September Weather in Keystone, United States

Last updated: February 24, 2025

In Keystone, United States, September brings a refreshing transition into autumn, characterized by a maximum temperature of 22°C (72°F) and a minimum of -11°C (13°F). The average temperature settles at a cool 7°C (46°F), creating a crisp atmosphere perfect for outdoor activities. Despite experiencing nine days of precipitation with a total of 43 mm (1.7 in), the air remains relatively humid at 65%, adding a touch of freshness to the early fall scenery. This month presents a blend of invigorating temperatures and occasional rain, making it an ideal time to enjoy the stunning natural beauty of Keystone.

September Temperature in Keystone

In September, Keystone, United States, experiences a significant transition as summer winds down and autumn approaches. The temperatures begin to cool, with average lows around -11°C (13°F) and highs reaching up to 22°C (72°F), marking a pleasant but crisp time of year. This month showcases a shift from the warmth of summer, evident in the average temperature of 7°C (46°F), offering a refreshing embrace of early fall. As daylight shortens, the scenery begins to change, providing a beautiful backdrop to the steadily cooling climate. This trend towards cooler temperatures prepares the region for the much harsher winter months ahead.

September Weather in Keystone: Daily Temperature Highs, Lows, and Averages

The chart illustrates September temperature patterns in Keystone, showcasing daily minimum, maximum, and average temperatures. It highlights the typical range of temperatures throughout the month, providing insights into weather trends.
JanFebMarAprMayJunJulAugSepOctNovDec131422273546525145302013
Data source: Temperature data for Keystone 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 Keystone

In September, Keystone experiences a modest precipitation total of 43 mm (1.7 in), reflecting a slight dip compared to the wetter months of summer. With 9 days of rain, the month is characterized by sporadic showers rather than continuous downpours, contributing to a gentler transition into autumn. This decline in precipitation follows a peak in July, which recorded the highest rainfall of the year at 83 mm (3.3 in). As the region shifts from the vibrancy of summer to the crispness of fall, September's rainfall sets the stage for October's average of 56 mm (2.2 in), marking a trend of increasing moisture as the season progresses. Overall, this period showcases a balanced mix of dry and wet days, maintaining Keystone's charm while hinting at the upcoming seasonal changes.

September Precipitation Trends in Keystone

The chart displays monthly precipitation patterns in Keystone, 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.
JanFebMarAprMayJunJulAugSepOctNovDec415967788150835343564143
Data source: Precipitation data for September generated using information from the Copernicus Climate Change Service.
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September Humidity in Keystone

In September, Keystone experiences a slight increase in humidity, reaching 65%, which marks a notable shift from the drier summer months. As the year progresses from the sweltering days of July and August, where humidity levels dipped to their lowest at 52% and 56% respectively, September gently reintroduces a touch of moisture to the air. This transition signals the gradual onset of fall, increasing humidity levels toward the year's end. The trend shows a steady rise in humidity from the blissfully dry summer into a more balanced autumn, suggesting a shift that many in the region may find refreshing as they prepare for the cooler months ahead.

September Average Humidity in Keystone

This chart illustrates the average monthly humidity levels in Keystone, 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.
JanFebMarAprMayJunJulAugSepOctNovDec88%87%81%76%67%58%52%56%65%76%82%89%
Data source: Humidity data for September generated using information from the Copernicus Climate Change Service.

September UV Index in Keystone

As September rolls in, Keystone, United States, continues to experience a high UV Index at 11, categorizing it as extreme with a burn time of just 10 minutes for unprotected skin. This trend of heightened UV exposure from the summer months tapers slightly, but remains significant. The UV Index elevated steadily from a moderate 4 in January to its peak at 14 in June, demonstrating a clear seasonal pattern. Despite the drop in intensity compared to the peak summer months, it’s crucial for residents and visitors alike to remain vigilant. With the sun still strong, take precautions to protect your skin—particularly on clear days. For more details on UV exposure, check out Keystone UV Index.

