September Weather in Palo Alto, United States

Last updated: February 24, 2025

In Palo Alto, United States, September showcases a vibrant climate with maximum temperatures soaring to 39°C (103°F), while the average stays pleasantly warm at 18°C (66°F). The month experiences a comfortable minimum of 10°C (50°F), providing a refreshing contrast to the daytime heat. Notably, precipitation is minimal with just 2 mm (0.1 in) recorded over 0 rainy days, allowing residents and visitors to enjoy the dry, sun-soaked days. With a humidity level of 55%, the overall atmosphere remains inviting, making September an ideal time to explore the beautiful surroundings of Palo Alto.

September Temperature in Palo Alto

In September, Palo Alto basks in the warmth of transitioning seasons, experiencing mild evenings and hot days that reach up to 39°C (103°F). This month marks a distinct peak in temperatures, with an average of 18°C (66°F) and a minimum of 10°C (50°F), making it a stark contrast to the cooler months preceding it. As summer wanes, residents enjoy lingering heat, often reminiscent of early August, while the climate begins to shift towards the cooler autumn temperatures. This month showcases not only the vibrancy of late summer but also hints at the crisp days ahead, making it a unique time to savor all that Palo Alto has to offer.

September Weather in Palo Alto: Daily Temperature Highs, Lows, and Averages

The chart illustrates September temperature patterns in Palo Alto, showcasing daily minimum, maximum, and average temperatures. It highlights the typical range of temperatures throughout the month, providing insights into weather trends.
JanFebMarAprMayJunJulAugSepOctNovDec525354565863636565625652
Data source: Temperature data for Palo Alto generated using information from the Copernicus Climate Change Service.
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The average of daily temperatures (24 hours) calculated across all days in the month.

September Precipitation in Palo Alto

In Palo Alto, September marks a transition towards the wetter months, albeit with minimal precipitation, as the city recorded only 2 mm (0.1 in) of rain, typically spread over no days. This scant amount mirrors the summer months of July and August, which also experienced no rainfall. As we look at the broader trend throughout the year, it's evident that the dry season stretches from late spring through early fall, with January and February presenting the most substantial rainfall, accumulating nearly 205 mm (8.1 in) combined. As September gives way to October, residents can anticipate a slight increase in precipitation, signaling the arrival of the rainy season and a refreshing shift in the weather pattern.

September Precipitation Trends in Palo Alto

The chart displays monthly precipitation patterns in Palo Alto, with values for each month alongside September's data, which is highlighted for easy comparison. It shows the amount of precipitation, providing insights into how September's conditions compare to those of other months.
JanFebMarAprMayJunJulAugSepOctNovDec1051008633242012244788
Data source: Precipitation data for September generated using information from the Copernicus Climate Change Service.
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September Humidity in Palo Alto

As summer fades into autumn, September in Palo Alto experiences a slight uptick in humidity with levels averaging around 55%. This marks a gradual increase from the peak summer months, where humidity dipped to a refreshing 54% in August. The trend of decreasing humidity from the start of the year, peaking at 84% in February, reveals a steady shift toward drier air that persists through the warmer months. However, this subtle rise in September suggests a transition period as the region prepares for the wetter, more humid months ahead, particularly in October, when humidity levels are expected to climb back to 62%. Overall, September signals a bridge between the crisp, dry air of summer and the more humid fall and winter seasons.

September Average Humidity in Palo Alto

This chart illustrates the average monthly humidity levels in Palo Alto, with September's data highlighted for easy comparison. It shows the fluctuation in humidity throughout the year, allowing you to see how September's humidity levels compare to other months.
JanFebMarAprMayJunJulAugSepOctNovDec82%84%82%78%71%64%59%54%55%62%78%80%
Data source: Humidity data for September generated using information from the Copernicus Climate Change Service.

September UV Index in Palo Alto

As September rolls into Palo Alto, the UV Index remains notably high at 9, placing it in the very high exposure category. This indicates that while summer begins to taper off, the sun’s intensity is still significant, demanding careful sun safety measures, with a burn time of just 15 minutes for unprotected skin. Following the peak summer months, where extremes of 11 were recorded, September's UV levels take a slight dip but remain unchanged from August, revealing a trend of sustained sun exposure this time of year. With the arrival of autumn, it's essential to stay vigilant and protect yourself from the sun’s rays. For more details on UV levels throughout the year, check out Palo Alto UV Index.

