Wouldn't it be northern-hemisphere-centric? Which makes sense -- I mean, there isn't like a uniform standard for astological signs. Even within the northern hemisphere there are different labels. The trend I'd look for is an association between strength and being born near the beginning of the local harvest season, or so. But this stuff is all ad-hoc folk observations and the practitioners are motivated to add extra history, to give it the appearance of "ancient wisdom." So I'm not motivated enough to gather the data to back it up, haha.

Edit: It also would be the case, I guess, that summers being generally pleasant-ish would be regional, right? Like some hotter regions can have pretty lean summers I think.