September is one of the cooler months of the year in Halfmoon Bay, with average highs near 10.6°C and lows around 7.5°C (mean 9.1°C). It is about average for rainfall, averaging 98 mm of rainfall and 82% relative humidity across the period.
September is one of the cooler months of the year in Halfmoon Bay. Across 45 years of records, the warmest September in Halfmoon Bay was 1989 — when the month averaged 10.6°C — while the coldest was 1992 at 7.3°C. Typical September days reach 10.6°C with nights near 7.5°C. The long-term September temperature trend is broadly flat.
September is about average for rainfall in Halfmoon Bay, averaging 98 mm of rainfall. The wettest September on record was 2010 with 166 mm, while the driest was 1989 at 26 mm.
Relative humidity in Halfmoon Bay during September averages 82%. The most humid September was 1991 at 85%, and the least humid was 2025 at 79%.
September receives about 3.2 kWh/m²/day of solar energy in Halfmoon Bay. The sunniest September was 1985 at 3.8 kWh/m²/day, and the cloudiest were 1995, 2010 and 2020 at 2.9.
Average wind speed in Halfmoon Bay during September is 4.5 m/s. The windiest September was 2025 at 5.8 m/s, while the calmest was 1984 at 3.2 m/s.
Ranked against the other 11 months using 1981–2025 averages.
| Metric | 1980s | 1990s | 2000s | 2010s | 2020s | Change |
|---|---|---|---|---|---|---|
| Avg Temp | 9.2°C | 8.5°C | 9.3°C | 9.0°C | 9.3°C | +0.1°C |
| Avg High | 10.8°C | 10.0°C | 10.9°C | 10.5°C | 10.9°C | +0.1°C |
| Avg Low | 7.5°C | 7.0°C | 7.8°C | 7.4°C | 7.7°C | +0.2°C |
| Avg Rain | 97 mm | 80 mm | 110 mm | 94 mm | 118 mm | +20 mm |
September temperatures in Halfmoon Bay have stayed relatively stable across the record.
September has ranged from a record low of 3.1°C (1997) to a record high of 16.9°C (2023).
Rainfall in September is about average for rainfall, averaging 98 mm, with a wettest year of 166 mm.
A quieter season to visit Halfmoon Bay
If your dates are flexible, the more temperate months of February and March tend to be the most comfortable for visiting Halfmoon Bay. That said, September — with days averaging 10.6°C and nights around 7.5°C (mean 9.1°C) and higher rainfall (98 mm) — can still be rewarding for travellers who don't mind the cooler conditions, and often means fewer crowds.
| Year | Avg High | Avg Temp | Avg Low | Hottest Day | Coldest Day | Rainfall | Humidity | Wind | Solar |
|---|---|---|---|---|---|---|---|---|---|
| 2025 | 11.0°C | 9.3°C | 7.7°C | 14.0°C | 5.2°C | 153 mm | 79% | 5.8 m/s | 3.1 |
