It is still hot in September due to a combination of meteorological factors and climate trends. Typically, September is a transitional month when the sun shifts its intensity toward the Southern Hemisphere, leading to cooling in the Northern Hemisphere. However, persistent high-pressure ridges can trap warm, muggy air and delay this cooling effect. Warm nights also prevent relief from daytime heat, keeping average temperatures higher overall. Additionally, climate change has intensified heat waves, making warm Septembers more common and intense than in the past. For example, in parts of the United States, September temperatures have been several degrees above normal in recent years due to these factors.

In some regions, other local influences like the warming of nearby ocean surfaces or specific wind patterns, such as Santa Ana winds in Los Angeles, contribute to the persistence of high temperatures through September and into October. In Los Angeles, sea surface warming and dry downslope winds create an environment for hotter late summer and early fall days.

Globally, September 2023 was notable for record-breaking warm temperatures driven chiefly by unusually warm oceans, which continue to affect atmospheric temperatures worldwide. This adds to the trend of hotter Septembers influenced by climate change and natural phenomena like El Niño.

Therefore, being hot in September can be explained by a mix of persistent weather patterns, local geographical factors, and the broader impact of global warming.