Maximum UV Index Levels in Palo Alto for September

The chart shows the monthly maximum UV Index (UVI) for Palo Alto in September, representing the highest UV radiation level. The UVI measures the intensity of ultraviolet radiation from the sun, indicating the potential risk of sunburn and UV-related health issues.
JanFebMarAprMayJunJulAugSepOctNovDec3589101111109743
Data source: Palo Alto September UVI data generated using information from the Copernicus Climate Change Service.

UV Risk Categories

  •  Extreme (11+): Avoid the sun, stay in shade.
  •  Very High (8-10): Limit sun exposure.
  •  High (6-7): Use SPF 30+ and protective clothing.
  •  Moderate (3-5): Midday shade recommended.
  •  Low (0-2): No protection needed.

September Sunshine in Palo Alto

As September unfolds in Palo Alto, the sun continues to shine brightly, offering an impressive 299 hours of sunlight, making it one of the more luminous months of the year. This marks a slight decline from the summer peak in June and July, which basked in 355 and 349 hours of golden rays, respectively. However, it outshines the earlier months, reflecting a steady ascent from January's humble 151 hours. As autumn approaches, the daylight gradually diminishes, yet September still feels the warmth of summer's tail end, reminding residents and visitors alike of the vibrant days just past while gently ushering in the changing seasons. With such radiant days, September stands out as a perfect time for outdoor activities, embracing the lingering warmth of summer before the crispness of fall takes hold.

Monthly Sunshine Duration in Palo Alto: September Highlighted

The September sunshine duration represents the total number of hours Palo Alto experiences direct sunlight during the month of September, excluding cloudy or obstructed periods. This chart compares the sunshine duration of September with the other months of the year.
JanFebMarAprMayJunJulAugSepOctNovDec151h216h263h296h341h355h349h320h299h276h225h194h
Data source: Palo Alto September Sunshine duration data generated using information from the Copernicus Climate Change Service.

Chance of Sun in Palo Alto During September

In Palo Alto, September boasts a 58% chance of sunny weather, reflecting a trend of warmer, drier conditions following the summer months. This percentage is a slight dip from August's 61%, yet it still falls within a favorable range compared to the spring and early summer months. Throughout the year, the likelihood of sunny days peaks in May at 62%, and while September does not quite reach that height, it remains significantly higher than the winter months, such as January's 39% and December's 35%. As the year transitions into autumn, September offers a delightful blend of lingering summer warmth and gentle beginnings of fall, making it an appealing time for outdoor activities under mostly sunny skies.

Percentage of Sunny Days in September in Palo Alto

The chart illustrates the percentage of sunny days in Palo Alto throughout the year, with September highlighted. It highlights the variation in sunshine across each month and provides an overview of the typical frequency of sunny days, offering insights into how often Palo Alto experiences clear skies throughout the year.
JanFebMarAprMayJunJulAugSepOctNovDec39%55%58%45%62%57%48%61%58%53%46%35%
Data source: Palo Alto September chance of sun data, generated using information from the Copernicus Climate Change Service.

Daylight Hours in Palo Alto in September

As September arrives in Palo Alto, the days begin to shorten, offering around 12 hours of daylight—a gentle transition from the long, sun-drenched days of summer, which stretched to a peak of 14 hours in June and July. This gradual decline, consistent with the rhythm of the seasons, reflects a natural ebb toward autumn, with daylight reducing to 11 hours in October and slipping further as the year progresses. Notably, September marks the closing of summer's high point, serving as a bridge to the cooler months where daylight diminishes steadily until the year's end. Embracing this shift, residents can savor the softer light and milder evenings, while looking forward to the cozy days of fall.

Average Daylight Hours in Palo Alto for September

The chart illustrates the variation in daylight duration in Palo Alto throughout the year, with September highlighted. It shows the monthly changes in daylight hours, calculated by summing the hours between sunrise and sunset.
JanFebMarAprMayJunJulAugSepOctNovDec9h 54m10h 49m11h 59m13h 11m14h 11m14h 41m14h 25m13h 31m12h 22m11h 10m10h 8m9h 36m
Data source: Palo Alto daylight hours, calculated by summing the hours between sunrise and sunset for each day.

