September is one of the warmer months of the year in Cornwall, with average highs near 21.7°C and lows around 10.5°C (mean 16.1°C). It is about average for rainfall, averaging 88 mm of rainfall and 76% relative humidity across the period.
September is one of the warmer months of the year in Cornwall. Across 45 years of records, the warmest September in Cornwall was 2015 — when the month averaged 19.6°C — while the coldest was 1995 at 13.3°C. Typical September days reach 21.7°C with nights near 10.5°C. The long-term trend is warming, about 0.8°C per decade.
September is about average for rainfall in Cornwall, averaging 88 mm of rainfall. The wettest September on record was 1981 with 161 mm, while the driest was 2023 at 24 mm.
Relative humidity in Cornwall during September averages 76%. The most humid September was 1981 at 82%, and the least humid was 2025 at 68%.
September receives about 3.9 kWh/m²/day of solar energy in Cornwall. The sunniest September was 2016 at 4.5 kWh/m²/day, and the cloudiest was 1990 at 3.1.
Average wind speed in Cornwall during September is 0.4 m/s. The windiest Septembers were 1992, 1997 and 2000, among others at 0.5 m/s, while the calmest were 2023, 2024 and 2025, among others at 0.4 m/s.
Ranked against the other 11 months using 1981–2025 averages.
| Metric | 1980s | 1990s | 2000s | 2010s | 2020s | Change |
|---|---|---|---|---|---|---|
| Avg Temp | 14.9°C | 15.0°C | 16.4°C | 17.1°C | 17.5°C | +2.6°C |
| Avg High | 20.3°C | 20.3°C | 22.1°C | 22.8°C | 23.4°C | +3.1°C |
| Avg Low | 9.5°C | 9.6°C | 10.8°C | 11.4°C | 11.5°C | +2°C |
| Avg Rain | 99 mm | 96 mm | 94 mm | 88 mm | 50 mm | -49 mm |
September temperatures in Cornwall have risen by about 2.7°C between 1981–1985 and 2021–2025.
September has ranged from a record low of -1°C (1991) to a record high of 35°C (2023).
Rainfall in September is about average for rainfall, averaging 88 mm, with a wettest year of 161 mm.
Daytime highs and overnight lows have shifted differently — highs rose about 1°C per decade while lows rose 0.6°C per decade, widening the gap between day and night.
One of the best times to visit Cornwall
September is one of the best times to visit Cornwall. Expect days averaging 21.7°C and nights around 10.5°C (mean 16.1°C) — warmer than Cornwall's yearly average — with higher rainfall (88 mm), so conditions are generally comfortable for sightseeing and outdoor plans.
| Year | Avg High | Avg Temp | Avg Low | Hottest Day | Coldest Day | Rainfall | Humidity | Wind | Solar |
|---|---|---|---|---|---|---|---|---|---|
| 2025 | 24.3°C | 17.8°C | 11.3°C | 29.1°C | 3.8°C | 52 mm | 68% | 0.4 m/s | 4.4 |
| 2024 | 24.3°C | 18.4°C | 12.5°C | 29.6°C | 6.4°C | 44 mm | 75% | 0.4 m/s | 4.3 |
