The calendar can declare autumn while the afternoon still feels
like July. That is not a mistake: it is the result of three different
ways of defining a season.
On 1 September, British weather forecasters begin talking about
autumn. Leaves may still be green. Schools may still be on holiday. A
warm spell may have people searching for the fan they put away too
soon.
The date feels especially odd after a very hot August. In 2026,
provisional Met Office figures made the point dramatically: the UK had
just experienced its hottest summer in a temperature record stretching
back to 1884, with an average temperature of 16.5°C.
Yet summer ended, officially, at midnight on 31 August.
“Officially” is doing a lot of work there. Meteorologists,
astronomers and everyday culture each use a different clock.
The neat season used by
weather records
Meteorological summer in the Northern Hemisphere is June, July and
August. Autumn is September, October and November; winter is December
through February; spring is March through May.
This arrangement is deliberately plain. Each season contains three
complete calendar months, so observations can be grouped and compared
without the start date moving from year to year.
Imagine trying to calculate a century of autumn rainfall when autumn
begins at a different hour on 22 or 23 September and ends at another
changing moment in December. It can be done, but it creates unnecessary
complexity. Fixed months make monthly and seasonal statistics easier to
compare between places and periods.
Meteorological seasons are therefore administrative tools based on
the annual temperature cycle. In Britain, the warmest three-month block
is usually June to August. Calling it summer creates a useful,
consistent dataset.
It does not compel the atmosphere to behave seasonally on
command.
The astronomical season
Astronomical autumn begins at the September equinox, usually on 22 or
23 September in the Northern Hemisphere.
Earth’s axis is tilted by about 23.5 degrees relative to its orbit
around the Sun. During part of the year, the Northern Hemisphere leans
towards the Sun and receives longer days and more direct sunlight.
During the other part, it leans away.
At an equinox, neither hemisphere is tilted strongly towards or away
from the Sun. Day and night are often described as equal, although
refraction and the way sunrise is defined mean they are not precisely
twelve hours everywhere.
Astronomical seasons follow these points in Earth’s orbit: the two
equinoxes and two solstices. Their dates shift slightly because a year
is not an exact number of days and our leap-year calendar is an
approximation designed to keep civil time aligned with the orbit.
This definition explains the changing light. It is less convenient
for comparing monthly weather.
Then there is the
season people actually feel
Human seasons are untidy. They are made from school terms, harvests,
clothes, television schedules, flowers, birds and the smell of the
air.
For one person, autumn begins when the evenings first become
noticeably dark. For another, it is the first fallen conker, the return
of a coat or the moment a supermarket replaces barbecue charcoal with
Halloween sweets.
Ecologists sometimes use phenology: the study of recurring natural
events such as flowering, leaf fall, migration and egg-laying. These
biological calendars shift with local conditions. Spring can arrive
earlier in a sheltered southern garden than on a northern hillside. A
drought can make trees shed leaves for stress rather than because autumn
has arrived.
This lived season is real, but it is difficult to standardise. Its
boundaries vary by species, place and observer.
How can an
entire country have one temperature?
When the Met Office reports a UK seasonal average, it does not mean
that every place experienced 16.5°C or that the thermometer sat at that
temperature all day.
Weather stations record conditions at many locations. Scientists
quality-check the observations and use them to estimate values across a
grid covering the country. Daily maximum and minimum temperatures
contribute to mean temperature, and the gridded values can then be
averaged across the UK and across the three-month season.
A national mean is consequently a summary of a huge number of
measurements. It smooths over the cool coast, the hot city afternoon,
the upland night and the wet western valley. That makes it valuable for
comparison, but it should not be mistaken for anyone’s personal
summer.
The first announcement is often provisional because late observations
and quality checks can still alter the final figure slightly.
Records are not all the
same record
“Hottest summer” can refer to several measures. There is the highest
mean temperature, the highest average maximum, the warmest average
night-time minimum or the single hottest day. One year does not
necessarily lead every list.
The length of the record matters too. Reliable national series have
starting dates determined by the availability and quality of past
observations. Saying “since records began” is incomplete unless the
record and its start date are identified.
For the 2026 announcement, the relevant UK mean-temperature series
begins in 1884. The Met Office also placed the result in a longer
climate context. Its analysis found that human-caused climate change had
made a summer this hot far more likely than it would have been in a
natural climate.
One hot season is weather. A sustained shift in the odds and averages
is climate.
Why the argument never
quite goes away
People sometimes treat the two formal definitions as rivals: either
autumn starts on 1 September or it starts at the equinox. In fact they
answer different questions.
If you are comparing rainfall and temperature across decades, the
meteorological season is usually the useful one. If you are explaining
daylight and Earth’s orbit, use the astronomical season. If you are
deciding whether it feels like autumn, neither institution can overrule
you.
The discrepancy is a reminder that categories are tools. We draw a
line through a continuous natural cycle because lines help us count,
compare and communicate.
Summer does not end like a television programme. The Sun does not
observe our calendar, and warm air can cross 1 September without filling
in a form. What changes is the question we are asking of it.
Quick facts
- Meteorological autumn runs from 1 September to 30 November.
- Astronomical autumn begins at the September equinox, usually on 22
or 23 September. - The dates of astronomical seasons vary slightly because the calendar
and Earth’s orbit do not divide perfectly. - The UK’s 2026 summer mean was provisionally reported as 16.5°C, the
highest in a series beginning in 1884. - A national mean combines many measurements and is not the
temperature experienced in every location.
The Knowledge Back Catalogue 
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