September is one of the warmer months of the year in Thornton, with average highs near 15.7°C and lows around 11.5°C (mean 13.6°C). It is about average for rainfall, averaging 95 mm of rainfall and 85% relative humidity across the period.
September is one of the warmer months of the year in Thornton. Across 45 years of records, the warmest September in Thornton was 2023 — when the month averaged 15.8°C — while the coldest was 1986 at 11.3°C. Typical September days reach 15.7°C with nights near 11.5°C. The long-term trend is warming, about 0.3°C per decade.
September is about average for rainfall in Thornton, averaging 95 mm of rainfall. The wettest September on record was 2025 with 184 mm, while the driest was 1986 at 17 mm.
Relative humidity in Thornton during September averages 85%. The most humid September was 1985 at 89%, and the least humid was 1991 at 81%.
September receives about 2.9 kWh/m²/day of solar energy in Thornton. The sunniest Septembers were 1986 and 1991 at 3.4 kWh/m²/day, and the cloudiest were 1993 and 2000 at 2.5.
Average wind speed in Thornton during September is 4.6 m/s. The windiest September was 1983 at 5.8 m/s, while the calmest was 2014 at 2.9 m/s.
Ranked against the other 11 months using 1981–2025 averages.
| Metric | 1980s | 1990s | 2000s | 2010s | 2020s | Change |
|---|---|---|---|---|---|---|
| Avg Temp | 13.0°C | 13.3°C | 14.0°C | 13.6°C | 14.5°C | +1.5°C |
| Avg High | 14.9°C | 15.4°C | 16.0°C | 15.7°C | 16.7°C | +1.8°C |
| Avg Low | 11.1°C | 11.1°C | 11.9°C | 11.6°C | 12.3°C | +1.2°C |
| Avg Rain | 95 mm | 83 mm | 92 mm | 101 mm | 113 mm | +17 mm |
September temperatures in Thornton have risen by about 1.3°C between 1981–1985 and 2021–2025.
September has ranged from a record low of 6°C (1993) to a record high of 23°C (2021).
Rainfall in September is about average for rainfall, averaging 95 mm, with a wettest year of 184 mm.
One of the best times to visit Thornton
September is one of the best times to visit Thornton. Expect days averaging 15.7°C and nights around 11.5°C (mean 13.6°C) — warmer than Thornton's yearly average — with higher rainfall (95 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 🌧️ | 16.0°C | 13.9°C | 11.8°C | 18.5°C | 7.9°C | 184 mm | 84% | 4.9 m/s | 2.8 |
| 2024 | 15.8°C | 13.6°C | 11.4°C | 21.1°C | 7.5°C | 91 mm | 85% | 4.6 m/s | 2.7 |
| 2023 🔥 | 18.0°C | 15.8°C | 13.6°C | 22.9°C | 9.9°C | 146 mm | 86% | 4.4 m/s | 2.8 |
| 2022 | 16.9°C | 14.5°C | 12.2°C | 20.3°C | 8.5°C | 106 mm | 84% | 4.4 m/s | 3 |
| 2021 | 17.8°C | 15.5°C | 13.3°C | 23.0°C | 10.3°C | 78 mm | 86% | 3.6 m/s | 2.8 |
| 2020 | 15.8°C | 13.5°C | 11.3°C | 19.8°C | 6.8°C | 71 mm | 85% | 4.6 m/s | 3.1 |
| 2019 | 15.7°C | 13.8°C | 11.9°C | 19.0°C | 9.7°C | 141 mm | 85% | 4.7 m/s | 2.9 |
| 2018 | 15.2°C | 13.2°C | 11.2°C | 19.5°C | 7.5°C | 93 mm | 83% | 5.2 m/s | 2.8 |
| 2017 | 15.2°C | 13.3°C | 11.5°C | 18.9°C | 8.7°C | 146 mm | 87% | 4.7 m/s | 2.7 |
| 2016 | 17.2°C | 15.2°C | 13.2°C | 21.6°C | 10.6°C | 93 mm | 86% | 4.7 m/s | 2.9 |
| 2015 | 15.1°C | 12.7°C | 10.2°C | 18.0°C | 8.5°C | 52 mm | 85% | 3.8 m/s | 3.3 |
| 2014 | 17.1°C | 14.5°C | 12.0°C | 19.4°C | 10.0°C | 17 mm | 85% | 2.9 m/s | 3.1 |
| 2013 | 15.2°C | 13.2°C | 11.1°C | 19.3°C | 8.7°C | 76 mm | 86% | 4.5 m/s | 2.6 |
| 2012 | 14.3°C | 12.5°C | 10.7°C | 18.3°C | 6.4°C | 155 mm | 86% | 5.3 m/s | 2.8 |
| 2011 | 15.9°C | 14.1°C | 12.3°C | 20.5°C | 9.9°C | 116 mm | 86% | 5.5 m/s | 2.8 |
| 2010 | 15.8°C | 13.7°C | 11.6°C | 18.9°C | 6.3°C | 125 mm | 86% | 5 m/s | 2.8 |
| 2009 | 15.5°C | 13.7°C | 11.8°C | 19.1°C | 8.7°C | 58 mm | 86% | 4.7 m/s | 2.8 |
| 2008 | 15.4°C | 13.4°C | 11.4°C | 17.9°C | 8.9°C | 113 mm | 88% | 4.4 m/s | 2.6 |
