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September Weather in Portsmouth, United States

Last updated: June 24, 2025

In Portsmouth, United States, the average temperature in September typically hovers around 24°C (75°F). Expect a range of temperatures, with a minimum of 12°C (54°F) and a maximum of 35°C (95°F). This month also sees an average precipitation of 127 mm (5.0 in) over approximately 13 days, and a notable average humidity of 78%.

September Temperature in Portsmouth

In September in Portsmouth, the average temperature is 24°C (75°F), with maximums reaching 35°C (95°F) and minimums dropping to as low as 12°C (54°F). This pleasant weather makes it an ideal time for outdoor activities and exploring the local attractions.

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. In Portsmouth, September typically has an average temperature of 24°C (75°F), with highs reaching up to 35°C (95°F) and lows dropping to around 12°C (54°F).
JanFebMarAprMayJunJulAugSepOctNovDec424751606876807974665347
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

In Portsmouth in September, the total precipitation amount is 127 mm (5.0 in) spread over 13 days.

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. In Portsmouth, September typically receives around 127 mm (5.0 inches) of precipitation spread over approximately 13 days.
JanFebMarAprMayJunJulAugSepOctNovDec90997997121121142169127917889
Data source: Precipitation data for September generated using information from the Copernicus Climate Change Service.
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September Humidity in Portsmouth

In Portsmouth in September humidity is 78%. With an average temperature of 24°C (75°F), this humidity feels like a warm blanket wrapping around you, making travel sticky and heavy but also inviting, as long as you're prepared with plenty of water and light clothing.

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, helping you understand how conditions vary by season. In September, average humidity in Portsmouth is approximately 78%, offering insight into how humid the weather typically is during this month.
JanFebMarAprMayJunJulAugSepOctNovDec84%83%82%81%80%82%81%81%78%80%84%84%
Data source: Humidity data for September generated using information from the Copernicus Climate Change Service.

September UV Index in Portsmouth

In Portsmouth in September, the maximum UV index is 9, which corresponds to a very high Exposure Category. The time to burn is just 15 minutes, so it's essential to take precautions. For detailed information, you can check the Hourly UV Index in Portsmouth.

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. In September, the maximum UV index in Portsmouth typically reaches 9, which falls under the very high exposure category.
JanFebMarAprMayJunJulAugSepOctNovDec4689101011109853
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.

How September Weather in Portsmouth Compares to Other Months

Weather in Portsmouth varies notably across the year, with each month offering distinct climate conditions. This page compares September’s weather to other months in Portsmouth, focusing on differences in temperature, rainfall, humidity, and UV levels.

This table compares monthly weather conditions in Portsmouth, showing how September’s temperature, precipitation, humidity, and UV index levels differ from those of other months.
 TemperaturePrecipitationHumidityUV
January Weather6°C (43°F)90 mm (3.5 inches)84%moderate
February Weather8°C (47°F)99 mm (3.9 inches)83%high
March Weather10°C (51°F)79 mm (3.1 inches)82%very high
April Weather16°C (60°F)97 mm (3.8 inches)81%very high
May Weather20°C (69°F)121 mm (4.7 inches)80%very high
June Weather24°C (76°F)121 mm (4.8 inches)82%very high
July Weather27°C (81°F)142 mm (5.6 inches)81%extreme
August Weather26°C (79°F)169 mm (6.6 inches)81%very high
September Weather24°C (75°F)127 mm (5.0 inches)78%very high
October Weather19°C (66°F)91 mm (3.6 inches)80%very high
November Weather11°C (53°F)78 mm (3.1 inches)84%moderate
December Weather8°C (47°F)89 mm (3.5 inches)84%moderate

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