Many New Yorkers pride themselves on being efficient, even when it comes to casual conversation on the street. So "How's it going?" and "See ya" are common ways to acknowledge someone you know and to say goodby...quickly.
But during this cold spell (we're talking serious cold, but you've no doubt already being made aware of it through the media hype) the way many people--even total strangers--say goodby is to say "Stay warm..."
And the "stay warm" comment is said, well, warmly, affectionately. It's a kind of "we're all in this together" kind of acknowledgement. I've enjoyed these few days, seeing people bundled up in often creative fashion with layers of whatever clothing--or sometimes bedding--they have on hand to wrap around themselves.
Best of all, though, is that "stay warm" comment from the most unlikely sources, including the concierge in our office building who usually has nothing to say. How ironic: the cold New York weather has thawed him out!