September Weather in Portsmouth, United States

Last updated: February 24, 2025

In September, Portsmouth experiences a delightful transition into autumn, with an average temperature of 18°C (65°F). Daily highs can reach a maximum of 30°C (86°F), while the nights cool down to a minimum of 7°C (46°F). The month is marked by an average humidity level of 71%, providing a crisp, refreshing atmosphere. Visitors can expect approximately 78 mm (3.1 in) of rain over 8 days, making it a varied month that balances warm summer remnants with the onset of fall.

September Temperature in Portsmouth

As September rolls in, Portsmouth experiences a delightful transition in temperatures, offering a refreshing blend of warmth and coolness. The average temperature hovers around 18°C (65°F), providing a comfortable backdrop for outdoor activities, while daily highs reach a pleasant 30°C (86°F), reminiscent of late summer days. The minimum temperature dips to 7°C (46°F), hinting at the approaching autumn chill. This month serves as a bridge between the scorching summer and the cooler months ahead, making it a perfect time to enjoy the lingering warmth while preparing for the colorful foliage that October brings.

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

The chart illustrates September temperature patterns in Portsmouth, showcasing daily minimum, maximum, and average temperatures. It highlights the typical range of temperatures throughout the month, providing insights into weather trends.
JanFebMarAprMayJunJulAugSepOctNovDec303236465565717165554334
Data source: Temperature data for Portsmouth 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 Portsmouth

September in Portsmouth sees a moderate precipitation total of 78 mm (3.1 in), aligning closely with the summer's drier months. With only 8 rainy days, it marks a relatively low frequency of precipitation, highlighting a transition as the region shifts from summer warmth towards autumn chill. Comparing this to August's 71 mm (2.8 in), September maintains a steady output, suggesting a stabilized weather pattern. However, as the year progresses into October, rainfall expectations increase significantly, as indicated by the notable rise to 142 mm (5.6 in), emphasizing the onset of the wetter season ahead. Overall, September serves as a bridge, balancing the lingering summer dryness while hinting at the wetter days to come.

September Precipitation Trends in Portsmouth

The chart displays monthly precipitation patterns in Portsmouth, 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.
JanFebMarAprMayJunJulAugSepOctNovDec79989414383881237178143108133
Data source: Precipitation data for September generated using information from the Copernicus Climate Change Service.
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September Humidity in Portsmouth

September in Portsmouth welcomes a gentle rise in humidity, sitting at 71%—a noticeable shift from the drier summer months of July and August, when humidity levels dipped to 67% and 66%, respectively. This slight increase marks a transition as the region begins to prepare for the wetter autumn season. Comparatively, September's humidity is notably lower than the winter months, where levels peak to around 84% in November and 86% in January. As the muggy summer heat gradually gives way to cooler breezes, September offers a brief yet refreshing balance before the humidity climbs again, setting the stage for the seasonal changes ahead.

September Average Humidity in Portsmouth

This chart illustrates the average monthly humidity levels in Portsmouth, 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.
JanFebMarAprMayJunJulAugSepOctNovDec86%84%82%80%72%72%67%66%71%79%84%81%
Data source: Humidity data for September generated using information from the Copernicus Climate Change Service.

September UV Index in Portsmouth

As summer transitions into fall, September in Portsmouth, United States, brings a UV Index of 7, indicating a high exposure category with a burn time of 25 minutes. This reduction from the peak UV levels seen in July and August highlights the gradual decrease in sun intensity as cooler weather approaches. While still important to protect your skin during this month, the UV Index is more manageable compared to the scorching days of mid-summer. By staying informed about the changing UV levels, you can enjoy the early autumn sunshine while safeguarding your health. For more detailed information, visit Portsmouth UV Index.

