I've lived northeast US for 5 years with UC, then part-time back and forth from Florida to northeast, then full-time Florida for the past 6 years. Climate has no impact on my UC. Food does.
I believe more UC is found in northern countries because of genetics than climate. That is northern Europeans have the proclivity towards IBD and as they spread to Canada and US they brought their genes with them
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But IBD is now becoming more frequent in other populations now and countries now that rarely had it. So either the genes are becoming intermixed or something else is causing UC frequency to show up in other races and ethnic populations.
Month back I posted an article suggesting that Round-up, an herbicide, was possibly a factor. And today someone else posted a similar scientific paper making the same projection (maybe the same one as earlier).