Proton and Atom LT vs Nano Air

We'll get to compare the Proton LT vs Nano Air and Atom LT vs Nano Air. 

In terms of warmth, all three are the same. But what about other departments?

But first, you'd probably better go ahead and read the foreword of my Atom LT vs Proton LT article. I lay out some important facts about the Arcteryx's motivation behind launching Proton LT there. 

If you don't have time, then let me summarize the part that concerns you: 

1. Atom LT started to lose its share when the advanced fabrics and insulation pieces were released into the market (such as Patagonia's Nano Air). 
2. Arcteryx's response against these higher technologies was to release a superior replacement to the Atom LT. And that was Proton LT. 

This information is important for this comparison because it might be wiser to compare the Nano Air to the Proton LT instead of the Atom LT.

Packing

Air is packable whereas the Atom and Proton LT aren't. 

The differences are:

Breathability

This makes up the single biggest downside of Atom LT.

Perfectly breathing FullRange insulation prevents you from overheating during any high output activity.

Atom LT, on the other hand, was released years before the Nano Air. So it isn't equipped with one of those advanced breathable active insulation pieces.

That's why its insulation can't keep up with the breathability of the cutting-edge jackets such as Nano Air or Proton LT. 

But Atom LT comes with a well-known feature: Side fleece panels. Thanks to these panels under the arms, air can circulate much more and keeps you less sweaty when you're physically active.

This helps the Atom LT with its breathability, but undoubtedly not as much as that of the other two. 

Also, all these jackets are DWR coated. 

Warmth & Weather Resistance

Side fleece panels of Atom LT become a disadvantage when it's cold, windy and/or rainy. They'll let some foul weather in. 

However, in spite of these notable disadvantages, Atom LT still stays slightly warmer than the Nano Air because of the Air's somewhat "fragile" construction. 

Absence of any side fleece panels simply doesn't cut it for Air in this manner. 

In terms of warmth, Proton LT is positioned somewhere in between. 

When it comes to weather resistance, on the other hand, relatively outdated outer fabric of Atom LT holds it back. Like Atom LT, Nano Air is also held back, but by its highly breathable face. 

Proton LT seems to have found the sweet spot, and performs noticeably better than each of them. 

Conclusion

In short:

• Getting the Nano Air (2019 version) would probably be a better decision if you're looking for an athletic piece to be used for high output activities ONLY.
• If you're going to use it in the neighborhood when it's cold out as well, then I'd recommend the Atom LT - but I'd also consider getting Proton LT as well (so read on).
• You can look at Proton LT as a more durable, comfortable and weather resistant but less breathable alternative to the Nano Air. If you'll be using the jacket both in the town when it's cold out AND during high output activities, Proton LT can really be one of your best choices.