It is still hot in September primarily due to a combination of persistent high-pressure systems that delay the usual cooling trend, climate change driving overall warmer temperatures, and cyclic climate patterns like El Niño. High pressure can cause warm air masses to linger, resulting in unusually warm and sticky weather, sometimes pushing temperature records for September. Additionally, global warming has raised baseline temperatures, making warm September days more common and intense than in the past. Cyclical ocean- atmosphere phenomena like El Niño also contribute by injecting extra heat into the atmosphere, exacerbating warm conditions worldwide, including in September. This means that sweating it out in September is increasingly becoming the new normal in many regions.
More specifically, the jet stream patterns in some years do not bring the usual cool air southward, allowing summer-like heat to persist well into September. Warm nights with less cooling at night also keep daily average temperatures high. This delayed cooling effect is linked to a "ridge of high pressure" that stalls over areas, trapping heat. The warmth in ocean waters linked to El Niño and warming oceans overall enhances this effect by influencing atmospheric conditions.
In summary, the reasons it can still be so hot in September include:
- Persistent high-pressure systems trapping warm air.
- Long-term climate change raising average temperatures.
- El Niño and warm ocean temperatures boosting atmospheric heat.
- Jet stream patterns limiting cooler air from reaching some regions.
- Warmer nights reducing relief from daytime heat.
Together, these factors explain why September can feel much like summer despite the calendar indicating early fall.
