Last updated: February 24, 2025
September in Mill Valley, United States is characterized by pleasantly warm temperatures, peaking at a maximum of 36°C (98°F) while averaging around 18°C (65°F). The month experiences remarkably low precipitation, with only 2 mm (0.1 in) recorded over 0 days, making it an ideal time for outdoor activities. Despite the warmth, the air retains a high humidity level of 86%, adding a touch of tropical feel to the otherwise dry conditions. This combination of warmth and dryness creates an inviting ambiance for both locals and visitors alike, perfect for exploring the scenic beauty of Mill Valley.
As September unfolds in Mill Valley, average temperatures begin to exhibit a warm embrace, with a notable minimum of 9°C (49°F) and a striking maximum reaching up to 36°C (98°F). This month marks a transition where the summer warmth continues to linger, elevating the average temperature to 18°C (65°F), the highest of the cooler seasons. Such warmth, particularly over the latter half of the month, invites both residents and visitors to revel in outdoor activities before autumn makes its appearance. This trend of fluctuating temperatures sets the stage for a gradual cool down, echoing the rhythm of the changing seasons while allowing the lingering warmth of late summer to be savored just a little longer.
In September, Mill Valley typically experiences a significant lull in precipitation, recording just 2 mm (0.1 in) over the month and notably, experiencing zero precipitation days. This reflects a broader trend of diminishing rainfall as the summer months draw to a close; from May to August, the region recorded only a total of 3 mm (0.1 in) of rain, signaling a dry spell. As the area transitions into the wetter months, this dry September sets the stage for a gradual increase in rainfall starting in October, when precipitation rises to a more substantial 30 mm (1.2 in), marking a stark contrast to the arid conditions of summer. The impending seasonal shift hints at a refreshing change, as the region prepares for the lushness that autumn brings.
As summer transitions to fall in Mill Valley, the humidity levels in September remain relatively high at 86%, slightly lower than the sweltering 87% recorded in both July and August. This decrease marks a continuation of the gradual decline in humidity observed from the midsummer peak, which consistently hovers around the mid-80s throughout the warmer months. The trend showcases that while September maintains a lingering warmth, it also hints at the impending shift towards drier conditions in October, where humidity drops to 80%. Overall, the yearly humidity pattern suggests a gentle ebb and flow, with late summer providing a brief spike before the onset of autumn's crispness.
In September, Mill Valley experiences a UV Index of 9, falling into the very high exposure category, which means that even brief sun exposure can lead to skin damage. Notably, as summer transitions into fall, the UV levels remain elevated, dropping only slightly from the intense summer months of July and June, which both peaked at an index of 11. Continuing the trend of high UV radiation, September's burn time is around 15 minutes, making it essential for residents and visitors to take precautions against sun exposure. As the days shorten and temperatures cool, it’s a reminder that UV rays can still pose a threat. For detailed insights on UV conditions throughout the year, visit Mill Valley UV Index.
UV Risk Categories
As September unfolds in Mill Valley, the days are still illuminated with a brilliant glow, capturing the essence of the late summer sun. With 312 hours of sunshine, this month showcases a delightful transition, retaining much of the warmth from August’s peak of 348 hours. The upward trend from the early months of the year, where sunlight lingered around 148 hours in January, consistently builds as the year progresses, hitting the highest point in July with 386 hours. Now, as days begin to shorten, September maintains a generous supply of sunlight before the dip in October to 280 hours. This gradual ebb and flow of sunlight not only marks the changing seasons but also invites residents and visitors alike to relish the lingering warmth before the autumn chill sets in.
In September, Mill Valley boasts a 62% chance of sunny weather, marking a transition from the vibrant summer months into the gentler embrace of autumn. This slight dip in sunshine probability from August’s 60% reflects a broader trend of dwindling sunny days as the year progresses. Prior to September, the highest likelihood of sun peaks in May at 75%, illustrating the region's warm and sunny summer allure. As the seasons shift, residents and visitors can enjoy a pleasant blend of sunshine while anticipating the cooler, cloudier days that lie ahead in October and November. Overall, September offers a delightful window for outdoor activities, balancing warmth with the hints of fall.
