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September Weather in Portsmouth, United States

Last updated: June 24, 2025

In September, the average temperature in Portsmouth, United States hovers around 20°C (67°F), creating a pleasant climate for outdoor activities. Expect minimums of 11°C (52°F) and maximums reaching 27°C (80°F). However, be prepared for some rain, as 110 mm (4.3 in) of precipitation is likely over 10 days. With average humidity at 75%, this month offers a refreshing atmosphere, perfect for exploring!

September Temperature in Portsmouth

In September, in Portsmouth, the average temperature is 20°C (67°F). The maximum can reach 27°C (80°F), and the minimum can be as low as 11°C (52°F). It's a comfortable time for traveling, making it ideal for exploring the area's 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 20°C (67°F), with highs reaching up to 27°C (80°F) and lows dropping to around 11°C (52°F).
JanFebMarAprMayJunJulAugSepOctNovDec353638465464727267594839
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 precipitation amount is 110 mm (4.3 in) over a period of 10 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 110 mm (4.3 inches) of precipitation spread over approximately 10 days.
JanFebMarAprMayJunJulAugSepOctNovDec9612111914713310410489110169117147
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 75%. With an average temperature of 20°C (67°F), this humidity feels like a warm, enveloping blanket that makes outdoor activities more strenuous, causing travelers to feel sticky and fatigued faster.

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 75%, offering insight into how humid the weather typically is during this month.
JanFebMarAprMayJunJulAugSepOctNovDec84%84%82%79%75%78%76%76%76%75%80%80%
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 8, which corresponds to a very high Exposure Category. This means that the time to burn is only 15 minutes. For more detailed information, you can refer to 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 8, which falls under the very high exposure category.
JanFebMarAprMayJunJulAugSepOctNovDec3568910998642
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 Weather1°C (35°F)95 mm (3.8 inches)84%moderate
February Weather2°C (36°F)121 mm (4.8 inches)84%moderate
March Weather3°C (39°F)119 mm (4.7 inches)82%high
April Weather8°C (46°F)147 mm (5.8 inches)79%very high
May Weather12°C (55°F)132 mm (5.2 inches)75%very high
June Weather18°C (64°F)104 mm (4.1 inches)78%very high
July Weather22°C (72°F)104 mm (4.1 inches)76%very high
August Weather22°C (73°F)89 mm (3.5 inches)76%very high
September Weather20°C (67°F)110 mm (4.3 inches)75%very high
October Weather15°C (60°F)169 mm (6.7 inches)75%high
November Weather9°C (48°F)117 mm (4.6 inches)80%moderate
December Weather4°C (39°F)147 mm (5.8 inches)80%low

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