Last updated: February 24, 2025
In Boulder, United States, September showcases a dynamic weather spectrum, with maximum temperatures soaring to 33°C (92°F) and average conditions settling around a comfortable 16°C (61°F). The month experiences a noticeable chill with minimum temperatures dropping to -4°C (25°F). Although the region enjoys sunny days, it also sees 42 mm (1.6 in) of precipitation spread over 8 days, contributing to a humidity level of 65%. This blend of warmth and moisture creates a vibrant atmospheric tapestry, perfect for outdoor enthusiasts and nature lovers alike.
In September, Boulder experiences a refreshing transition as summer begins to fade, yet it maintains a delightful warmth. The month starts with crisp minimum temperatures around -4°C (25°F), gradually combining with a comfortable average of 16°C (61°F) that invites outdoor activities. Peak temperatures can still reach a balmy 33°C (92°F), a gentle reminder of summer’s lingering embrace. As the days grow shorter and cooler, the shift in climate contrasts sharply with the frigid conditions of winter months ahead, creating a season marked by both warmth and the anticipation of colder days. This blend of temperatures signifies a perfect time for hiking and enjoying the stunning fall foliage that Boulder has to offer.
In September, Boulder experiences a noteworthy transition as summer draws to a close, marked by an average precipitation of 42 mm (1.6 in) spread over 8 days. This represents a decline from the heavier rains of May and June, which saw 134 mm (5.3 in) and 74 mm (2.9 in) respectively. As the landscape begins to prepare for fall, September introduces a refreshing but moderate wetness, suggesting a gradual shift in weather patterns. While the month is considerably drier than the preceding summer months, the rainfall still plays a crucial role in sustaining the region's natural beauty, paving the way for the vibrant colors of autumn foliage. This trend highlights an expected tapering of precipitation leading into the drier months of October and November, where an average of 48 mm (1.9 in) and 30 mm (1.2 in) is anticipated.
As summer transitions into fall, September in Boulder sees a notable shift in humidity levels. The month begins with a slight increase to 65%, marking a rise from the drier summer months of June and July, where humidity dipped to 58% and 52% respectively. This uptick reflects the city's gradual move away from the dry heat of summer towards the more humid conditions of autumn. Compared to the lower humidity levels experienced in August at 56%, September's figures indicate a return to a more moderate climate. As the days cool down and the foliage begins to change, residents can expect a refreshing shift in the air—a blend of lingering warmth and the onset of autumn's crispness.
In September, Boulder experiences a UV Index of 10, categorizing it as very high and suggesting a burn time of just 15 minutes for unprotected skin. This month follows a summer pattern where the UV Index peaked at 12 in June and July. As the seasons transition, September still retains significant UV radiation, though it marks a slight decrease from the extreme levels of the previous months. This trend underscores the importance of sun protection even as fall approaches. Whether you're enjoying the outdoors or simply running errands, be mindful of your exposure to ensure skin safety. For more in-depth information, visit Boulder UV Index.
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September in Boulder, United States, shines with 309 hours of sunlight, marking a gentle shift as summer gives way to autumn. This month reflects a slight decline from the sun-drenched heights of July and June, which boast 378 and 374 hours, respectively. However, September still stands out with significantly more sunshine than the winter months, such as January with only 185 hours. The gradual tapering of daylight serves as a reminder of the changing seasons, allowing residents to savor the lingering warmth while preparing for the crisp air of fall. As leaves begin their colorful transformation, September's sunny days create a picturesque backdrop for outdoor adventures and vibrant fall festivities.
In Boulder, September marks a delightful transition into autumn, boasting a notable 69% chance of sunny weather. This upward trend in sunny days begins to emerge in late summer; with August already enjoying a solid 65% sunshine rate, the arrival of September promises even more radiant skies. As the year progresses, Boulder experiences varying levels of sunlight, peaking in the late summer months and continuing to hold a strong likelihood of clear weather into October. With the warmth of September sunshine, it's a fantastic time for outdoor activities and enjoying the vibrant fall colors before cooler temperatures set in.