As summer fades into autumn, September in Mill Valley marks a gentle transition in daylight duration. With about 12 hours of daylight, the month reflects a gradual decline from the peak of 14 hours experienced in June and July. This pattern of decreasing daylight is part of the yearly rhythm, as the days gradually shorten through October, eventually settling into the 9 hours typical of winter in December. September's light offers a beautiful balance, allowing ample time for outdoor activities while hinting at the cozy evenings to come. It's a month where summer's warmth lingers, providing a soothing backdrop for the shifting seasons.
As summer transitions to fall, September in Mill Valley showcases a gentle breeze with an average wind speed of 3.1 m/s (7 mph). This slight dip follows a peak in May, where winds reached 4.8 m/s (11 mph), hinting at the season’s gradual change. The winds begin to ease from the vibrant gusts of earlier months, with August also reflecting a calm at 3.5 m/s (8 mph). The trend of decreasing wind speeds continues into October, suggesting a shift towards the softer, more tranquil atmosphere characteristic of autumn. As we move from the lively beats of summer to the serene whispers of fall, September serves as a delicate bridge, inviting reflection and appreciation of nature's rhythm.
The weather in Mill Valley changes throughout the year, offering a unique experience in each season. Below is a month-by-month comparison of September's weather with other months, highlighting differences in temperature, precipitation, and other key factors. Click on each section to discover seasonal trends in Mill Valley and find detailed weather information for every month.
In September, temperatures in Mill Valley range from a cool minimum of 9°C (49°F) to a warm maximum of 36°C (98°F), with an average of 18°C (65°F). This month typically experiences very little precipitation, averaging only 2 mm (0.1 in) with no rain days, while humidity remains relatively high at 86%. In stark contrast, January features much cooler temperatures, with minimums dipping to 0°C (33°F) and maximums reaching only 23°C (73°F), resulting in an average temperature of 10°C (51°F). Rainfall increases significantly during this month, accumulating to 111 mm (4.4 in) over 8 days, while humidity is slightly lower at 85%. For more details on the weather in January, you can check Mill Valley weather in January.
September and February present strikingly different weather conditions. In September, temperatures range from a chilly 9°C (49°F) at night to a sweltering 36°C (98°F) during the day, with very little precipitation—only 2 mm (0.1 in)—and an impressive humidity level of 86%. In contrast, February sees cooler temperatures with a minimum of 2°C (35°F) and a maximum of 23°C (73°F), accompanied by significantly higher precipitation of 116 mm (4.6 in) spread over 8 days. While September boasts dry heat, February's wetter climate brings a refreshing change, making it an interesting comparison for weather enthusiasts. For more insights on February weather, check out Mill Valley weather in February.
When comparing the weather in September to that in March, it's evident that September tends to be much warmer and drier. With temperatures ranging from a minimum of 9°C (49°F) to a scorching maximum of 36°C (98°F), September averages around 18°C (65°F) with virtually no rainfall—only 2 mm recorded across the month. In stark contrast, March experiences cooler temperatures, peaking at 24°C (75°F) and averaging 12°C (54°F), but it sees considerable rainfall totaling 80 mm (3.1 in), spread over about 10 days. Humidity levels are relatively similar, with September at 86% and March slightly lower at 82%. For more detailed insights, you can check Mill Valley weather in March.
When comparing the weather in September and April, significant differences emerge in temperature and precipitation. September boasts a much higher average temperature, ranging from a chilly minimum of 9°C (49°F) to a scorching maximum of 36°C (98°F), while April's temperatures are more moderate, with a minimum of 4°C (40°F) and a maximum of 28°C (83°F). Additionally, September experiences notably low precipitation at just 2 mm (0.1 in) over no rainy days, whereas April sees a more substantial 35 mm (1.4 in) of rain across five days. Humidity levels are slightly higher in September at 86%, compared to April's 79%. For a closer look at April’s weather specifics, you can visit Mill Valley weather in April.