Maximum UV Index Levels in Keystone for September

The chart shows the monthly maximum UV Index (UVI) for Keystone 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.
JanFebMarAprMayJunJulAugSepOctNovDec469121314131211754
Data source: Keystone 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 Keystone

As September rolls in, Keystone, United States, enjoys a delightful transition from summer to fall, basking in 314 hours of sunshine. This month retains much of the warmth and brightness of August, which recorded 357 hours, while still showing a significant dip from the sun-drenched peak of June and July, each at 382 hours. The gradual decrease in daylight hours hints at the impending winter months, yet September's sun remains a cherished gift, marking a time when outdoor activities can still flourish. With each subsequent month leading to cooler temperatures—October offers 280 hours and November plummets to 247 hours—September serves as a beautiful bridge, allowing residents and visitors alike to soak in the last remnants of summer while embracing the vibrant colors of fall.

Monthly Sunshine Duration in Keystone: September Highlighted

The September sunshine duration represents the total number of hours Keystone 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.
JanFebMarAprMayJunJulAugSepOctNovDec183h232h291h316h355h382h382h357h314h280h247h237h
Data source: Keystone September Sunshine duration data generated using information from the Copernicus Climate Change Service.

Chance of Sun in Keystone During September

In Keystone, United States, the weather in September is particularly promising for sun-seekers, boasting a 65% chance of sunny days. This marks a steady increase from the summer months, where August holds a respectable 61%. As the year progresses, the likelihood of sunshine peaks during this month, showcasing a trend of increasing clear skies as summer transitions into fall. Following September, the chance of sun slightly dip in October to 54%, yet it remains significantly higher than the winter months. This makes September an ideal time to enjoy outdoor activities under the vibrant autumn backdrop, harnessing the last rays of summer warmth before cooler temperatures set in.

Percentage of Sunny Days in September in Keystone

The chart illustrates the percentage of sunny days in Keystone 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 Keystone experiences clear skies throughout the year.
JanFebMarAprMayJunJulAugSepOctNovDec48%38%47%52%42%52%48%61%65%54%60%53%
Data source: Keystone September chance of sun data, generated using information from the Copernicus Climate Change Service.

Daylight Hours in Keystone in September

As September unfolds in Keystone, United States, the days begin to gently shorten, offering 12 hours of daylight—a subtle shift from the vibrant summer months. This decline marks the transition from the sun-drenched peak of summer, where July basked in a full 14 hours, toward the cozy embrace of autumn. Following the gradual increase in daylight from the depths of winter, when January barely sees 9 hours, September serves as a reminder of nature's rhythmic patterns. With daylight gradually decreasing in October to 11 hours, the cool air and early twilight invite a reflective mood, as the community prepares to welcome the enchanting hues of fall.

Average Daylight Hours in Keystone for September

The chart illustrates the variation in daylight duration in Keystone throughout the year, with September highlighted. It shows the monthly changes in daylight hours, calculated by summing the hours between sunrise and sunset.
JanFebMarAprMayJunJulAugSepOctNovDec9h 44m10h 43m11h 58m13h 16m14h 21m14h 53m14h 35m13h 37m12h 23m11h 6m9h 59m9h 25m
Data source: Keystone daylight hours, calculated by summing the hours between sunrise and sunset for each day.

September Winds in Keystone

As September arrives in Keystone, United States, the average wind speed settles at a gentle 2.0 m/s (4 mph), mirroring the calmness of the late summer months. This slight dip follows a gradual decrease from May’s breezier days, where winds peaked at 2.4 m/s (5 mph) before tapering down through the sweltering summer. Interestingly, September's winds reflect a consistent pattern seen in the preceding months, establishing a tranquil atmosphere suitable for outdoor pursuits. Looking ahead, a rise in wind speeds is anticipated as the season transitions into October, rekindling the dynamic breezes characteristic of autumn. This evolving wind pattern not only enhances the natural landscape but also adds a refreshing chill to the air, invigorating the community as it prepares for the changing seasons.

