Summary
In September 2025, the Netherlands experienced a striking seasonal transition that many described as an overnight change from summer warmth to autumn chill. Verified data from the KNMI (Royal Netherlands Meteorological Institute) confirm that between 20 and 21 September 2025, temperatures dropped sharply across the country. This abrupt cooling occurred just before the autumn equinox on 22 September, giving the impression that the season had changed in a single day.
This article explores that transformation through science and experience—how Earth’s geometry, atmospheric behavior, and even the cultural rhythm of daylight saving time (DST, daylight saving time) combine to make autumn’s arrival feel so sudden.
Context and Scope
This blog focuses on the verified meteorological and astronomical evidence of the seasonal shift in the Netherlands during September 2025, under the Europe/Amsterdam (CEST/CET) time zone. It integrates:
- Official KNMI data confirming the sharp cooling from 20 to 21 September 2025.
- The timing and meaning of the autumn equinox.
- The atmospheric mechanisms behind strong day–night temperature contrasts.
- The schedule of the Dutch daylight saving time change in October 2025.
- The historical case of Samoa’s 30 September clock change.
The temporal scope extends from 19 September to 26 October 2025, encompassing the observed weather event, its scientific causes, and the symbolic clock change marking autumn’s advance.
Factual Narrative
A Verified Shift: From Summer Warmth to Autumn Chill
According to the KNMI’s official report for September 2025, summer briefly returned on 19 and 20 September before a cold front swept through. Thunderstorms brought the transition to an end, followed by a notable drop in temperature from 21 September onward.
“On 19 and 20 September the summer showed itself. On 20 September the warmth was driven away by thunderstorms from the west. From 21 September it cooled considerably, especially at night.”
(translated from KNMI press release, 30 September 2025)
The KNMI’s monthly summary reinforced this:
“From 21 September it became much cooler, with daytime temperatures around 15 to 16 °C.”
(translated from KNMI Monthly Overview, September 2025)
Daily temperature data from Schiphol Airport show a clear contrast:
- 20 September: maximum 23.2 °C, minimum 16.9 °C (late-summer warmth).
- 21 September: maximum 18.1 °C, minimum 9.8 °C (autumn chill).
This cold shift occurred just one day before the autumn equinox—22 September 2025 at 20:19 (CEST)—reinforcing the feeling of a seamless handover between seasons.
The Equinox: Balance and Change
The equinox marks the moment when Earth’s axis is perfectly perpendicular to the Sun, creating nearly equal day and night lengths. After this, the Northern Hemisphere tilts gradually away from the Sun, reducing both daylight hours and solar energy received at the surface.
Although the equinox itself does not cause an instant temperature drop, the jet stream—a high-altitude current that guides weather systems—often brings polar air southward around this time. When such a cold front aligns with the equinox, as in 2025, the result is a powerful illusion: it feels as though the Sun itself decided it was time for autumn.
The Dutch Time Change: The Cultural Clock of the Season
In the Netherlands, the transition from Central European Summer Time (CEST) to Central European Time (CET) happens on the last Sunday of October. In 2025, this will occur at 03:00 on 26 October, when clocks move back one hour.
Though purely administrative, this change amplifies the emotional sense of seasonal descent—darker evenings, longer nights, and a slower rhythm that mirrors the cooling climate. It marks not only a technical adjustment but also a shared cultural signal that autumn has fully arrived.
The Daily Rhythm of Early Autumn
After late September, the Netherlands enters a phase of strong daily contrasts. Nights grow cold under clear skies as the ground loses heat rapidly—a process known as radiative cooling. Mornings are often misty, while afternoons can still feel warm as sunlight heats the surface between 10:00 and 17:00.
This balance shifts through October and November. By December, the low solar angle and shorter days make daytime warmth rare, and the air remains consistently cold, marking the arrival of true winter.
When 30 September Once Marked a Time Change
Although no European country changes its clocks on 30 September, Samoa once did. In 2012, Samoa began its daylight saving period on 30 September, moving clocks forward by one hour. The experiment ended in 2021, when the government abolished DST altogether.
This historical example shows how different nations approach the relationship between sunlight and social time—and why the European Union (EU) standardizes its changes at the ends of March and October, minimizing confusion and disruption.
Entities & Roles Index
- Netherlands — Location of the verified temperature shift and seasonal transition.
- KNMI (Royal Netherlands Meteorological Institute) — Official Dutch meteorological authority confirming the event.
- European Union (EU) — Regional body coordinating daylight saving schedules.
- Samoa — Nation that previously changed clocks on 30 September.
- Jet stream — Fast upper-atmosphere air current that steers weather systems.
- Cold front — Boundary where cold air replaces warm air, causing sudden cooling.
- Sun / Earth axis — Astronomical mechanism defining the equinox and seasonal change.
Chronology
- 19–20 September 2025: Late-summer warmth across the Netherlands.
- 20 September 2025: Thunderstorms break the warm spell.
- 21 September 2025: Significant cooling; temperatures drop sharply overnight.
- 22 September 2025 (20:19 CEST): Autumn equinox.
- 5 October 2025: Strong day–night contrasts persist.
- 26 October 2025: Dutch time change to CET.
- 30 September 2012: Samoa’s last daylight saving start.
Practical Conclusions
- KNMI data confirm that temperatures in the Netherlands fell markedly between 20 and 21 September 2025, marking the true onset of autumn.
- The autumn equinox occurred one day later, amplifying the perception of an “overnight” seasonal change.
- Radiative cooling and clear skies explain the wide day–night temperature differences typical of early autumn.
- The Dutch time change on 26 October 2025 will mark the transition to winter time (CET).
- Historical examples like Samoa’s 30 September shift illustrate the variety of global daylight saving practices.
Sources
- KNMI – Monthly Overview September 2025 (in Dutch) — Confirms warm 19–20 September and cooling from 21 September onward.
- KNMI – Press Release, 30 September 2025 (in Dutch) — Describes thunderstorms on 20 September and the following cooling.
- KNMI – Weather Observations Portal — Provides daily temperature data for Schiphol and other Dutch stations.
- Time and Date – Equinoxes and Solstices for Amsterdam — Lists equinox time: 22 September 2025 at 20:19 CEST.
- Time and Date – Time Change in the Netherlands 2025 — Confirms Dutch daylight saving time schedule for 26 October 2025.
- Time and Date – Samoa Removes DST — Details Samoa’s 2012 start and 2021 abolition of DST.
- IANA TZ Database Archive — Provides historical daylight saving rules for Samoa.
Appendix
- “Equinox” — The moment when Earth’s axis is perpendicular to the Sun, creating nearly equal day and night.
- “CET (Central European Time)” — The standard winter time zone in most of Western Europe.
- “CEST (Central European Summer Time)” — The one-hour-ahead summer schedule used in the EU.
- “Jet stream” — Fast upper-level wind that guides weather systems.
- “Cold front” — Boundary where a cold air mass replaces warmer air.
- “Radiative cooling” — Nighttime heat loss from Earth’s surface into space.
- “Samoa DST 2012” — Example of a 30 September daylight saving shift, later discontinued.
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