What's the opposite of a nip in the air?
Nip is for cold, what do we say for a slight hotness in the air?
"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.
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.
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.
You can describe slightly warm weather as balmy much as slightly cold weather is nippy, a synonymous idiom with 'a nip in the air/to the air'.
One possible antonym of nip is glow.
One definition of glow, according to Merriam-Webster, is "to experience a sensation of or as if of heat". Moreover, this meaning of "experiencing heat" is supported by Merriam Webster's thesaurus; glow is listed as a near synonym of warmth.
Therefore, to express warmth in the air, you could say:
There is a glow to the air.
A quick search on Google Books reveals instances of glow in the air and glow to the air in print, including the Pulitzer Prize–winning Children of Crisis.
Another possible option is fire or fieriness.
There was a fieriness in the air.
There was a fire to the air.
Other possibilities include a burn to the air (which is used in this Google Books search result), calidity in the air, a heat in the air, and a torridity to the air.
I'll try close as in "it's a close, hazy day." It means uncomfortably hot in a humid, airless, stuffy way. I use the word sometimes. Invariably, the hearer says, "What did you say?" Apparently, close isn't spoken much, but neither is nip in the air.
The opposite of a nip in the air? Warm breeze
If you are very brave you could use lilt. I think that it conveys the meaning that you want, but I don't think it has been used in this exact context.
There was a lilt in the air, and we could tell that summer was well on it's way.
Spring is in the air.
"PM Update: A touch of warmth for summer’s final day Friday", Washington DC weather page
"yeah, but it's a dry heat," Arizona
"Here come those Santa Ana winds again," Los Angeles, according to Steely Dan
I've never heard the exact term 'a nip in the air' but here in the UK we say variations of:
"It's a bit nippy today."
Nippy being the commonly used term here.
A nippy scene in London, UK goes something like this:
It's around zero degrees outside. You can see every particle of your breath in the air; and after ten minutes without gloves on your hands are becoming numb. The slight breeze soon feels like ocean waves washing over you and whipping your rosy, numb cheeks. You walk quickly with your chin tucked beneath your coat, and your hands in your pockets. You can bear it for a few hours if you have to, but want to get inside as soon as possible.
As a single word opposite of this, you could use stuffy.
You could say:
"It's a bit stuffy today."
A stuffy scene in London, UK goes something like this:
The moment you step out of the moderately air conditioned building you were in, you're greeted by a thick wall of heat. Once you step into it, it does not relent. The shade only serves to keep the direct heat off of your skin, while the thick heat remains. After half an hour of brisk walking, you're gulping down water while begging for a whisper of breeze. You can bear it for a few hours if you have to, but want to get inside as soon as possible.
How about a tropic hint?
Emily Dickenson's "Psalm of the Day", a poem about summer, has this image:
Arctic creature, dimly stirred // By tropic hint,
An alternative, possibly more suited to everyday speech, would be a hint of summer: "There was a hint of summer in the air." (I see Sven Yargs has made a similar suggestion.)
Since nip is a biting cold, you need something that describes the action of the hot.
"Heat hung in the air" is a possibility. It gives the sense of the air holding the temperature without seeming it seeming like a scorching day.
It also has the benefit of "in the air" can be changed or removed. "A heat hung over them..."