Average September Wind Speed in Keystone

The chart displays monthly wind speed patterns in Keystone, 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.
JanFebMarAprMayJunJulAugSepOctNovDec2.73.22.52.62.42.01.82.02.0 m/s2.52.82.9
Data source: Keystone September wind data, generated using information from the Copernicus Climate Change Service.

How September Weather in Keystone Compares to Other Months

The weather in Keystone 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 Keystone and find detailed weather information for every month.

September and January present striking contrasts in weather. In September, temperatures range from a minimum of -11°C (13°F) to a comfortable maximum of 22°C (72°F), with an average of 7°C (46°F). The month sees moderate precipitation of 43 mm (1.7 in) across approximately 9 days, accompanied by a relative humidity of about 65%. Conversely, January experiences much colder conditions, with a minimum temperature plunging to -26°C (-15°F) and a maximum of only 5°C (42°F), resulting in an average of -11°C (13°F). Despite slightly lower precipitation at 41 mm (1.6 in) spread over 10 days, the humidity rises significantly to 88%. For more details on January weather, visit Keystone weather in January.

September and February present stark contrasts in weather conditions. September showcases a wider temperature range, with a minimum of -11°C (13°F) and a pleasant maximum of 22°C (72°F), while February is much colder, with temperatures plunging as low as -28°C (-19°F) and averaging -10°C (14°F). Precipitation levels are also higher in February, with 59 mm (2.3 in) over 14 days compared to September's 43 mm (1.7 in) over just 9 days. Additionally, February is notably more humid, with humidity levels reaching 87%, compared to September's 65%. For more details on February weather, check out Keystone weather in February.

When comparing the weather in September and March, notable differences emerge in temperature and precipitation. September typically sees milder conditions with an average temperature of 7°C (46°F), ranging from a minimum of -11°C (13°F) to a maximum of 22°C (72°F). In contrast, March is considerably colder, with an average temperature of -5°C (22°F) and lows dipping to -25°C (-12°F). Furthermore, March experiences higher precipitation levels at 67 mm (2.7 in) over 14 days, while September has less moisture at 43 mm (1.7 in) across 9 days. Additionally, March tends to be more humid with an 81% humidity level compared to September's 65%. For more detailed insights, check the Keystone weather in March.

When comparing the weather in September and April, it's clear that September experiences milder temperatures, with an average of 7°C (46°F) and a maximum reaching 22°C (72°F), compared to April's colder average of -3°C (27°F) and a maximum of 10°C (50°F). Additionally, September tends to be drier, with only 43 mm (1.7 in) of precipitation over about 9 days, while April sees increased precipitation, totaling 78 mm (3.1 in) across 13 days. Humidity levels are also higher in April at 76%, compared to 65% in September. For more detailed information, you can check Keystone weather in April.

In comparing the weather of September and May, September generally features milder temperatures, with an average of 7°C (46°F) and a maximum of 22°C (72°F), while May tends to be cooler, with an average of 2°C (35°F) and a maximum of 17°C (63°F). September experiences less precipitation overall, measuring 43 mm (1.7 in) over about 9 days, compared to May's 81 mm (3.2 in) across 14 days. Humidity levels are similar in both months, hovering around 65-66%. For more details on May weather specifically, you can refer to Keystone weather in May.

When comparing the weather in September and June, both months exhibit cool temperatures, but September tends to be slightly warmer overall. In September, temperatures range from a minimum of -11°C (13°F) to a maximum of 22°C (72°F), with an average of 7°C (46°F). In contrast, June shows a minimum of -9°C (15°F) and a slightly higher maximum of 24°C (75°F), resulting in a similar average temperature of 8°C (46°F). Precipitation is more frequent in June, with 50 mm (2.0 in) over 10 days, compared to 43 mm (1.7 in) over 9 days in September. Additionally, June has a lower humidity level of 58%, while September experiences higher humidity at 65%. For more details on the weather in June, you can visit Keystone weather in June.