In September, the weather tends to be warmer, with temperatures ranging from a minimum of 9°C (49°F) to a maximum of 36°C (98°F) and an average of 18°C (65°F). The month boasts very little precipitation, with only 2 mm (0.1 in) recorded over no rainy days and relatively high humidity at 86%. In contrast, May experiences cooler conditions, with temperatures starting from 7°C (45°F) and peaking at 30°C (86°F), averaging around 15°C (60°F). It is also a more humid month with 20 mm (0.8 in) of rainfall spread over 2 days, contributing to a slightly lower humidity level of 83%. For more details, visit Mill Valley weather in May.
September and June exhibit relatively mild weather patterns, with both months experiencing minimal precipitation and similar humidity levels. In September, temperatures range from a minimum of 9°C (49°F) to a maximum of 36°C (98°F), averaging around 18°C (65°F). In contrast, June sees slightly warmer temperatures, with a minimum of 10°C (51°F) and a maximum of 32°C (90°F), averaging 17°C (63°F). Despite the differences in temperature extremes, both months maintain an average precipitation of 2 mm (0.1 in) over zero rainy days. For a more detailed look at June's weather specifics, you can explore Mill Valley weather in June.
In comparing the weather of September and July, September tends to be slightly warmer with average temperatures reaching 18°C (65°F) and a maximum of 36°C (98°F), while July's average is a cooler 17°C (63°F) with a maximum of 30°C (87°F). Both months experience minimal precipitation, with just 2 mm (0.1 in) and no rainy days recorded. Humidity levels remain quite similar, with September at 86% and July at 87%. For more detailed information, you can check Mill Valley weather in July.
In September, the weather features a broader temperature range, with minimums of 9°C (49°F) and maximums hitting 36°C (98°F), compared to August's more moderate minimum of 11°C (52°F) and a maximum of 32°C (90°F). Both months experience minimal precipitation, with September receiving just 2 mm (0.1 in) and August slightly lower at 1 mm (0.1 in), both with zero rainy days. Humidity remains similar, with September at 86% and August at 87%. For more details about August weather, you can check Mill Valley weather in August.
In September, the weather features a wide range in temperatures, with a minimum of 9°C (49°F) and a maximum soaring to 36°C (98°F), accompanied by high humidity levels at 86% and virtually no precipitation. In contrast, October sees a slight decline in temperatures, with minimums dropping to 6°C (44°F) and maximums reaching 31°C (88°F). Additionally, October experiences an increase in rainfall, totaling 30 mm (1.2 in) over two days, while humidity levels decrease to 80%. For a more detailed overview of October weather in the area, check out Mill Valley weather in October.
In September, the weather is generally warm with temperatures ranging from a minimum of 9°C (49°F) to a scorching maximum of 36°C (98°F), resulting in an average temperature of 18°C (65°F). This month sees very little precipitation, with only 2 mm recorded and no rainy days, along with high humidity at 86%. In contrast, November experiences much cooler temperatures, with a minimum of 3°C (37°F) and a maximum of 25°C (77°F), averaging at 13°C (55°F). Additionally, November is significantly wetter, with 48 mm of rainfall over 5 days, and a slightly lower humidity of 79%. For a closer look at November weather, you can check out Mill Valley weather in November.
September and December present markedly different weather patterns. In September, temperatures range from a cool 9°C (49°F) at night to a scorching max of 36°C (98°F) during the day, providing an average temperature of 18°C (65°F). The month is characterized by its dry conditions, with only 2 mm of precipitation recorded and no rainy days, resulting in high humidity levels at 86%. In contrast, December brings a much cooler atmosphere, with temperatures dropping to a minimum of 3°C (38°F) and averaging around 10°C (51°F), peaking at a maximum of 22°C (72°F). This month is significantly wetter, with 98 mm of precipitation spread over 9 days, while the humidity decreases slightly to 79%. For a detailed look at what to expect, check out Mill Valley weather in December.
As September unfolds in Mill Valley, the weather presents a delightful transition from the heat of summer to the gentle embrace of autumn. With maximum temperatures reaching up to 36°C (98°F), the days can still feel quite warm, yet the average of 18°C (65°F) offers a pleasant reprieve. Uniquely, September experiences absolutely no precipitation, making it a great time for outdoor activities, unlike the wetter months that follow. The air remains humbly humid at 86%, adding a touch of softness to the vibrant scenery. For a deeper understanding of the seasonal nuances, explore more at Mill Valley Climate.