As summer fades into autumn, September in Boulder sees a transition in daylight hours, with an average of 12 hours of sunlight per day. This slight decline from the summer peak of 14 hours reflects the shifting seasons, as days gradually shorten. Just a month prior, Boulder basked in ample sunlight, while September's shorter days hint at the approaching fall months of October and November, which will drop to 11 and 9 hours of daylight, respectively. This pattern emphasizes the rhythm of nature, where longer, sun-drenched days during the summer give way to crisp, cooler evenings, inviting residents to embrace the changing landscape as nature transitions into its autumn palette.
As September rolls into Boulder, the average wind speed stalls at a gentle 2.2 m/s (5 mph), continuing the trend of mild breezes seen throughout the summer months. The winds remain relatively calm compared to the sharper gusts that appear in the late fall and winter, where speeds climb to a crisp 3.0 m/s (7 mph) by December. This slight dip in wind activity from earlier in the year hints at the onset of autumn, creating a serene atmosphere perfect for outdoor enjoyment. While the winds may not be fierce, they carry the subtle changes of the season, weaving through the vibrant colors as the foliage begins to shift. September's breeziness invites locals and visitors alike to savor the tranquility before winter's more robust blusters take hold.
The weather in Boulder 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 Boulder and find detailed weather information for every month.
September and January present contrasting weather patterns. In September, temperatures range from a chilly minimum of -4°C (25°F) to a warm maximum of 33°C (92°F), with an average of 16°C (61°F). This month sees about 42 mm (1.6 in) of precipitation over 8 days and a humidity level of 65%. In stark contrast, January temperatures drop significantly, with a minimum of -24°C (-12°F) and a maximum of 15°C (59°F), averaging just -3°C (26°F). Although it has lower precipitation at 25 mm (1.0 in) over 5 days, January boasts a higher humidity level of 88%. For more detailed insights into January's conditions, visit Boulder weather in January.
September and February exhibit stark contrasts in weather. September features a wide temperature range, with minimums of -4°C (25°F) and maximums reaching up to 33°C (92°F), averaging around a comfortable 16°C (61°F). It is relatively mild with moderate humidity at 65% and a total precipitation of 42 mm (1.6 in) over 8 days. In contrast, February presents a much colder climate, with minimum temperatures plummeting to -26°C (-14°F) and an average of -4°C (26°F), while the maximum only reaches 20°C (69°F). Although February sees slightly less precipitation at 36 mm (1.4 in) over 9 days, the humidity is notably higher at 87%. For more specific details about February weather conditions, you can visit Boulder weather in February.
September and March showcase stark contrasts in weather patterns. In September, temperatures range from a chilly minimum of -4°C (25°F) to a warm maximum of 33°C (92°F), with an average of 16°C (61°F). The month experiences relatively mild precipitation, averaging 42 mm (1.6 in) over about 8 days and 65% humidity. In contrast, March is much colder, with temperatures dipping as low as -21°C (-5°F) and peaking at 23°C (73°F), averaging just 2°C (35°F). This month is wetter, with 62 mm (2.5 in) of precipitation spread over 11 days, accompanied by a higher humidity level of 81%. For a more detailed overview of March weather, you can check Boulder weather in March.
The weather in September offers a more temperate experience, with average temperatures ranging from a chilly -4°C (25°F) to a warm 33°C (92°F), and a total precipitation of 42 mm (1.6 in) over about 8 days. In contrast, April is notably cooler, with average temperatures spanning from -17°C (2°F) to a maximum of 24°C (75°F), accompanied by higher precipitation of 68 mm (2.7 in) over 12 days. Additionally, the humidity in April is slightly higher at 76%, compared to September's 65%. For a closer look at April's weather, you can explore Boulder weather in April.
September typically offers a wider range of temperatures compared to May, with minimums around -4°C (25°F) and maximums reaching as high as 33°C (92°F). In contrast, May starts cooler, with minimums around -9°C (17°F), and peaks at a milder 27°C (81°F). While September enjoys an average temperature of 16°C (61°F) with relatively low precipitation of 42 mm (1.6 in) over about 8 days, May experiences significantly higher rainfall at 134 mm (5.3 in) over 16 days. Both months have similar humidity levels, hovering around 65-66%. For more detailed weather information, you can check Boulder weather in May.
