December is one of the cooler months of the year in Simi Valley, with average highs near 15.2°C and lows around 3.2°C (mean 9.2°C). It is a relatively wet time of year, averaging 66 mm of rainfall and 61% relative humidity across the period.
December is one of the cooler months of the year in Simi Valley. Across 45 years of records, the warmest December in Simi Valley was 2024 — when the month averaged 13.2°C — while the coldest was 1987 at 6°C. Typical December days reach 15.2°C with nights near 3.2°C. The long-term trend is warming, about 0.6°C per decade.
December is a relatively wet time of year in Simi Valley, averaging 66 mm of rainfall. The wettest December on record was 2021 with 225 mm, while the driest was 2017 at 0 mm.
Relative humidity in Simi Valley during December averages 61%. The most humid December was 1983 at 77%, and the least humid was 2017 at 28%.
December receives about 2.8 kWh/m²/day of solar energy in Simi Valley. The sunniest December was 2017 at 3.3 kWh/m²/day, and the cloudiest was 1987 at 2.4.
Average wind speed in Simi Valley during December is 2.6 m/s. The windiest December was 1982 at 3.2 m/s, while the calmest was 2022 at 1.9 m/s.
Ranked against the other 11 months using 1981–2025 averages.
| Metric | 1980s | 1990s | 2000s | 2010s | 2020s | Change |
|---|---|---|---|---|---|---|
| Avg Temp | 8.5°C | 8.5°C | 8.9°C | 9.6°C | 11.2°C | +2.7°C |
| Avg High | 14.3°C | 14.4°C | 14.8°C | 15.7°C | 17.4°C | +3.1°C |
| Avg Low | 2.7°C | 2.5°C | 3.0°C | 3.5°C | 5.1°C | +2.4°C |
| Avg Rain | 54 mm | 62 mm | 51 mm | 68 mm | 110 mm | +56 mm |
December temperatures in Simi Valley have risen by about 2.9°C between 1981–1985 and 2021–2025.
December has ranged from a record low of -5.9°C (1990) to a record high of 26.4°C (2017).
Rainfall in December is a relatively wet time of year, averaging 66 mm, with a wettest year of 225 mm.
A quieter season to visit Simi Valley
If your dates are flexible, the more temperate months of June and September tend to be the most comfortable for visiting Simi Valley. That said, December — with days averaging 15.2°C and nights around 3.2°C (mean 9.2°C) and moderate rainfall (66 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 | 19.1°C | 13.0°C | 6.8°C | 25.4°C | 2.0°C | 193 mm | 67% | 2.5 m/s | — |
| 2024 🔥 | 20.4°C | 13.2°C | 6.0°C | 24.8°C | 2.0°C | 2 mm | 50% | 2.3 m/s | 2.9 |
| 2023 | 17.8°C | 11.9°C | 6.1°C | 24.0°C | 2.9°C | 90 mm | 62% | 2.2 m/s | 2.7 |
| 2022 | 14.7°C | 9.2°C | 3.8°C | 23.3°C | -1.5°C | 119 mm | 73% | 1.9 m/s | 2.5 |
| 2021 🌧️ | 14.8°C | 9.2°C | 3.6°C | 26.1°C | -1.2°C | 225 mm | 68% | 2.1 m/s | 2.6 |
| 2020 | 17.4°C | 10.8°C | 4.1°C | 24.1°C | -0.3°C | 30 mm | 43% | 2.7 m/s | 3 |
| 2019 | 13.9°C | 8.8°C | 3.7°C | 21.6°C | -1.2°C | 126 mm | 75% | 2.4 m/s | 2.5 |
| 2018 | 15.7°C | 9.5°C | 3.3°C | 21.4°C | -1.8°C | 47 mm | 65% | 2.6 m/s | 2.9 |
| 2017 | 20.5°C | 13.0°C | 5.5°C | 26.4°C | -1.5°C | 0 mm | 28% | 2.3 m/s | 3.3 |
| 2016 | 15.6°C | 9.5°C | 3.4°C | 22.3°C | -1.9°C | 100 mm | 64% | 2.6 m/s | 2.8 |
| 2015 | 15.1°C | 8.7°C | 2.2°C | 26.2°C | -3.1°C | 19 mm | 59% | 2.8 m/s | 3 |
| 2014 | 15.3°C | 10.1°C | 5.0°C | 22.2°C | -3.4°C | 104 mm | 73% | 2.2 m/s | 2.6 |
| 2013 | 16.9°C | 9.8°C | 2.7°C | 23.4°C | -4.0°C | 6 mm | 45% | 2.4 m/s | 3.2 |
| 2012 | 13.7°C | 8.3°C | 2.9°C | 21.5°C | -2.5°C | 48 mm | 72% | 2.2 m/s | 2.7 |
| 2011 | 15.4°C | 8.7°C | 2.0°C | 23.1°C | -1.6°C | 13 mm | 49% | 2.3 m/s | 3.2 |
| 2010 | 14.8°C | 9.7°C | 4.6°C | 24.0°C | -2.7°C | 216 mm | 73% | 2.5 m/s | 2.5 |
| 2009 | 13.7°C | 7.8°C | 2.0°C | 19.9°C | -4.5°C | 70 mm | 62% | 2.3 m/s | 2.8 |
| 2008 | 13.8°C | 7.8°C | 1.9°C | 25.8°C | -3.1°C | 52 mm | 65% | 2.4 m/s | 2.8 |
