The opposite of a "nip" in the air?

Solution 1:

"Nip in the air" is an idiom, so there may not be a direct opposite idiom describing the quality of warm air.

The closest I can think of is it was toasty. But this is usually used to describe an object or place, not the air. You might say, "the room was toasty" or "my hands are toasty in those gloves".

Another seasonal (Spring) phrase might be summer is in the air.

And then there is the simple, it was warm or maybe there is a warmth in the air.

When it's very hot, you can say, there was a sweltering heat, but that doesn't describe the air being mildly warm in the same way that nip describes it being mildly cool.

Solution 2:

Four Google Books results refine in various ways the "summer in the air" idea that Javid Jamae suggests in his answer. Bill Reynolds, Glory Days (1998), cites "the promise of summer in the air"; Ethan Canin, America America (2008), mentions "a touch of summer in the air"; Stephen Leacock, Literary Lapses (1924), refers to "a feeling of young summer in the air"; and Rudyard Kipling, The Light That Failed (1890) has "the pulse of summer in the air." A "pulse" of heat in the air makes an especially nice counterpart to a "nip" of cold in the air, I think.

Solution 3:

Etymonline attests that 'nippy' refers to 'a "biting" chill in the air'. It seems likely that this is simply a contraction of 'nip in the air'.

This conveys the sensation of the weather more than anything - the mild shock of sudden cold on the skin. I think that 'balmy' perhaps conveys the same feeling for mild hotness: the warm, almost liquid feeling of the late-summer sun.

Full disclosure: I'm English, and the fetishization of weather is practically a national sport here.