September is one of the warmer months of the year in Siwan, with average highs near 33.9°C and lows around 22.1°C (mean 28°C). It is a relatively wet time of year, averaging 175 mm of rainfall and 84% relative humidity across the period.
September is one of the warmer months of the year in Siwan. Across 45 years of records, the warmest September in Siwan was 1983 — when the month averaged 31.5°C — while the coldest was 1984 at 24.4°C. Typical September days reach 33.9°C with nights near 22.1°C. The long-term trend is warming, about 0.2°C per decade.
Ranked against the other 11 months using 1981–2025 averages.
| Metric | 1980s | 1990s | 2000s | 2010s | 2020s | Change |
|---|---|---|---|---|---|---|
| Avg Temp | 28.0°C | 27.6°C | 27.9°C | 28.1°C | 28.9°C | +0.9°C |
| Avg High | 34.5°C | 33.4°C | 33.5°C | 34.2°C | 34.2°C | -0.3°C |
| Avg Low | 21.5°C | 21.8°C | 22.3°C | 21.9°C | 23.6°C | +2.1°C |
| Avg Rain | 185 mm | 167 mm | 165 mm | 189 mm | 162 mm | -23 mm |
September temperatures in Siwan have risen by about 1.3°C between 1981–1985 and 2021–2025.
September has ranged from a record low of 16.3°C (1994) to a record high of 38.7°C (1983).
Rainfall in September is a relatively wet time of year, averaging 175 mm, with a wettest year of 409 mm.
Daytime highs and overnight lows have shifted differently — highs rose about 0°C per decade while lows rose 0.4°C per decade, narrowing the gap between day and night.
A good time to visit Siwan
September is a good time to visit Siwan, with days averaging 33.9°C and nights around 22.1°C (mean 28°C, warmer than Siwan's yearly average) and higher rainfall. November and February are marginally more temperate, but September still suits most travellers.
| Year | High | Low | Avg | Rainfall | Humidity | Wind | Solar |
|---|---|---|---|---|---|---|---|
| 2025 | 33.6°C | 24.7°C | 29.1°C | 128 mm | 85% | 1.5 m/s | 4.5 |
| 2024 | 35.9°C | 23.9°C | 29.9°C | 189 mm | 85% | 2.1 m/s | 4.4 |
| 2023 | 34.8°C | 24.5°C | 29.6°C | 108 mm | 83% | 2.1 m/s | 4.3 |
| 2022 | 34.5°C | 23.1°C | 28.8°C | 147 mm | 82% | 1.8 m/s | 3.8 |
| 2021 | 32.9°C | 24.3°C | 28.6°C | 137 mm | 88% | 2.1 m/s | 4.5 |
| 2020 | 33.6°C | 21.3°C | 27.5°C | 264 mm | 86% | 1.5 m/s | 4 |
| 2019 | 35.8°C | 21.3°C | 28.5°C | 287 mm | 84% | 2.2 m/s | 3.7 |
| 2018 | 35.1°C | 22.4°C | 28.8°C | 103 mm | 81% | 2.2 m/s | 4.8 |
| 2017 | 33.8°C | 23.7°C | 28.8°C | 60 mm | 84% | 1.5 m/s | 4.8 |
| 2016 | 35.3°C | 23.7°C | 29.5°C | 239 mm | 86% | 1.5 m/s | 3.8 |
| 2015 | 34.5°C | 19.3°C | 26.9°C | 59 mm | 79% | 1.7 m/s | 5.1 |
| 2014 | 33.9°C | 20.7°C | 27.3°C | 239 mm | 85% | 1.8 m/s | 4.6 |
| 2013 | 33.3°C | 22.9°C | 28.1°C | 241 mm | 82% | 1.5 m/s | 4.9 |
| 2012 | 33.3°C | 20.8°C | 27.0°C | 251 mm | 84% | 1.7 m/s | 4.6 |
| 2011 | 34.0°C | 22.3°C | 28.2°C | 266 mm | 86% | 2.2 m/s | 4.6 |
| 2010 | 33.2°C | 21.6°C | 27.4°C | 149 mm | 86% | 2.1 m/s | 4.4 |
| 2009 | 33.3°C | 23.7°C | 28.5°C | 110 mm | 85% | 1.9 m/s | 5.1 |
| 2008 | 32.8°C | 21.5°C | 27.2°C | 198 mm | 87% | 1.7 m/s | 5.1 |
| 2007 | 33.0°C | 22.2°C | 27.6°C | 309 mm | 88% | 2.2 m/s | 4.3 |