Maximum UV Index Levels in Portsmouth for September

The chart shows the monthly maximum UV Index (UVI) for Portsmouth 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.
JanFebMarAprMayJunJulAugSepOctNovDec2457910997532
Data source: Portsmouth 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 Portsmouth

In September, Portsmouth basks in a delightful 285 hours of sunshine, marking a gentle transition from the sun-drenched summer months. This month, while slightly dimmer than August’s peak 337 hours, still captures a vibrant ambiance, as the warmth of late summer lingers. The charted trend shows an impressive climb in sunshine hours from the dreary depths of winter, starting with just 150 hours in January and culminating in the sun-soaked heights of summer. As the year progresses, September's figure remains robust compared to earlier months, indicating that while the days may be growing shorter, the sun still reigns brightly, inviting residents and visitors alike to savor the lingering warmth before the cooler autumn months set in.

Monthly Sunshine Duration in Portsmouth: September Highlighted

The September sunshine duration represents the total number of hours Portsmouth 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.
JanFebMarAprMayJunJulAugSepOctNovDec150h183h256h245h295h330h335h337h285h221h193h176h
Data source: Portsmouth September Sunshine duration data generated using information from the Copernicus Climate Change Service.

Chance of Sun in Portsmouth During September

As summer transitions to fall, September in Portsmouth offers a refreshing change in weather, with a 44% chance of sunny days. This slight uptick from the summer months—where sunny probabilities lingered around 32% for both July and August—provides a welcome boost for outdoor enthusiasts. Interestingly, the month sits between the sunnier March (61%) and November (54%), indicating that while September may not be the sunniest month, it offers a comfortable blend of sunny days to enjoy before the chill of autumn sets in. As temperatures cool and the foliage begins to change, residents and visitors alike can look forward to occasional bright days perfect for leisurely strolls and fall activities.

Percentage of Sunny Days in September in Portsmouth

The chart illustrates the percentage of sunny days in Portsmouth 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 Portsmouth experiences clear skies throughout the year.
JanFebMarAprMayJunJulAugSepOctNovDec42%35%61%40%38%40%32%32%44%38%54%32%
Data source: Portsmouth September chance of sun data, generated using information from the Copernicus Climate Change Service.

Daylight Hours in Portsmouth in September

As summer fades into autumn, September in Portsmouth sees a pleasant balance in daylight, with roughly 12 hours of sun illuminating the days. After basking in the longest days of June, the gradual shift becomes apparent, as daylight begins its subtle retreat. The month serves as a transitional phase, with the vibrant hues of early fall emerging, while days start to shorten, a characteristic trend that continues into the cooler October. This ebb and flow of daylight not only impacts local activities but also heralds the cozy, golden tones of autumn that residents and visitors eagerly anticipate.

Average Daylight Hours in Portsmouth for September

The chart illustrates the variation in daylight duration in Portsmouth throughout the year, with September highlighted. It shows the monthly changes in daylight hours, calculated by summing the hours between sunrise and sunset.
JanFebMarAprMayJunJulAugSepOctNovDec9h 23m10h 31m11h 57m13h 26m14h 40m15h 18m14h 57m13h 51m12h 26m10h 58m9h 41m9h 2m
Data source: Portsmouth daylight hours, calculated by summing the hours between sunrise and sunset for each day.

September Winds in Portsmouth

As September rolls into Portsmouth, the average wind speed picks up slightly from the languid summer months, reaching 3.9 m/s (9 mph). This gentle increase signals a transition from the calm of August, where winds barely kissed the shores at 3.3 m/s (7 mph). Reflecting on the year, we see that winter months typically bring stronger breezes, peaking with January and February both averaging 4.7 m/s (11 mph). However, the overall trend shows a gradual decline in wind speed during the warmest months, reaching its nadir in July. As fall approaches, the winds begin to recover, hinting at the brisker days ahead while still offering a pleasant backdrop for enjoying the coastal charm of Portsmouth in early autumn.

Average September Wind Speed in Portsmouth

The chart displays monthly wind speed patterns in Portsmouth, 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.
JanFebMarAprMayJunJulAugSepOctNovDec4.74.75.24.44.24.13.53.33.9 m/s4.44.64.6
Data source: Portsmouth September wind data, generated using information from the Copernicus Climate Change Service.

