Boston weather in September
In September, Boston has daytime highs averaging 23 °C, nights around 15 °C, with 7.8 wet days, and a sea temperature of 17 °C. Figures are 1991–2020 climate normals.
- 22.7°C Average high
- 14.6°C Average low
- 7.8days Wet days
- 90mm Rainfall
- 17.0°C Sea temperature
- 13h Daylight
Daily temperatures through September
How September develops
Averages for the first, middle and last part of the month, from daily 1991–2020 normals. Chance of rain is the share of years in which that calendar day recorded more than 1 mm.
| Days | Avg high | Avg low | Chance of rain |
|---|---|---|---|
| 1–10 September | 24.7 °C | 16.2 °C | 24 % |
| 11–20 September | 22.6 °C | 14.4 °C | 28 % |
| 21–30 September | 21.0 °C | 13.2 °C | 26 % |
- Average highs fall from 24.7 °C early in September to 21.0 °C by the end of it.
What to expect
| Measure | Typical September | Share of the month |
|---|---|---|
| Days above 25 °C | 8.3 of 30 | 28 % |
| Days above 30 °C | 1.3 of 30 | 4 % |
| Days below 0 °C | 0 of 30 | 0 % |
| Days with more than 1 mm rain | 7.8 of 30 | 26 % |
Conditions in September
| Measure | Average |
|---|---|
| Mean temperature | 18 °C |
| Record high (1991–2020) | 36 °C |
| Record low (1991–2020) | 4.0 °C |
| Relative humidity | 75 % |
| Wind speed | 18 km/h |
| Cloud cover | 52 % |
Where Boston ranks in September
| Ranking | Position | Value |
|---|---|---|
| Places with the best weather in North America in September | 28th of 50 | 90/100 |
September compared with the months around it
| Month | Avg high | Rainfall | Cloud |
|---|---|---|---|
| July | 27.8 °C | 74 mm | 55 % |
| August | 26.9 °C | 86 mm | 52 % |
| September | 22.7 °C | 90 mm | 52 % |
| October | 16.3 °C | 108 mm | 53 % |
| November | 10.2 °C | 94 mm | 56 % |
See all twelve months for Boston
- September is the 4th warmest month here.
- It is the 6th driest month of the year.
- It ranks 1st for least cloud cover.
- Days last about 13 hours from sunrise to sunset.
- It rains on 7.8 of 30 days — 26 % of the month.
- Nights are about 8.1 °C cooler than the middle of the day.
- The hottest September day on record since 1991 reached 36 °C, 13 °C above the usual daytime high.
- Beach score: 56 / 100 — see how this is calculated.
QuestionsSeptember in Boston, answered
How warm is Boston in September?
Daily highs average 23 °C and nights fall to about 15 °C, a spread of 8.1 °C between afternoon and dawn. That makes September the 4th warmest month of the year here.
Does it rain much in Boston in September?
September averages 90 mm of rain, spread over about 8 of the 30 days — 26 % of the month. Cloud cover averages 52 % of the sky.
Is the sea warm enough to swim in September?
The sea averages 17 °C off Boston in September. Most people find water below 20 °C bracing and above 25 °C comfortable for a long swim; this figure is a monthly surface average, so a sheltered bay on a still afternoon can be warmer.
How long are the days in September?
Boston averages 13 hours between sunrise and sunset in September. Day length is computed from latitude and date, so it is exact rather than modelled — but it says nothing about how much of that time is sunny.
Data sources and method
- ERA5 / ERA5-Land reanalysis — Copernicus Climate Change Service (C3S), ECMWF. Copernicus Licence
- Open-Meteo — Open-Meteo.com. CC BY 4.0
- NOAA OISST v2.1 — NOAA Physical Sciences Laboratory. Public domain
- GeoNames — GeoNames.org. CC BY 4.0
- Normal period
- 1991–2020
- Dataset version
- era5-1991-2020-scores1.1
- Last processed
Measured values come from ERA5 reanalysis and are reproduced unchanged. Calculated by us: daylight hours, beach and comfort scores, monthly rankings, threshold day counts, chance of a wet day by date, ten-day averages within the month. Full formulas are on the methodology page.
Contains modified Copernicus Climate Change Service information 2026. Neither the European Commission nor ECMWF is responsible for any use that may be made of the Copernicus information or data it contains.