When comparing the weather in September and June, we see some notable differences. September experiences a broader temperature range, with a minimum of -4°C (25°F) and a maximum of 33°C (92°F), resulting in an average temperature of 16°C (61°F). In contrast, June has slightly higher average temperatures, ranging from a minimum of 3°C (37°F) to a maximum of 34°C (93°F), with an average of 17°C (63°F).
Precipitation is also more pronounced in June, with 74 mm (2.9 in) over 14 days compared to September's 42 mm (1.6 in) over 8 days, indicating a wetter month. Additionally, the humidity in September reaches 65%, which is higher than June's 58%. For more detailed insight into June's weather specifics, you can check Boulder weather in June.
When comparing the weather in September to that of July, we observe notable differences in temperature and precipitation. September experiences a wider temperature range, with minimums dropping as low as -4°C (25°F) and maximums reaching up to 33°C (92°F), averaging 16°C (61°F). In contrast, July is generally warmer, with minimum temperatures at 7°C (45°F) and maximums peaking at 34°C (93°F), averaging 20°C (68°F). Additionally, September sees lower precipitation levels, with 42 mm (1.6 in) spread over 8 days, compared to July's 71 mm (2.8 in) over 16 days. Humidity levels also differ, as September has a higher average humidity of 65%, compared to July’s 52%. For more details, check out Boulder weather in July.
In comparing the weather between September and August, September experiences a broader temperature range with minimums dipping to -4°C (25°F) and an average of 16°C (61°F), while August retains warmer minimums around 7°C (45°F) and an average of 19°C (67°F). Both months share similar maximum temperatures, peaking at around 33°C (91-92°F). However, September sees less precipitation with 42 mm (1.6 in) over 8 days compared to August's 67 mm (2.7 in) spread across 13 days, resulting in September being drier overall. Additionally, September's humidity is slightly higher at 65%, compared to August’s 56%. For more detailed information, you can check Boulder weather in August.
September typically enjoys milder temperatures, with averages around 16°C (61°F) and maximums reaching up to 33°C (92°F). In contrast, October sees a significant drop, with average temperatures declining to 7°C (45°F) and maximums peaking at 26°C (79°F). Precipitation is slightly higher in October at 48 mm (1.9 in) over 7 days, compared to September’s 42 mm (1.6 in) over 8 days. Additionally, October experiences increased humidity at 76%, up from September's 65%. For a more detailed look at weather patterns, you can check Boulder weather in October.
September and November weather showcase a stark contrast in temperatures and conditions. September typically experiences milder temperatures, ranging from a minimum of -4°C (25°F) to a maximum of 33°C (92°F), with an average around 16°C (61°F). It also sees a moderate level of precipitation at 42 mm (1.6 in) over approximately 8 days, alongside a relatively comfortable humidity of 65%. In contrast, November brings significantly colder weather, with temperatures dipping as low as -16°C (3°F) and averaging just 2°C (36°F), while reaching a maximum of 22°C (71°F). The month sees slightly less precipitation at 30 mm (1.2 in) across 6 days, but the humidity rises to a higher level of 82%. For more detailed information, check out Boulder weather in November.
September weather is characterized by a wider temperature range, with minimums around -4°C (25°F) and highs reaching up to 33°C (92°F), providing a warm average of 16°C (61°F). In contrast, December presents a much cooler climate, with average temperatures plummeting to -2°C (29°F) and maximums capping at 19°C (67°F). Additionally, December sees less precipitation at 19 mm (0.8 in) over only 4 days, compared to September’s 42 mm (1.6 in) across 8 days. The humidity also rises in December to 89%, contributing to a distinct chill that marks the transition to winter. For more details on December weather, you can visit Boulder weather in December.
As September rolls in, Boulder experiences a refreshing transition marked by mild temperatures and moderate humidity, making it a delightful time for outdoor activities. The warmth lingers from summer with occasional cooler days, allowing for a comfortable climate that contrasts with the chill of the upcoming fall months. With an average temperature that invites exploration, combined with a moderate amount of precipitation spread across several days, September strikes a balance that is both vibrant and inviting. This month serves as a beautiful bridge between the heat of summer and the crispness of autumn, offering a unique charm that sets it apart from the rest of the year. To learn more about what to expect throughout the seasons, check out Boulder Climate.