September is one of the warmer months of the year in Allentown, with average highs near 25°C and lows around 12.8°C (mean 18.9°C). It is a relatively wet time of year, averaging 114 mm of rainfall and 73% relative humidity across the period.
September is one of the warmer months of the year in Allentown. Across 45 years of records, the warmest September in Allentown was 2005 — when the month averaged 21.6°C — while the coldest was 1984 at 16.1°C. Typical September days reach 25°C with nights near 12.8°C. The long-term trend is warming, about 0.6°C per decade.
September is a relatively wet time of year in Allentown, averaging 114 mm of rainfall. The wettest September on record was 2011 with 317 mm, while the driest was 2019 at 20 mm.
Relative humidity in Allentown during September averages 73%. The most humid September was 2011 at 88%, and the least humid was 1983 at 59%.
September receives about 4.2 kWh/m²/day of solar energy in Allentown. The sunniest September was 2007 at 5 kWh/m²/day, and the cloudiest was 2018 at 3.1.
Average wind speed in Allentown during September is 0.4 m/s. The windiest Septembers were 1999 and 2010 at 0.5 m/s, while the calmest were 2023, 2024 and 2025, among others at 0.4 m/s.
Ranked against the other 11 months using 1981–2025 averages.
| Metric | 1980s | 1990s | 2000s | 2010s | 2020s | Change |
|---|---|---|---|---|---|---|
| Avg Temp | 18.1°C | 18.2°C | 18.8°C | 20.0°C | 19.8°C | +1.7°C |
| Avg High | 24.4°C | 24.3°C | 24.9°C | 26.1°C | 25.6°C | +1.2°C |
| Avg Low | 11.8°C | 12.1°C | 12.8°C | 13.9°C | 13.9°C | +2.1°C |
| Avg Rain | 100 mm | 118 mm | 122 mm | 118 mm | 109 mm | +9 mm |
September temperatures in Allentown have risen by about 1.6°C between 1981–1985 and 2021–2025.
September has ranged from a record low of 1.6°C (1989) to a record high of 36.1°C (2010).
Rainfall in September is a relatively wet time of year, averaging 114 mm, with a wettest year of 317 mm.
One of the best times to visit Allentown
September is one of the best times to visit Allentown. Expect days averaging 25°C and nights around 12.8°C (mean 18.9°C) — warmer than Allentown's yearly average — with higher rainfall (114 mm), so conditions are generally comfortable for sightseeing and outdoor plans.
| Year | Avg High | Avg Temp | Avg Low | Hottest Day | Coldest Day | Rainfall | Humidity | Wind | Solar |
|---|---|---|---|---|---|---|---|---|---|
| 2025 | 27.3°C | 20.9°C | 14.5°C | 31.1°C | 9.6°C | 62 mm | 67% | 0.4 m/s | 4.4 |
| 2024 | 25.9°C | 19.9°C | 14.0°C | 30.6°C | 8.5°C | 34 mm | 72% | 0.4 m/s | 4.1 |
| 2023 | 25.4°C | 19.9°C | 14.3°C | 35.6°C | 8.4°C | 163 mm | 77% | 0.4 m/s | 4.1 |