| 2007 | 13.4°C | 7.6°C | 1.9°C | 24.5°C | -3.7°C | 42 mm | 63% | 2.9 m/s | 3 |
| 2006 | 15.8°C | 9.4°C | 3.0°C | 22.3°C | -1.9°C | 24 mm | 53% | 2.9 m/s | 3.1 |
| 2005 | 16.5°C | 10.5°C | 4.5°C | 23.8°C | 1.0°C | 24 mm | 62% | 2.4 m/s | 2.7 |
| 2004 | 14.9°C | 9.4°C | 3.8°C | 24.7°C | -1.6°C | 156 mm | 65% | 2.7 m/s | 2.9 |
| 2003 | 14.5°C | 8.8°C | 3.1°C | 21.9°C | -2.9°C | 43 mm | 65% | 2.3 m/s | 2.7 |
| 2002 | 14.0°C | 8.6°C | 3.1°C | 18.6°C | -1.9°C | 69 mm | 72% | 2 m/s | 2.8 |
| 2001 | 13.5°C | 8.0°C | 2.4°C | 18.7°C | -1.2°C | 31 mm | 69% | 2.5 m/s | 2.7 |
| 2000 | 18.0°C | 11.4°C | 4.7°C | 23.7°C | -0.7°C | 1 mm | 45% | 2.5 m/s | 2.9 |
| 1999 | 16.6°C | 10.1°C | 3.7°C | 22.5°C | -0.7°C | 5 mm | 42% | 3.1 m/s | 3.1 |
| 1998 | 14.7°C | 8.2°C | 1.8°C | 23.9°C | -4.3°C | 16 mm | 54% | 3 m/s | 3 |
| 1997 | 13.0°C | 7.3°C | 1.7°C | 21.3°C | -2.0°C | 130 mm | 65% | 3 m/s | 2.9 |
| 1996 | 14.5°C | 9.5°C | 4.5°C | 21.0°C | -0.4°C | 121 mm | 71% | 3 m/s | 2.6 |
| 1995 | 16.3°C | 10.2°C | 4.1°C | 24.8°C | -0.9°C | 49 mm | 66% | 3 m/s | 2.6 |
| 1994 | 14.3°C | 8.4°C | 2.4°C | 21.0°C | -1.8°C | 31 mm | 65% | 2.3 m/s | 2.8 |
| 1993 | 14.9°C | 8.5°C | 2.0°C | 20.9°C | -1.3°C | 31 mm | 57% | 2.7 m/s | 3 |
| 1992 | 11.8°C | 6.5°C | 1.2°C | 18.2°C | -2.9°C | 135 mm | 70% | 2.9 m/s | 2.8 |
| 1991 | 15.0°C | 9.1°C | 3.2°C | 21.0°C | -1.7°C | 95 mm | 58% | 2.7 m/s | 2.9 |
| 1990 | 13.1°C | 6.7°C | 0.2°C | 21.6°C | -5.9°C | 3 mm | 44% | 3.1 m/s | 3 |
| 1989 | 17.9°C | 10.8°C | 3.7°C | 24.0°C | -1.2°C | 1 mm | 36% | 2.6 m/s | 3 |
| 1988 | 13.3°C | 7.7°C | 2.2°C | 23.7°C | -3.9°C | 103 mm | 63% | 2.8 m/s | 2.6 |
| 1987 ❄️ | 11.0°C | 6.0°C | 1.0°C | 20.1°C | -4.4°C | 88 mm | 71% | 2.8 m/s | 2.4 |
| 1986 | 16.2°C | 9.7°C | 3.2°C | 23.5°C | -0.3°C | 12 mm | 58% | 2.1 m/s | 2.7 |
| 1985 | 15.0°C | 9.3°C | 3.6°C | 21.1°C | -2.7°C | 17 mm | 61% | 2.8 m/s | 2.9 |
| 1984 | 11.6°C | 6.3°C | 1.1°C | 16.8°C | -3.6°C | 117 mm | 74% | 2.9 m/s | 2.5 |
| 1983 | 13.3°C | 8.2°C | 3.1°C | 20.3°C | -2.1°C | 83 mm | 77% | 3.1 m/s | — |
| 1982 | 13.5°C | 7.8°C | 2.1°C | 19.9°C | -2.3°C | 45 mm | 65% | 3.2 m/s | — |
| 1981 | 16.8°C | 10.5°C | 4.1°C | 22.2°C | 0.0°C | 15 mm | 64% | 2.9 m/s | — |
Actual recorded December data for each year, 1981–2025 — sourced from NASA POWER.
The average high in December is around 15.2°C with lows near 3.2°C (mean 9.2°C), based on 45 years of NASA POWER data.
December averages 66 mm of rainfall in Simi Valley, a relatively wet time of year. The wettest on record was 225 mm in 2021.
The hottest December reached 26.4°C in 2017. The coldest dipped to -5.9°C in 1990.
Comparing the earliest years on record (1981–1985) with the most recent (2021–2025), average December temperatures have risen by about 2.9°C. Daytime highs moved from 14°C to 17.4°C and overnight lows from 2.8°C to 5.3°C over the same span.
December is one of the cooler months of the year in Simi Valley and a relatively wet time of year, so plan for around 15.2°C daytime highs and 3.2°C at night.
Yes — December is the coldest month of the year in Simi Valley, averaging 9.2°C.
For December in Simi Valley, pack warm layers and a jacket for chilly mornings, with typical highs of 15.2°C and lows of 3.2°C. Bring an umbrella or light rain jacket too.
Relative humidity in December averages 61% — moderately humid.
Average wind speed in December is 2.6 m/s. The windiest December on record averaged 3.2 m/s (1982).
The wettest December on record was 2021 with 225 mm. On average, December sees 66 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 Simi Valley behaves each December, 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 December specifically, along with year-by-year records and how the month has changed over the decades.