| 2023 | 25.0°C | 18.5°C | 11.9°C | 35.0°C | 5.9°C | 24 mm | 71% | 0.4 m/s | 4.3 |
| 2022 | 22.1°C | 16.3°C | 10.5°C | 31.1°C | 3.5°C | 67 mm | 74% | 0.4 m/s | 3.8 |
| 2021 | 23.2°C | 17.7°C | 12.3°C | 27.9°C | 6.2°C | 73 mm | 71% | 0.4 m/s | 3.8 |
| 2020 | 21.7°C | 16.0°C | 10.3°C | 28.6°C | 1.2°C | 42 mm | 70% | 0.4 m/s | 3.6 |
| 2019 | 22.4°C | 16.3°C | 10.2°C | 28.4°C | 5.6°C | 77 mm | 75% | 0.4 m/s | 3.9 |
| 2018 | 23.9°C | 18.4°C | 12.9°C | 32.8°C | 5.6°C | 65 mm | 74% | 0.4 m/s | 3.8 |
| 2017 | 23.1°C | 17.6°C | 12.1°C | 32.4°C | 5.9°C | 52 mm | 81% | 0.4 m/s | 4.3 |
| 2016 | 24.8°C | 18.4°C | 12.0°C | 33.4°C | 3.4°C | 53 mm | 68% | 0.4 m/s | 4.5 |
| 2015 🔥 | 25.6°C | 19.6°C | 13.6°C | 33.3°C | 6.3°C | 85 mm | 71% | 0.4 m/s | 4.2 |
| 2014 | 21.8°C | 15.8°C | 9.9°C | 28.4°C | -0.1°C | 67 mm | 78% | 0.4 m/s | 3.8 |
| 2013 | 20.6°C | 14.9°C | 9.3°C | 30.5°C | 2.7°C | 109 mm | 78% | 0.4 m/s | 3.9 |
| 2012 | 21.7°C | 16.0°C | 10.4°C | 30.1°C | 3.4°C | 134 mm | 73% | 0.4 m/s | 3.8 |
| 2011 | 22.6°C | 17.5°C | 12.3°C | 28.2°C | 4.3°C | 86 mm | 81% | 0.4 m/s | 4 |
| 2010 | 21.3°C | 16.4°C | 11.4°C | 31.8°C | 5.6°C | 149 mm | 79% | 0.4 m/s | 3.3 |
| 2009 | 20.6°C | 14.9°C | 9.2°C | 25.5°C | 3.0°C | 66 mm | 78% | 0.4 m/s | 4.4 |
| 2008 | 21.8°C | 16.1°C | 10.4°C | 29.8°C | 3.0°C | 74 mm | 80% | 0.4 m/s | 3.8 |
| 2007 | 23.2°C | 17.1°C | 11.0°C | 32.1°C | 4.0°C | 74 mm | 72% | 0.4 m/s | 4.4 |
| 2006 | 20.1°C | 14.8°C | 9.6°C | 26.6°C | 3.0°C | 113 mm | 82% | 0.4 m/s | 3.3 |
| 2005 | 22.8°C | 17.3°C | 11.9°C | 29.3°C | 4.5°C | 146 mm | 78% | 0.4 m/s | 4.1 |
| 2004 | 22.4°C | 16.7°C | 11.1°C | 26.9°C | 4.1°C | 88 mm | 80% | 0.4 m/s | 4.2 |
| 2003 | 23.1°C | 17.6°C | 12.1°C | 29.8°C | 6.8°C | 85 mm | 75% | 0.4 m/s | 4 |
| 2002 | 24.7°C | 18.5°C | 12.3°C | 33.9°C | 4.3°C | 93 mm | 70% | 0.4 m/s | 4.3 |
| 2001 | 22.3°C | 16.6°C | 11.0°C | 32.2°C | 4.2°C | 98 mm | 72% | 0.4 m/s | 4.1 |
| 2000 | 19.5°C | 14.3°C | 9.0°C | 28.9°C | -0.6°C | 104 mm | 79% | 0.5 m/s | 3.6 |
| 1999 | 23.5°C | 17.8°C | 12.1°C | 34.6°C | 4.3°C | 156 mm | 76% | 0.4 m/s | 3.5 |
| 1998 | 21.2°C | 16.0°C | 10.7°C | 27.2°C | 4.0°C | 98 mm | 80% | 0.4 m/s | 3.9 |
| 1997 | 19.8°C | 14.5°C | 9.3°C | 25.5°C | 0.8°C | 91 mm | 79% | 0.5 m/s | 3.4 |
| 1996 | 21.0°C | 16.3°C | 11.6°C | 27.8°C | 4.6°C | 117 mm | 82% | 0.4 m/s | 3.7 |
| 1995 ❄️ | 19.4°C | 13.3°C | 7.2°C | 27.3°C | 1.5°C | 66 mm | 71% | 0.4 m/s | 4 |
| 1994 | 20.2°C | 14.8°C | 9.4°C | 26.3°C | 4.7°C | 68 mm | 80% | 0.4 m/s | 3.8 |
| 1993 | 19.0°C | 13.8°C | 8.6°C | 28.0°C | 0.2°C | 125 mm | 79% | 0.4 m/s | 3.4 |
| 1992 | 20.1°C | 14.8°C | 9.6°C | 27.5°C | 1.2°C | 94 mm | 79% | 0.5 m/s | 3.4 |
