The handover month
Every September, something almost invisible happens across the entire northern half of the planet. As the Earth continues its orbit, our hemisphere tips gradually away from the sun, and the balance of energy shifts: less solar warmth arriving each day, and a longer dark night to shed what accumulated. The atmospheric machinery responds. The Azores High — that great slab of settled, sinking air that guarded our summer — begins to retreat south and weaken. In its place, the jet stream, which spent July and August meandering lazily to our north, starts to dip south and gather pace. By the end of the month, it’s aimed squarely at the British Isles.
The result, for us, is a month of two faces. Early September can be glorious: still-warm seas, soft light, gentle mornings. Late September can feel like somebody flipped a switch — the first proper gales arrive, trees begin shedding, and the rain gauge at Monkton Heathfield wakes up after a sleepy August. Meteorologists call it the transition season. Gardeners call it the beginning of the tidy-up. Anyone who keeps a weather station simply calls it interesting again.
Sea lag: why the coast stays warm while the fields cool
Here’s a small September curiosity worth knowing, and it’s physics, not folklore. From mid-August onward, our land cools quickly as nights lengthen, but the sea holds its heat stubbornly — water warms and cools far more slowly than earth and air. Roughly 70% of the planet’s surface is ocean, and in September all that stored summer heat becomes the hemisphere’s memory of the season just gone.
Locally, that means coastal Somerset often basks in sea breezes feeding off 16–17°C Channel waters while inland parishes like ours, fifteen miles from the coast, are already reaching for a jumper by dusk. That temperature difference between warm sea and cooling land is also fuel. As the first autumn low-pressure systems sweep in, that stored sea heat can intensify rainfall — one reason early-autumn storms sometimes catch everyone off guard with their vigour.
The first gales
Ask anyone who’s lived on the Levels long enough: the first real gale of the season usually arrives in the last ten days of September. A week or two before the storm season proper opens in October, it’s when you first hear the wind in the chimney again and find the wheelie bin has relocated overnight. Watch the wind gauge on the Today page in the second half of the month. When the daily peak starts knocking on 60 km/h — a number we barely flirted with all summer — the hemisphere’s changeover has formally reached Somerset. The tree that loses a branch in the first September gale was probably weakened all along; the wind merely holds the audit.
Dew, mist and the shortening mornings
August nights in Somerset can still feel muggy. September nights cannot. As ground temperatures fall, the air nearest the surface reaches its dew point more easily, and mornings acquire that low sparkle of condensation across grass and cars. This is also the dawn of mist season. The Vale of Taunton and the quiet lanes around Monkton Heathfield are classic mist country — clear skies overnight, damp ground from the year’s growing rainfall totals, and cooling air. Keep an eye on the humidity readings through the month: the gap between afternoon and early-morning values widens noticeably, and on calm, clear nights you can watch relative humidity climb towards saturation in real time before the mist forms in the dips.
The borrowed warmth: summer’s last word
Almost every September delivers one final flourish — a short spell of warm, sunny, settled weather. In older European tradition it was called St Luke’s little summer, after the saint’s feast day in October; North Americans coined “Indian summer.”
Whatever the name, the mechanism is the same and it’s planetary in scale: the jet stream briefly bulges north, allowing continental or Azorean warmth back in for three or four days before the poleward energy drain resumes. These spells are precious precisely because they’re finite. The smart September forecaster treats them as a gift rather than a trend: enjoy the warmth, but don’t be surprised when the next Atlantic frontal system sweeps it away with a band of rain and a 20-degree swing in the maximum temperature.
What your rain gauge will tell you
For our corner of Somerset, September rainfall averages somewhere in the region of 70–80 mm, typically spread across 10–13 rain days. But averages deceive: the month divides cleanly. Early September can be almost dry; late September can produce a single frontal system that deposits a fifth of the month’s rain in one wet afternoon.
This year, keep an eye on the “days since significant rainfall” figure on the Today page. Watching it reset as the first autumn fronts arrive is one of the small pleasures of running a home station — summer’s long, dry counts versus autumn’s frequent refreshing.
And there’s another September phenomenon worth mentioning, and it’s the most planetary of all. The remains of Atlantic hurricanes, exhausted of their tropical fury, occasionally ride the jet stream across the ocean and arrive on our shores as deep areas of ordinary-but-persistent low pressure — no longer hurricanes, but capable of drenching rain. It’s one of the quieter ways the tropics touch Somerset.
Enjoying September’s weather, whatever it brings
The best way to think about September is as a negotiation between two seasons, with the planet, not the forecast, as referee. Morning can belong to autumn and afternoon to summer, all in the same day. Dress for the morning, bring hope for the afternoon. Take the early walks while they last — the soft early-autumn light around Monkton Heathfield, hedgerows heavy with blackberries, the first tinge of yellow in the horse chestnuts. If a settled spell arrives, make the most of it. And when the first gale rattles through, there’s a satisfaction in watching the gauges respond: the wind speed climbing, the pressure falling, the rain ticking in.
Winter is not here yet. But September is when you first hear the hemisphere clearing its throat.