (1961, 1990), (1991, 2020), (1996, 2025)]:
    g, n = gemiddeld(van, tot)
    print(f"{van}-{tot}: {g:5.2f} °C  ({n} jaren)")
oud, _ = gemiddeld(1901, 1930); nieuw, _ = gemiddeld(1996, 2025)
print(f"\nVerschil 1996-2025 t.o.v. 1901-1930: {nieuw - oud:+.2f} °C")
print("Warmste vijf jaren:", ", ".join(f"{j} ({t:.2f})" for j, t in sorted(jaar.items(), key=lambda x: -x[1])[:5]))
print("Koudste vijf jaren:", ", ".join(f"{j} ({t:.2f})" for j, t in sorted(jaar.items(), key=lambda x: x[1])[:5]))
Output
45,656 dagen opgehaald, 125 volledige jaren (1901-2025)

1901-1930:  8.93 °C  (30 jaren)
1931-1960:  9.28 °C  (30 jaren)
1961-1990:  9.40 °C  (30 jaren)
1991-2020: 10.57 °C  (30 jaren)
1996-2025: 10.77 °C  (30 jaren)

Verschil 1996-2025 t.o.v. 1901-1930: +1.84 °C
Warmste vijf jaren: 2024 (11.80), 2023 (11.79), 2014 (11.71), 2020 (11.69), 2022 (11.57)
Koudste vijf jaren: 1963 (7.87), 1940 (8.00), 1919 (8.05), 1962 (8.13), 1956 (8.15)

The sources2

KNMI daily weather dataIn depth

KNMINetherlands, weather station

Daily values from over fifty weather stations: temperature, rainfall, sunshine and wind. The De Bilt series starts in 1901.

OpenAPIDownloadClimate and environment
Up to date
updated 3 days ago

ERA5 climate reanalysis

Copernicus (ECMWF)World, approx. 30 km

A reconstructed hourly weather record for the whole earth since 1940. Coarse (30 km) but without gaps: useful where no station stood.

APIDownloadClimate and environment
Reachable
daily

More examples with this data

Solar radiation on a flat surface in De Bilt (KNMI measurements): an average of 1,038 kWh per m² a year in 1996–2025, 8% more than in 1958–1987

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Global mean temperature at 2 metres (ERA5): highest on average in July, lowest in January

Climate and environment · september 2026

Summer days (25 °C or warmer) in De Bilt (KNMI measurements): an average of 29 days a year in 1996–2025, 87% more than in 1901–1930

Climate and environment · 2025

Also in the indexes

Datasets from data.overheid.nl, the National Georegister, CBS, Eurostat and city portals that fit this question.

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