This BBC story speculates that Delta became the main variant in part due to the cold weather. But I'm not sure any one factor can be singled out.
Re:The UK’s delta surge is collapsing
by mariodiana on Friday August 06, 2021 @09:23AM
(#28086795)
I saw a doctor on YouTube (just a private guy with his own channel) apply this same reasoning to Florida. Right now is Florida's "winter": meaning, people tend to stay indoors with the windows closed (to avoid the oppressive heat of the July and August).
Edit:
https://www.reuters.com/article/us-health-coronavirus-obesit...