| 2022 | 25.6°C | 19.6°C | 13.6°C | 32.0°C | 5.4°C | 89 mm | 71% | 0.4 m/s | 4.3 |
| 2021 | 24.2°C | 19.2°C | 14.2°C | 28.6°C | 8.2°C | 216 mm | 83% | 0.4 m/s | 4.2 |
| 2020 | 25.1°C | 19.0°C | 12.8°C | 30.2°C | 3.6°C | 92 mm | 72% | 0.4 m/s | 4.1 |
| 2019 | 27.9°C | 21.0°C | 14.2°C | 33.6°C | 7.2°C | 20 mm | 66% | 0.4 m/s | 4.3 |
| 2018 | 25.1°C | 20.5°C | 15.9°C | 33.2°C | 8.6°C | 198 mm | 86% | 0.4 m/s | 3.1 |
| 2017 | 25.9°C | 19.9°C | 13.9°C | 33.7°C | 7.6°C | 59 mm | 73% | 0.4 m/s | 4.4 |
| 2016 | 27.9°C | 21.2°C | 14.6°C | 34.7°C | 7.0°C | 83 mm | 64% | 0.4 m/s | 4.5 |
| 2015 | 27.7°C | 21.1°C | 14.6°C | 35.4°C | 9.8°C | 99 mm | 67% | 0.4 m/s | 4.5 |
| 2014 | 26.1°C | 19.6°C | 13.2°C | 33.0°C | 6.1°C | 38 mm | 67% | 0.4 m/s | 4.4 |
| 2013 | 24.7°C | 17.7°C | 10.8°C | 33.8°C | 3.8°C | 50 mm | 69% | 0.4 m/s | 4.9 |
| 2012 | 24.3°C | 18.6°C | 12.9°C | 33.3°C | 5.2°C | 153 mm | 76% | 0.4 m/s | 4.2 |
| 2011 🌧️ | 24.0°C | 19.4°C | 14.8°C | 29.2°C | 5.2°C | 317 mm | 88% | 0.4 m/s | 3.2 |
| 2010 | 27.0°C | 20.5°C | 13.9°C | 36.1°C | 7.7°C | 161 mm | 63% | 0.5 m/s | 4.5 |
| 2009 | 22.0°C | 17.0°C | 11.9°C | 27.0°C | 5.2°C | 92 mm | 81% | 0.4 m/s | 4 |
| 2008 | 25.4°C | 19.5°C | 13.5°C | 34.2°C | 6.0°C | 156 mm | 74% | 0.4 m/s | 4.3 |
| 2007 | 27.0°C | 20.2°C | 13.4°C | 32.0°C | 6.3°C | 32 mm | 68% | 0.4 m/s | 5 |
| 2006 | 21.8°C | 16.8°C | 11.8°C | 26.7°C | 5.3°C | 139 mm | 83% | 0.4 m/s | 3.6 |
| 2005 🔥 | 28.8°C | 21.6°C | 14.5°C | 34.4°C | 5.7°C | 28 mm | 60% | 0.4 m/s | 4.9 |
| 2004 | 24.6°C | 19.2°C | 13.7°C | 28.3°C | 6.3°C | 227 mm | 82% | 0.4 m/s | 4 |
| 2003 | 23.3°C | 18.3°C | 13.3°C | 26.8°C | 5.9°C | 210 mm | 84% | 0.4 m/s | 3.7 |
| 2002 | 27.1°C | 20.3°C | 13.5°C | 35.5°C | 8.0°C | 120 mm | 65% | 0.4 m/s | 4.5 |
| 2001 | 24.8°C | 17.9°C | 11.0°C | 30.6°C | 4.3°C | 105 mm | 66% | 0.4 m/s | 4.6 |
| 2000 | 23.7°C | 17.3°C | 11.0°C | 31.7°C | 1.9°C | 107 mm | 73% | 0.4 m/s | 4.1 |
| 1999 | 24.3°C | 18.5°C | 12.8°C | 32.6°C | 4.7°C | 273 mm | 77% | 0.5 m/s | 4 |
| 1998 | 26.6°C | 19.8°C | 13.1°C | 32.4°C | 4.4°C | 65 mm | 68% | 0.4 m/s | 4.3 |
| 1997 | 23.6°C | 17.5°C | 11.5°C | 30.0°C | 4.0°C | 81 mm | 71% | 0.4 m/s | 4.2 |
| 1996 | 24.2°C | 18.6°C | 13.1°C | 31.4°C | 5.6°C | 134 mm | 78% | 0.4 m/s | 3.7 |
| 1995 | 25.0°C | 18.3°C | 11.7°C | 32.5°C | 4.5°C | 81 mm | 63% | 0.4 m/s | 4.3 |
| 1994 | 22.9°C | 17.0°C | 11.1°C | 28.6°C | 6.4°C | 93 mm | 76% | 0.4 m/s | 4.2 |
| 1993 | 23.4°C | 18.1°C | 12.7°C | 32.1°C | 2.1°C | 184 mm | 78% | 0.4 m/s | 3.3 |
| 1992 | 23.9°C | 18.0°C | 12.0°C | 29.4°C | 2.9°C | 114 mm | 74% | 0.4 m/s | 3.8 |
| 1991 | 24.9°C | 18.4°C | 11.9°C | 34.3°C | 2.4°C | 86 mm | 66% | 0.4 m/s | 4.3 |