September Winds in Palo Alto

As September unfolds in Palo Alto, the gentle caress of the wind becomes even more noticeable, with an average speed of just 2.0 m/s (4 mph), marking it as the calmest month of the year. This decline in wind speed from the summer months suggests a transition into the quieter tones of autumn, following a gradual increase that peaked in May at 3.0 m/s (7 mph). The pattern observed throughout the year shows that while winter months often bring a slight resurgence, the winds consistently retreat during the summer and into early fall, creating a serene atmosphere perfect for enjoying the changing leaves and pleasant weather. As the year progresses, one can anticipate the calming influence of September’s breezes, reflecting the shifting rhythms of nature in this picturesque region.

Average September Wind Speed in Palo Alto

The chart displays monthly wind speed patterns in Palo Alto, with values for each month shown alongside September's data, which is highlighted for easy comparison. It illustrates the average wind speed, offering insights into how September's wind conditions compare to the rest of the year.
JanFebMarAprMayJunJulAugSepOctNovDec2.22.52.62.63.02.72.32.12.0 m/s2.41.82.4
Data source: Palo Alto September wind data, generated using information from the Copernicus Climate Change Service.

How September Weather in Palo Alto Compares to Other Months

The weather in Palo Alto 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 Palo Alto and find detailed weather information for every month.

September and January present stark contrasts in weather. In September, daytime temperatures can soar to a sweltering maximum of 39°C (103°F), with an average of 18°C (66°F) and minimal rainfall, making it a dry and relatively humid month at 55%. In sharp contrast, January experiences a cooler climate with average temperatures settling around 11°C (52°F) and a maximum of 24°C (75°F). This month is significantly wetter, with total precipitation reaching 105 mm (4.1 in) over approximately 7 days, accompanied by a higher humidity level of 82%. For more detailed insights into January weather, you can explore Palo Alto weather in January.

When comparing the weather in September and February, there are notable differences in temperature and precipitation. September averages a mild 18°C (66°F) with temperatures ranging from a cool 10°C (50°F) to a scorching high of 39°C (103°F), yet it experiences very little rainfall, only 2 mm over the month. In contrast, February offers a much cooler climate with an average temperature of 11°C (53°F), fluctuating between 2°C (37°F) and a maximum of 24°C (76°F). Additionally, February is characterized by significant precipitation, totaling 100 mm (3.9 in) over about 8 days, along with higher humidity at 84%. For more insights on what to expect, check out Palo Alto weather in February.

September and March present notably contrasting weather patterns. In September, temperatures range from a minimum of 10°C (50°F) to a maximum of 39°C (103°F), offering warm conditions with an average of 18°C (66°F). This month is characterized by almost no precipitation, with just 2 mm recorded over zero days, and a comfortable humidity level of 55%. In contrast, March features much cooler temperatures, with a minimum of 4°C (40°F) and a maximum of 25°C (78°F), averaging at 12°C (54°F). It is also a wetter month, experiencing 86 mm (3.4 in) of precipitation over 11 days, accompanied by higher humidity at 82%. For more details on March weather, visit Palo Alto weather in March.

In comparing the weather of September and April, we notice a significant contrast in temperature and precipitation. September boasts a warmer climate with average temperatures reaching 18°C (66°F) and highs soaring to 39°C (103°F), while April is cooler, featuring an average temperature of 13°C (57°F) and a maximum of 30°C (87°F). Additionally, September enjoys dry conditions with minimal precipitation of just 2 mm over 0 days, whereas April experiences more rainfall, totaling 33 mm over 5 days, contributing to a higher humidity level of 78%. For more detailed insights, you can check Palo Alto weather in April.

September and May exhibit contrasting weather conditions. In September, temperatures range from a cool 10°C (50°F) at night to a scorching high of 39°C (103°F) during the day, with an average temperature of 18°C (66°F) and virtually no precipitation. The humidity level is a moderate 55%, making for warm and dry days. In contrast, May features milder temperatures, with a minimum of 8°C (46°F), an average of 15°C (59°F), and a maximum of 29°C (84°F). This month experiences more rainfall, with about 24 mm (0.9 in) spread over two days, and a higher humidity level at 71%. For more details on May's weather specifics, you can visit Palo Alto weather in May.