| 2024 | 10.8°C | 9.3°C | 7.7°C | 15.0°C | 4.1°C | 122 mm | 81% | 5.2 m/s | 3 |
| 2023 | 11.3°C | 9.7°C | 8.1°C | 16.9°C | 6.1°C | 90 mm | 81% | 4.6 m/s | 3.2 |
| 2022 | 10.9°C | 9.4°C | 7.8°C | 15.4°C | 4.4°C | 62 mm | 82% | 3.5 m/s | 3.3 |
| 2021 | 10.6°C | 9.0°C | 7.3°C | 13.6°C | 5.3°C | 147 mm | 82% | 5 m/s | 3.1 |
| 2020 | 10.7°C | 9.2°C | 7.7°C | 14.2°C | 3.2°C | 133 mm | 81% | 5.1 m/s | 2.9 |
| 2019 | 10.3°C | 8.9°C | 7.4°C | 13.2°C | 4.9°C | 66 mm | 81% | 5 m/s | 3.4 |
| 2018 | 11.0°C | 9.5°C | 7.9°C | 15.1°C | 5.4°C | 45 mm | 81% | 4.1 m/s | 3.4 |
| 2017 | 10.8°C | 9.2°C | 7.7°C | 16.5°C | 4.5°C | 120 mm | 85% | 4 m/s | 3.2 |
| 2016 | 11.2°C | 9.7°C | 8.1°C | 13.8°C | 4.8°C | 47 mm | 84% | 3.8 m/s | 3.3 |
| 2015 | 9.5°C | 7.9°C | 6.3°C | 13.4°C | 3.9°C | 81 mm | 83% | 4.2 m/s | 3.3 |
| 2014 | 10.7°C | 9.3°C | 7.8°C | 14.4°C | 4.8°C | 66 mm | 82% | 4 m/s | 3.3 |
| 2013 | 10.5°C | 8.9°C | 7.4°C | 14.1°C | 4.6°C | 109 mm | 82% | 4.5 m/s | 3.1 |
| 2012 | 11.1°C | 9.5°C | 7.9°C | 14.7°C | 4.7°C | 123 mm | 81% | 4.3 m/s | 3.1 |
| 2011 | 9.9°C | 8.4°C | 7.0°C | 13.2°C | 3.8°C | 119 mm | 83% | 4.6 m/s | 3.2 |
| 2010 🌧️ | 9.9°C | 8.4°C | 6.9°C | 13.6°C | 3.5°C | 166 mm | 82% | 5.2 m/s | 2.9 |
| 2009 | 10.9°C | 9.4°C | 7.9°C | 14.8°C | 5.7°C | 91 mm | 82% | 4.4 m/s | 3 |
| 2008 | 11.1°C | 9.6°C | 8.1°C | 16.0°C | 5.1°C | 137 mm | 82% | 4.5 m/s | 3 |
| 2007 | 10.7°C | 9.1°C | 7.5°C | 15.0°C | 4.7°C | 97 mm | 83% | 3.8 m/s | 3.2 |
| 2006 | 11.2°C | 9.6°C | 8.1°C | 15.1°C | 5.2°C | 132 mm | 81% | 5.1 m/s | 3.3 |
| 2005 | 11.1°C | 9.4°C | 7.8°C | 14.3°C | 3.7°C | 90 mm | 83% | 4.1 m/s | 3.2 |
| 2004 | 9.8°C | 8.4°C | 7.1°C | 13.5°C | 3.7°C | 118 mm | 82% | 4.9 m/s | 3 |
| 2003 | 9.9°C | 8.4°C | 6.9°C | 12.6°C | 3.9°C | 130 mm | 81% | 4.8 m/s | 3.1 |
| 2002 | 11.0°C | 9.3°C | 7.6°C | 13.9°C | 4.4°C | 142 mm | 81% | 5.4 m/s | 3 |
| 2001 | 12.2°C | 10.5°C | 8.8°C | 15.1°C | 6.5°C | 81 mm | 82% | 4.4 m/s | 3.2 |
| 2000 | 10.8°C | 9.3°C | 7.8°C | 13.9°C | 5.2°C | 78 mm | 81% | 5.1 m/s | 3.2 |
| 1999 | 11.2°C | 9.6°C | 7.9°C | 14.2°C | 4.8°C | 107 mm | 83% | 4.5 m/s | 3.1 |
| 1998 | 10.6°C | 9.1°C | 7.6°C | 14.7°C | 3.9°C | 115 mm | 81% | 5.1 m/s | 3.1 |
| 1997 | 9.2°C | 7.6°C | 5.9°C | 13.0°C | 3.1°C | 45 mm | 80% | 3.7 m/s | 3.3 |
| 1996 | 11.7°C | 10.0°C | 8.3°C | 15.6°C | 6.0°C | 67 mm | 83% | 3.6 m/s | 3.2 |
| 1995 | 9.4°C | 7.9°C | 6.4°C | 14.3°C | 3.2°C | 127 mm | 83% | 4.3 m/s | 2.9 |
| 1994 | 9.0°C | 7.5°C | 6.0°C | 12.2°C | 3.4°C | 120 mm | 83% | 4.5 m/s | 3.2 |
