The main reasons why it is often hot in September include:

• A persistent ridge of high pressure can trap warm air and delay the usual cooling that comes with the transition from summer to fall. This ridge keeps temperatures warmer than seasonal norms by suppressing cooler air masses from moving in and allowing warm, humid air to linger.

• September warmth has become more intense and frequent due to climate change. Rising average temperatures from fossil fuel emissions cause heat waves to extend longer into the fall, with warmer nights giving less relief from the heat.

• Natural climate patterns also contribute, such as El Niño, which raises ocean temperatures and, in turn, atmospheric temperatures. Warm ocean surfaces especially influence coastal and southern regions with more intense heat.

• In specific locations like Los Angeles, late summer and early fall see the disappearance of marine layers that cool the coast during summer, combined with offshore Santa Ana winds causing warm downslope air that spikes temperatures in September and October.

• Recent years have seen record-breaking September heat globally, partly due to these factors combined, indicating a new normal for late summer and early fall warmth in many parts of the world.

In summary, September can be very hot due to a combination of persistent high- pressure systems, climate change, natural ocean-atmosphere cycles like El Niño, and local phenomena such as coastal wind patterns that delay cooling after summer and push temperatures above typical seasonal averages.