Reason #457 to Make Aliyah
In the course of his all-in-one-breath stream-of-consciousness halacha survey course, my rav in Israel mentions around this time of year (or three weeks ago) that it's a minhag to wear white on Rosh HaShana because we're trying to appear as pure as possible when approaching G-d, therefore, "here is Israel, if you decide you're going to wear your black outfit on Rosh HaShana, you're very likely going to stick out like a sore thumb."
But here, quite aside from any mention of the cardinal rule (is that a Catholic phrase??) of not wearing white after Labor Day...even if it's sufficiently warm outside to wear a white-themed summer-y outfit, it's nearly a foregone conclusion that they're going to air-condition the shul to within an inch of your life.
So much for minhag...