| 1993 | 9.6°C | 8.3°C | 7.1°C | 14.3°C | 4.9°C | 71 mm | 81% | 4.6 m/s | 3.2 |
| 1992 ❄️ | 9.0°C | 7.3°C | 5.6°C | 12.8°C | 3.9°C | 42 mm | 82% | 3.5 m/s | 3.5 |
| 1991 | 10.4°C | 9.0°C | 7.6°C | 14.5°C | 6.1°C | 63 mm | 85% | 4.1 m/s | 3.2 |
| 1990 | 10.3°C | 8.8°C | 7.3°C | 13.1°C | 5.0°C | 48 mm | 80% | 4.5 m/s | 3.4 |
| 1989 🔥 | 12.5°C | 10.6°C | 8.7°C | 15.4°C | 5.0°C | 26 mm | 83% | 3.3 m/s | 3.6 |
| 1988 | 10.9°C | 9.4°C | 7.9°C | 14.1°C | 4.7°C | 116 mm | 85% | 4.3 m/s | 3 |
| 1987 | 10.1°C | 8.5°C | 6.8°C | 12.5°C | 4.1°C | 110 mm | 81% | 5.3 m/s | 3.1 |
| 1986 | 10.4°C | 8.9°C | 7.3°C | 14.6°C | 4.2°C | 93 mm | 82% | 4.4 m/s | 3.6 |
| 1985 | 11.8°C | 9.9°C | 8.0°C | 15.0°C | 3.7°C | 78 mm | 81% | 4.1 m/s | 3.8 |
| 1984 | 11.0°C | 9.2°C | 7.4°C | 13.8°C | 4.6°C | 99 mm | 84% | 3.2 m/s | 3.6 |
| 1983 | 10.0°C | 8.4°C | 6.8°C | 13.5°C | 4.5°C | 113 mm | 82% | 4.7 m/s | — |
| 1982 | 10.6°C | 9.1°C | 7.5°C | 12.6°C | 5.1°C | 89 mm | 81% | 3.7 m/s | — |
| 1981 | 10.3°C | 8.9°C | 7.4°C | 12.7°C | 4.8°C | 153 mm | 81% | 5.7 m/s | — |
Actual recorded September data for each year, 1981–2025 — sourced from NASA POWER.
The average high in September is around 10.6°C with lows near 7.5°C (mean 9.1°C), based on 45 years of NASA POWER data.
September averages 98 mm of rainfall in Halfmoon Bay, about average for rainfall. The wettest on record was 166 mm in 2010.
The hottest September reached 16.9°C in 2023. The coldest dipped to 3.1°C in 1997.
Comparing the earliest years on record (1981–1985) with the most recent (2021–2025), average September temperatures have risen by about 0.2°C. Daytime highs moved from 10.7°C to 10.9°C and overnight lows from 7.4°C to 7.7°C over the same span.
September is one of the cooler months of the year in Halfmoon Bay and about average for rainfall, so plan for around 10.6°C daytime highs and 7.5°C at night.
Not quite — September is the 4th coolest month in Halfmoon Bay, averaging 9.1°C.
For September in Halfmoon Bay, pack warm layers and a jacket for chilly mornings, with typical highs of 10.6°C and lows of 7.5°C. Bring an umbrella or light rain jacket too.
Relative humidity in September averages 82% — quite humid.
Average wind speed in September is 4.5 m/s. The windiest September on record averaged 5.8 m/s (2025).
The wettest September on record was 2010 with 166 mm. On average, September sees 98 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 Halfmoon Bay 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.