| 1990 | 23.9°C | 17.6°C | 11.3°C | 31.4°C | 2.6°C | 69 mm | 72% | 0.4 m/s | 4.2 |
| 1989 | 24.6°C | 18.8°C | 12.9°C | 34.3°C | 1.6°C | 129 mm | 75% | 0.4 m/s | 3.9 |
| 1988 | 23.4°C | 17.0°C | 10.5°C | 28.2°C | 4.9°C | 83 mm | 74% | 0.4 m/s | 4.4 |
| 1987 | 22.8°C | 17.6°C | 12.4°C | 26.8°C | 4.9°C | 216 mm | 83% | 0.4 m/s | 3.9 |
| 1986 | 24.5°C | 18.3°C | 12.1°C | 30.5°C | 3.7°C | 68 mm | 74% | 0.4 m/s | 3.9 |
| 1985 | 26.1°C | 19.5°C | 12.9°C | 33.5°C | 5.5°C | 161 mm | 66% | 0.4 m/s | 4.5 |
| 1984 ❄️ | 22.6°C | 16.1°C | 9.6°C | 29.4°C | 3.8°C | 37 mm | 74% | 0.4 m/s | 4.5 |
| 1983 | 26.9°C | 19.5°C | 12.1°C | 35.5°C | 3.8°C | 52 mm | 59% | 0.4 m/s | — |
| 1982 | 24.9°C | 18.4°C | 11.8°C | 30.8°C | 7.3°C | 71 mm | 69% | 0.4 m/s | — |
| 1981 | 24.1°C | 17.9°C | 11.8°C | 31.9°C | 3.4°C | 85 mm | 70% | 0.4 m/s | — |
Actual recorded September data for each year, 1981–2025 — sourced from NASA POWER.
The average high in September is around 25°C with lows near 12.8°C (mean 18.9°C), based on 45 years of NASA POWER data.
September averages 114 mm of rainfall in Allentown, a relatively wet time of year. The wettest on record was 317 mm in 2011.
The hottest September reached 36.1°C in 2010. The coldest dipped to 1.6°C in 1989.
Comparing the earliest years on record (1981–1985) with the most recent (2021–2025), average September temperatures have risen by about 1.6°C. Daytime highs moved from 24.9°C to 25.7°C and overnight lows from 11.6°C to 14.1°C over the same span.
September is one of the warmer months of the year in Allentown and a relatively wet time of year, so plan for around 25°C daytime highs and 12.8°C at night.
Not quite — September is the 9th coolest month in Allentown, averaging 18.9°C.
For September in Allentown, pack light layers you can adjust through the day, with typical highs of 25°C and lows of 12.8°C. Bring an umbrella or light rain jacket too.
Relative humidity in September averages 73% — moderately humid.
Average wind speed in September is 0.4 m/s. The windiest September on record averaged 0.5 m/s (1999).
The wettest September on record was 2011 with 317 mm. On average, September sees 114 mm.
All figures are derived from NASA POWER (Prediction Of Worldwide Energy Resources), based on MERRA-2 satellite reanalysis, covering 1981–2025.
Explore how the weather in Allentown behaves each September, based on 45 years of NASA POWER climate data from 1981 to 2025. This page breaks down average temperatures, rainfall, humidity, wind and solar radiation for September specifically, along with year-by-year records and how the month has changed over the decades.
