Weather in United Kingdom in September

September climate overview

Daytime highs across United Kingdom in September run from 17.5°C (Cornwall) to 18.6°C (London). Even in London only 3% of days reach 25°C. Nights average 11.9°C; the coldest 10% of nights in Cornwall fall to 10°C or below.

Rainfall in September

Expect around 10 rainy days (1 mm or more) in September, with London the driest (9 days) and Cornwall the wettest (11 days). The wettest September at London between 1991 and 2020 was 1998 with 108 mm; the driest was 1997 with 10 mm.

Sea temperature

The sea averages 16.0°C around United Kingdom in September. That is cold for swimming without a wetsuit.

Sunshine and daylight

Around 4.2 hours of sunshine a day on average, out of 12.4 hours of daylight (sunshine estimated from cloud cover).

September against the rest of the year in United Kingdom

September is the 4th warmest month of the twelve in United Kingdom. It is 2.1°C cooler than August and 3.5°C cooler in October. The hottest month here is July (20°C) and the coolest January (8°C).

Which part of United Kingdom in September?

• Most sunshine: London, 4.2 h a day.
• Driest: London, about 9 rainy days.
• Warmest sea: Cornwall, 16°C.
• Hottest: London, 19°C by day.
• Coolest (better for walking and cities): Cornwall, 18°C.

What to pack for United Kingdom in September

Based on the averages: light layers - warm in the sun, cooler in the shade; a warm layer for evenings, when it drops to around 12°C; a rain jacket - roughly 10 wet days.

Similar weather elsewhere in September

Within 1.5°C of United Kingdom's September daytime average: Bled (20°C), Krakow (19°C), Amsterdam (18°C, sea 17°C), Cape Town (18°C, sea 15°C). See the full ranking for September.

London for warmer days and fewer rainy days

London is the better fit if your priority is daytime warmth and fewer interruptions from rain. Its September average of 19°C is slightly warmer than Cornwall, though the difference is too small to make warmth alone the deciding factor. With 9 rainy days, it still pays to have indoor alternatives rather than filling every day with outdoor plans. Nights average 11°C, so a comfortable afternoon does not mean you can leave your jumper behind for the evening. Pack light layers and a waterproof jacket, and treat any genuinely hot spell as a bonus rather than the basis of your trip.

Cornwall for mild coastal days, not warm-water swimming

Cornwall suits travellers who are happy to spend time by the sea without expecting warm water or reliably dry days. Daytime temperatures average 18°C, while nights at 13°C are milder than London's. That makes the evening temperature less of a drop, but an extra layer is still sensible. The sea averages 16°C, which will feel cool for swimming; a wetsuit is worth considering if you plan to stay in the water. With 11 rainy days, Cornwall is the wetter option in this comparison. Keep coastal outings flexible and choose your time outdoors around the forecast rather than assuming beach weather.