| 1991 | 19.5°C | 14.0°C | 8.5°C | 28.2°C | -1.0°C | 79 mm | 73% | 0.4 m/s | 3.8 |
| 1990 | 19.7°C | 14.2°C | 8.6°C | 26.9°C | 1.6°C | 71 mm | 77% | 0.4 m/s | 3.1 |
| 1989 | 21.2°C | 15.6°C | 10.0°C | 29.0°C | -0.3°C | 89 mm | 76% | 0.4 m/s | 3.7 |
| 1988 | 20.6°C | 14.9°C | 9.1°C | 27.3°C | 1.6°C | 73 mm | 73% | 0.5 m/s | 3.9 |
| 1987 | 19.9°C | 15.0°C | 10.1°C | 25.9°C | 3.0°C | 108 mm | 79% | 0.4 m/s | 3.8 |
| 1986 | 18.6°C | 13.4°C | 8.3°C | 23.7°C | 2.7°C | 139 mm | 81% | 0.4 m/s | 3.3 |
| 1985 | 21.7°C | 15.8°C | 9.9°C | 28.1°C | 3.1°C | 106 mm | 76% | 0.4 m/s | 4.1 |
| 1984 | 19.6°C | 13.9°C | 8.3°C | 26.1°C | 1.6°C | 39 mm | 75% | 0.4 m/s | 4.4 |
| 1983 | 22.6°C | 16.8°C | 11.1°C | 30.8°C | 4.4°C | 70 mm | 72% | 0.4 m/s | — |
| 1982 | 20.0°C | 14.6°C | 9.3°C | 27.9°C | 3.9°C | 110 mm | 82% | 0.4 m/s | — |
| 1981 🌧️ | 18.6°C | 14.1°C | 9.5°C | 26.9°C | -0.8°C | 161 mm | 82% | 0.4 m/s | — |
Actual recorded September data for each year, 1981–2025 — sourced from NASA POWER.
The average high in September is around 21.7°C with lows near 10.5°C (mean 16.1°C), based on 45 years of NASA POWER data.
September averages 88 mm of rainfall in Cornwall, about average for rainfall. The wettest on record was 161 mm in 1981.
The hottest September reached 35°C in 2023. The coldest dipped to -1°C in 1991.
Comparing the earliest years on record (1981–1985) with the most recent (2021–2025), average September temperatures have risen by about 2.7°C. Daytime highs moved from 20.5°C to 23.8°C and overnight lows from 9.6°C to 11.7°C over the same span.
September is one of the warmer months of the year in Cornwall and about average for rainfall, so plan for around 21.7°C daytime highs and 10.5°C at night.
Not quite — September is the 9th coolest month in Cornwall, averaging 16.1°C.
For September in Cornwall, pack light layers you can adjust through the day, with typical highs of 21.7°C and lows of 10.5°C. Bring an umbrella or light rain jacket too.
Relative humidity in September averages 76% — quite humid.
Average wind speed in September is 0.4 m/s. The windiest September on record averaged 0.5 m/s (1988).
The wettest September on record was 1981 with 161 mm. On average, September sees 88 mm.
All figures are derived from NASA POWER (Prediction Of Worldwide Energy Resources), based on MERRA-2 satellite reanalysis, covering 1981–2025.
Explore how the weather in Cornwall behaves each September, based on 45 years of NASA POWER climate data from 1981 to 2025. This page breaks down average temperatures, rainfall, humidity, wind and solar radiation for September specifically, along with year-by-year records and how the month has changed over the decades.