In September, the weather tends to be a bit cooler compared to July, with average temperatures ranging from a chilly -11°C (13°F) at night to a pleasant 22°C (72°F) during the day. In contrast, July sees milder temperatures, starting at a minimum of -2°C (28°F) and peaking at 23°C (74°F). Additionally, September experiences less precipitation, with 43 mm (1.7 in) over about 9 days, while July is wetter, receiving 83 mm (3.3 in) across 15 days. September also has higher humidity at 65%, compared to July's 52%. For more detailed weather information, check out Keystone weather in July.

In August, temperatures in Keystone ranged from a minimum of 0°C (32°F) to a maximum of 22°C (73°F), with an average temperature of 11°C (52°F). The month saw 53 mm (2.1 in) of precipitation spread over 11 days, and the humidity was recorded at 56%.

In contrast, September brought slightly cooler temperatures, with a minimum of -11°C (13°F) and the same maximum as August at 22°C (72°F). The average temperature dipped to 7°C (46°F). Although precipitation decreased to 43 mm (1.7 in) over 9 days, the humidity rose to 65%.

Overall, September tends to be cooler and slightly wetter than August, signaling the transition from summer to fall. For a detailed overview of August weather, visit Keystone weather in August.

September typically experiences milder temperatures, with average highs reaching up to 22°C (72°F) and lows around -11°C (13°F), while October sees a significant drop, featuring average highs of only 17°C (63°F) and lows plunging to -24°C (-10°F). Alongside the cooler temperatures, October also brings higher humidity at 76% compared to September's 65%. Precipitation increases slightly in October, totaling 56 mm (2.2 in) over 9 days, compared to September's 43 mm (1.7 in) over the same number of days. For a detailed look at what to expect, view Keystone weather in October.

September and November present stark contrasts in weather conditions. In September, temperatures range from a minimum of -11°C (13°F) to a maximum of 22°C (72°F), with an average of 7°C (46°F) and moderate humidity at 65%. This month experiences about 43 mm (1.7 in) of precipitation over 9 days. In contrast, November sees significantly colder temperatures, with a minimum of -27°C (-17°F) and a maximum of just 10°C (50°F), yielding an average temperature of -7°C (20°F). The humidity rises to 82%, and precipitation is slightly lower at 41 mm (1.6 in) across 8 days. For more detailed insights, explore Keystone weather in November.

September weather in Keystone is mild, with temperatures ranging from a minimum of -11°C (13°F) to a maximum of 22°C (72°F), and an average of 7°C (46°F). The month sees about 43 mm (1.7 in) of precipitation over approximately 9 days, with humidity levels around 65%. In contrast, December brings much colder conditions, with temperatures dipping to a minimum of -29°C (-20°F) and a high of just 8°C (47°F), averaging -11°C (13°F). While the total precipitation remains similar at 43 mm (1.7 in), it occurs over about 10 days, and humidity rises significantly to 89%. For more information on December weather, check out Keystone weather in December.

Compare Keystone's Climate with Cities Worldwide

Conclusion: What to Expect From September Weather in Keystone

As September ushers in the early signs of autumn, Keystone's weather transforms dramatically compared to the warmer summer months. You can expect crisp mornings giving way to pleasantly mild afternoons, with temperatures ranging from chilly lows to comfortably warm highs. While the likelihood of precipitation increases with about nine rainy days throughout the month, the overall humidity settles at a moderate level, creating a refreshing contrast to the summer heat. This transition into fall not only enriches the landscape with vibrant colors but also makes it a delightful time for outdoor activities. For more in-depth insights on what to expect throughout the year, check out Keystone Climate.