How September Weather in Portsmouth Compares to Other Months

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

September generally offers a much warmer climate compared to January. With minimum temperatures around 7°C (46°F) and average highs reaching up to 30°C (86°F), September enjoys a pleasant mix of warmth and moderate humidity at 71%. In contrast, January is characterized by frigid temperatures, with lows plummeting to -19°C (-2°F) and an average of -1°C (30°F). Although both months see a similar level of precipitation (79 mm or 3.1 inches over 8 days), January's higher humidity at 86% contributes to a more biting cold. For more detailed information on the winter weather, check out Portsmouth weather in January.

September and February present vastly different weather experiences. In September, temperatures range from a mild 7°C (46°F) at night to a warm average of 18°C (65°F), peaking at 30°C (86°F) during the day. The humidity is relatively comfortable at 71%, with an average of 78 mm (3.1 inches) of rainfall over 8 days. In stark contrast, February is characterized by much colder conditions, with temperatures dipping as low as -16°C (4°F) and averaging around 0°C (32°F). The month sees a maximum temperature of 17°C (64°F) along with higher humidity at 84% and increased precipitation of 98 mm (3.9 inches) over 11 days. For a detailed look at February's weather, you can visit Portsmouth weather in February.

September and March present strikingly different weather patterns. In September, temperatures can range from a cool 7°C (46°F) to a warm 30°C (86°F), with an average temperature of 18°C (65°F) and relatively low humidity at 71%. Precipitation during this month totals 78 mm (3.1 in) over about 8 days. In contrast, March experiences much colder conditions, with temperatures fluctuating between -14°C (8°F) and a maximum of 17°C (63°F), resulting in a chilly average of just 2°C (36°F). Additionally, March sees slightly higher precipitation at 94 mm (3.7 in) spread across about 10 days, accompanied by a higher humidity level of 82%. For more details on March weather, visit Portsmouth weather in March.

In comparing the weather of September and April, it’s clear that September is generally warmer, with average temperatures ranging from a minimum of 7°C (46°F) to a maximum of 30°C (86°F), while April experiences cooler temperatures, with a minimum of -2°C (29°F) and a maximum of 24°C (76°F). However, April tends to be wetter, receiving a total of 143 mm (5.6 in) of precipitation over 13 days, compared to September’s 78 mm (3.1 in) over just 8 days. Additionally, April's humidity is slightly higher at 80%, compared to 71% in September. For a detailed look at the weather patterns, you can check Portsmouth weather in April.

September and May offer distinct weather experiences. In September, temperatures range from a chilly 7°C (46°F) at night to a warm 30°C (86°F) during the day, creating a comfortable average of 18°C (65°F). In contrast, May sees cooler conditions, with temperatures starting at a low of 2°C (37°F) and reaching a maximum of 27°C (82°F), averaging 13°C (55°F). Both months experience similar levels of precipitation, with September receiving 78 mm (3.1 in) over 8 days and May slightly higher at 83 mm (3.3 in) over 11 days. Humidity levels are comparable as well, hovering around 71% for September and 72% for May. For more detailed information on May weather, you can check Portsmouth weather in May.

When comparing the weather in September and June, both months experience a similar average temperature of 18°C (65°F) and maximum temperatures reaching up to 30°C (86°F). However, September tends to have slightly cooler minimum temperatures, averaging 7°C (46°F) compared to June's 8°C (47°F). Precipitation is slightly lower in September, with 78 mm (3.1 in) over 8 days, whereas June sees 88 mm (3.5 in) across 10 days. Humidity levels are nearly identical, with September at 71% and June at 72%. For more details, check out Portsmouth weather in June.