| 2007 | 15.4°C | 13.3°C | 11.3°C | 18.0°C | 6.3°C | 79 mm | 87% | 4.8 m/s | 2.9 |
| 2006 | 17.9°C | 15.6°C | 13.4°C | 22.1°C | 10.2°C | 92 mm | 87% | 4.5 m/s | 3.1 |
| 2005 | 16.5°C | 14.4°C | 12.3°C | 21.5°C | 8.4°C | 90 mm | 85% | 4.8 m/s | 2.9 |
| 2004 | 15.9°C | 14.0°C | 12.0°C | 20.3°C | 8.4°C | 102 mm | 87% | 5.7 m/s | 3.1 |
| 2003 | 16.2°C | 14.0°C | 11.8°C | 19.6°C | 8.5°C | 96 mm | 84% | 3.4 m/s | 3 |
| 2002 | 16.2°C | 14.0°C | 11.7°C | 19.3°C | 9.2°C | 51 mm | 84% | 3.1 m/s | 3.1 |
| 2001 | 15.1°C | 13.0°C | 10.8°C | 17.7°C | 8.2°C | 83 mm | 87% | 5.1 m/s | 2.6 |
| 2000 | 16.2°C | 14.1°C | 12.0°C | 20.7°C | 8.5°C | 160 mm | 87% | 4.4 m/s | 2.5 |
| 1999 | 16.8°C | 14.6°C | 12.4°C | 20.8°C | 10.0°C | 116 mm | 87% | 4.2 m/s | 3 |
| 1998 | 15.9°C | 13.8°C | 11.7°C | 20.2°C | 8.4°C | 87 mm | 88% | 4.5 m/s | 3 |
| 1997 | 15.6°C | 13.5°C | 11.5°C | 18.0°C | 8.7°C | 65 mm | 85% | 4.9 m/s | 2.9 |
| 1996 | 15.8°C | 13.4°C | 11.0°C | 19.5°C | 8.1°C | 42 mm | 83% | 4.5 m/s | 3 |
| 1995 | 15.5°C | 13.4°C | 11.3°C | 18.5°C | 7.9°C | 94 mm | 84% | 4.1 m/s | 3 |
| 1994 | 13.9°C | 12.1°C | 10.2°C | 17.1°C | 8.2°C | 107 mm | 86% | 4.8 m/s | 2.8 |
| 1993 | 14.5°C | 12.2°C | 9.9°C | 17.9°C | 6.0°C | 86 mm | 84% | 4.8 m/s | 2.5 |
| 1992 | 14.7°C | 12.8°C | 10.8°C | 19.5°C | 8.6°C | 99 mm | 86% | 5.3 m/s | 3 |
| 1991 | 16.6°C | 14.0°C | 11.4°C | 21.8°C | 6.5°C | 58 mm | 81% | 4.4 m/s | 3.4 |
| 1990 | 14.6°C | 12.7°C | 10.8°C | 18.0°C | 7.7°C | 72 mm | 82% | 4.9 m/s | 2.8 |
| 1989 | 15.7°C | 13.6°C | 11.4°C | 18.7°C | 9.6°C | 27 mm | 84% | 4.3 m/s | 2.8 |
| 1988 | 14.2°C | 12.6°C | 10.9°C | 20.7°C | 6.1°C | 104 mm | 85% | 5.4 m/s | 2.6 |
| 1987 | 14.6°C | 12.9°C | 11.1°C | 18.3°C | 6.6°C | 111 mm | 84% | 4.8 m/s | 3.1 |
| 1986 ❄️ | 13.4°C | 11.3°C | 9.2°C | 16.1°C | 6.1°C | 17 mm | 82% | 3.8 m/s | 3.4 |
| 1985 | 15.4°C | 13.5°C | 11.6°C | 18.9°C | 8.5°C | 106 mm | 89% | 4.7 m/s | 2.7 |
| 1984 | 14.8°C | 13.1°C | 11.4°C | 18.7°C | 8.6°C | 144 mm | 83% | 4.8 m/s | 2.7 |
| 1983 | 14.7°C | 12.8°C | 11.0°C | 17.3°C | 7.7°C | 118 mm | 85% | 5.8 m/s | — |
| 1982 | 15.5°C | 13.4°C | 11.3°C | 20.3°C | 8.3°C | 95 mm | 86% | 4.6 m/s | — |
| 1981 | 16.0°C | 14.0°C | 12.0°C | 19.9°C | 9.7°C | 137 mm | 84% | 4.9 m/s | — |
Actual recorded September data for each year, 1981–2025 — sourced from NASA POWER.
The average high in September is around 15.7°C with lows near 11.5°C (mean 13.6°C), based on 45 years of NASA POWER data.
September averages 95 mm of rainfall in Thornton, about average for rainfall. The wettest on record was 184 mm in 2025.
The hottest September reached 23°C in 2021. The coldest dipped to 6°C in 1993.
Comparing the earliest years on record (1981–1985) with the most recent (2021–2025), average September temperatures have risen by about 1.3°C. Daytime highs moved from 15.3°C to 16.9°C and overnight lows from 11.5°C to 12.5°C over the same span.
September is one of the warmer months of the year in Thornton and about average for rainfall, so plan for around 15.7°C daytime highs and 11.5°C at night.
Not quite — September is the 10th coolest month in Thornton, averaging 13.6°C.
For September in Thornton, pack light layers you can adjust through the day, with typical highs of 15.7°C and lows of 11.5°C. Bring an umbrella or light rain jacket too.
Relative humidity in September averages 85% — quite humid.
Average wind speed in September is 4.6 m/s. The windiest September on record averaged 5.8 m/s (1983).
The wettest September on record was 2025 with 184 mm. On average, September sees 95 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 Thornton 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.