In comparison, September typically showcases a wider temperature range, with a minimum of 10°C (50°F), an average of 18°C (66°F), and a maximum soaring to 39°C (103°F). This contrasts with June, where temperatures hover between a minimum of 10°C (49°F), an average of 17°C (63°F), and a maximum of 35°C (96°F). Both months share relatively low precipitation, with just 2 mm (0.1 in) recorded and zero rainy days. However, September experiences slightly lower humidity at 55%, compared to June's 64%. For more detailed insights about June's weather, check Palo Alto weather in June.

In comparing the weather of September and July, September tends to be warmer overall, with an average temperature of 18°C (66°F) and a maximum reaching up to 39°C (103°F). In contrast, July has a slightly lower average temperature of 17°C (63°F) and a maximum of 29°C (84°F). Interestingly, both months experience very little precipitation, with September recording only 2 mm and July completely dry at 0 mm. Humidity levels are similar as well, with September at 55% and July slightly higher at 58%. For more detailed insights, you can check Palo Alto weather in July.

In comparing the weather of September and August, September showcases a slightly wider temperature range, with minimums dropping to 10°C (50°F) and maximums reaching an impressive 39°C (103°F), while August maintains a more moderate range, from a minimum of 12°C (54°F) to a maximum of 35°C (96°F). Both months experience very little precipitation, with September recording only 2 mm (0.1 in) across no rainy days, similar to August's completely dry conditions, which can be explored further in this link: Palo Alto weather in August. Humidity remains relatively stable, with September at 55% and August at 54%, showcasing a consistent dry climate as summer transitions into fall.

September and October present a noticeable shift in weather patterns. In September, temperatures range from a minimum of 10°C (50°F) to a maximum of 39°C (103°F), with an average of 18°C (66°F) and very little precipitation—only 2 mm across 0 days. In contrast, October sees cooler temperatures, starting at a minimum of 7°C (45°F) and peaking at 33°C (92°F), with an average of 17°C (63°F). Additionally, October experiences more humidity at 62% and a significant increase in precipitation, totaling 24 mm over 1 day. For more detailed information, you can check out Palo Alto weather in October.

September and November display a remarkable contrast in weather patterns. In September, temperatures can soar with a minimum of 10°C (50°F) and an average of 18°C (66°F), peaking at a sweltering maximum of 39°C (103°F). The month is characterized by minimal precipitation, averaging just 2 mm (0.1 in) with no rainy days and a humidity level of 55%. In stark contrast, November experiences much cooler temperatures, with a minimum of 4°C (39°F), an average of 13°C (56°F), and a maximum of 27°C (80°F). Rainfall becomes more prominent in November, with a total precipitation of 47 mm (1.9 in) spread over five days, and humidity rises significantly to 78%. For a detailed look at the weather trends in November, visit Palo Alto weather in November.

September and December present contrasting weather patterns. In September, temperatures range from a mild 10°C (50°F) at night to a scorching maximum of 39°C (103°F) during the day, with an average of 18°C (66°F). The month is characterized by very little precipitation, only 2 mm (0.1 in), and a pleasant humidity of 55%. In stark contrast, December sees much cooler temperatures, with a minimum of 3°C (37°F), an average of 11°C (52°F), and a maximum of 22°C (72°F). The month is significantly wetter as well, with 88 mm (3.5 in) of precipitation over 9 days, and humidity levels rising to 80%. For more detailed information, you can visit Palo Alto weather in December.

Compare Palo Alto's Climate with Cities Worldwide

Conclusion: What to Expect From September Weather in Palo Alto

As September unfolds in Palo Alto, residents experience a unique transition marked by warm temperatures and low humidity. This month often boasts significantly higher daytime highs compared to the summer's peak, creating an inviting atmosphere for outdoor activities. Additionally, with virtually no precipitation, the dry climate allows for plenty of sunshine and clear skies, a delightful contrast to the wetter winter months. Overall, September serves as a pleasant bridge to the cooler fall season, making it an ideal time to explore the beauty of Palo Alto. For more details on weather patterns throughout the year, check out Palo Alto Climate.