In comparing the weather of September and July, we observe notable differences in temperature and precipitation patterns. September experiences milder temperatures, with a minimum of 7°C (46°F) and an average of 18°C (65°F), compared to July's warmer minimum of 13°C (56°F) and average of 22°C (71°F). Both months peak at similar maximum temperatures, with September reaching up to 30°C (86°F) and July slightly higher at 31°C (88°F). However, July sees more precipitation, totaling 123 mm (4.8 in) over 12 days, while September has 78 mm (3.1 in) over just 8 days. Additionally, September has a higher humidity level of 71%, compared to July's 67%. For a more detailed look at July weather, visit Portsmouth weather in July.

In comparing the weather of September to that of August, September experiences a more significant drop in minimum temperatures, averaging 7°C (46°F) compared to August's milder 13°C (55°F). However, September also sees a maximum temperature that is quite similar, with highs reaching 30°C (86°F) versus August's peak of 31°C (88°F). Precipitation levels in September are slightly higher at 78 mm (3.1 in) over 8 days, compared to August's 71 mm (2.8 in) over 9 days, indicating a slightly wetter month overall. Humidity in September is notably higher at 71%, compared to 66% in August, contributing to a more humid feel as the month progresses. For more detailed insights, you can check Portsmouth weather in August.

September and October present a noticeable shift in weather patterns. In September, temperatures range from a minimum of 7°C (46°F) to a maximum of 30°C (86°F), with an average of 18°C (65°F). The month sees relatively moderate precipitation levels of 78 mm (3.1 in) over 8 days, alongside a humidity level of 71%. In contrast, October experiences cooler temperatures, with a minimum of 1°C (34°F) and an average of 13°C (56°F), peaking at 25°C (78°F). Precipitation increases significantly to 142 mm (5.6 in), occurring over 11 days, and humidity rises slightly to 78%. For more details on daily variations, check out Portsmouth weather in October.

September and November present contrasting weather patterns. September generally enjoys warmer conditions, with temperatures ranging from a mild 7°C (46°F) to a maximum of 30°C (86°F), and an average of 18°C (65°F). This month sees significant sunshine and a moderate level of humidity at 71%, alongside 78 mm (3.1 in) of precipitation over 8 days. In stark contrast, November ushers in much cooler temperatures, with lows plummeting to -12°C (11°F) and an average of only 6°C (43°F). Though maximum temperatures can still reach up to 20°C (69°F), the month experiences higher humidity levels at 84% and more precipitation, totaling 108 mm (4.3 in) over 11 days. For further details, visit Portsmouth weather in November.

September and December showcase stark contrasts in weather conditions. In September, temperatures range from a minimum of 7°C (46°F) to a pleasant average of 18°C (65°F), with maximum highs reaching up to 30°C (86°F). This month experiences relatively moderate precipitation, totaling 78 mm (3.1 in) over about 8 days, accompanied by a humidity level of 71%. In contrast, December brings a chilly climate, with temperatures plunging to a minimum of -19°C (-1°F) and an average of just 1°C (34°F). The maximum temperature peaks at 15°C (59°F), and precipitation increases to 133 mm (5.2 in) over approximately 10 days, creating a more humid atmosphere at 81%. For those interested in detailed winter weather conditions, check out Portsmouth weather in December.

Compare Portsmouth's Climate with Cities Worldwide

Conclusion: What to Expect From September Weather in Portsmouth

As summer bids farewell, September in Portsmouth offers a delightful transition with milder temperatures and a comfortable average that beckons outdoor enthusiasts. Unlike the sweltering highs of July and August, September's warmth, peaking around 30°C, becomes more pleasant and manageable. However, with occasional rainfall totaling 78 mm across just 8 days, the month still maintains a refreshing balance that sets it apart from earlier summer months. The humidity level remains moderate at 71%, inviting visitors to enjoy the scenic vistas without the oppressive heat. Overall, September presents a charming mix of warmth and refreshing showers, marking a gentle shift into autumn. Discover more about the unique climate patterns throughout the year by visiting Portsmouth Climate.