| 2006 | 33.3°C | 22.5°C | 27.9°C | 201 mm | 85% | 2.1 m/s | 4.7 |
| 2005 | 33.3°C | 22.4°C | 27.8°C | 76 mm | 85% | 2.1 m/s | 4.9 |
| 2004 | 34.4°C | 22.1°C | 28.3°C | 65 mm | 80% | 1.7 m/s | 4.8 |
| 2003 | 33.7°C | 23.1°C | 28.4°C | 137 mm | 85% | 2.2 m/s | 4.3 |
| 2002 | 36.1°C | 22.6°C | 29.3°C | 101 mm | 77% | 2.5 m/s | 4.6 |
| 2001 | 32.9°C | 21.7°C | 27.3°C | 233 mm | 85% | 1.6 m/s | 4.3 |
| 2000 | 32.6°C | 21.3°C | 27.0°C | 219 mm | 87% | 2.5 m/s | 4.2 |
| 1999 | 33.4°C | 22.4°C | 27.9°C | 87 mm | 85% | 2.5 m/s | 4.9 |
| 1998 | 33.2°C | 23.7°C | 28.4°C | 221 mm | 87% | 2.2 m/s | 5.1 |
| 1997 | 32.8°C | 21.6°C | 27.2°C | 137 mm | 84% | 1.5 m/s | 4.5 |
| 1996 | 32.7°C | 23.5°C | 28.1°C | 173 mm | 85% | 1.5 m/s | 5.4 |
| 1995 | 32.9°C | 21.6°C | 27.3°C | 202 mm | 86% | 2.3 m/s | 5 |
| 1994 | 33.0°C | 16.3°C | 24.6°C | 182 mm | 84% | 2.3 m/s | 4.9 |
| 1993 | 33.2°C | 22.6°C | 27.9°C | 297 mm | 88% | 2.3 m/s | 4.6 |
| 1992 | 35.6°C | 21.2°C | 28.4°C | 58 mm | 77% | 2.1 m/s | 5 |
| 1991 | 32.8°C | 21.3°C | 27.0°C | 195 mm | 83% | 1.5 m/s | 4.4 |
| 1990 | 34.4°C | 23.3°C | 28.8°C | 120 mm | 87% | 2.2 m/s | 4.3 |
| 1989 | 36.7°C | 23.4°C | 30.1°C | 136 mm | 78% | 2 m/s | 4.4 |
| 1988 | 33.7°C | 22.6°C | 28.2°C | 134 mm | 85% | 1.8 m/s | 4.9 |
| 1987 🌧️ | 33.4°C | 20.6°C | 27.0°C | 409 mm | 89% | 2 m/s | 4.3 |
| 1986 | 34.6°C | 20.4°C | 27.5°C | 93 mm | 81% | 2.3 m/s | 5 |
| 1985 | 33.9°C | 22.7°C | 28.3°C | 275 mm | 88% | 1.9 m/s | 4.4 |
| 1984 ❄️ | 31.5°C | 17.3°C | 24.4°C | 247 mm | 86% | 2 m/s | 4.8 |
| 1983 🔥 | 38.7°C | 24.3°C | 31.5°C | 47 mm | 69% | 2.5 m/s | — |
| 1982 | 34.8°C | 19.1°C | 27.0°C | 124 mm | 82% | 2 m/s | — |
| 1981 | 33.3°C | 23.2°C | 28.3°C | 200 mm | 87% | 2 m/s | — |
Actual recorded September data for each year, 1981–2025 — sourced from NASA POWER.
The average high in September is around 33.9°C with lows near 22.1°C (mean 28°C), based on 45 years of NASA POWER data.
September averages 175 mm of rainfall in Siwan, a relatively wet time of year. The wettest on record was 409 mm in 1987.
The hottest September reached 38.7°C in 1983. The coldest dipped to 16.3°C in 1994.
Comparing the earliest years on record (1981–1985) with the most recent (2021–2025), average September temperatures have risen by about 1.3°C. Daytime highs moved from 34.4°C to 34.3°C and overnight lows from 21.3°C to 24.1°C over the same span.
September is one of the warmer months of the year in Siwan and a relatively wet time of year, so plan for around 33.9°C daytime highs and 22.1°C at night.
Not quite — September is the 7th coolest month in Siwan, averaging 28°C.
For September in Siwan, pack light, breathable clothing and sun protection, with typical highs of 33.9°C and lows of 22.1°C. Bring an umbrella or light rain jacket too.
Relative humidity in September averages 84% — quite humid.
Average wind speed in September is 2 m/s. The windiest September on record averaged 2.5 m/s (1983).
The wettest September on record was 1987 with 409 mm. On average, September sees 175 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 Siwan 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.
