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:
- 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).
- 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.
- Some of the Arc pieces you see in this post might be discontinued, and hence might be unavailable at arcteryx.com.
- If that's the case, then I'd recommend you to check out this page at REI. You might be able to find them there.
Approximate lowest temperatures you can wear each down jacket in the market without feeling uncomfortably cold are... As you see, there's a strong correlation between FP x FW and temperature ratings, but not more than, say, 80%. I explained why this happens here in this post of mine. These are only approximate values assuming: If you think you run warmer or colder than the average person, then shift temperature ranges accordingly.
Name Est. Temp (F/C) Insulation Material Insulation Amount Review Arc'teryx Alpha Parka (Men's - Women's) -17/-27 Down N/A N/A No M Feathered Friends Khumbu (Men's) -17/-27 Down N/A N/A No M Arc'teryx Ceres SV (Men's - Women's) -15/-26 Down N/A N/A Review Yes M Mountain Equipment K7 (Men's - Women's) -13/-25 Down N/A N/A No M Rab Positron Pro (Men's - Women's) -13/-25 Down N/A N/A No M W Patagonia Grade VII (Men's) -11/-24 Down N/A N/A No M Arc'teryx Firebee AR (Men's - Women's) -10/-23 Down N/A N/A Review Yes M W Mountain Equipment Exo (Men's) -10/-23 Down N/A N/A No M Arc'teryx Thorsen (Men's) -7/-22 ParkaD N/A N/A Review No M Patagonia Frozen Range (Men's - Women's) -6/-21 ParkaD N/A N/A Review No M W Arc'teryx Centrale (Women's) -5/-20 ParkaD N/A N/A Review No W Rab Neutrino Pro (Men's - Women's) -2/-19 Down N/A N/A No M W Arc'teryx Therme (Men's) -2/-19 ParkaD N/A N/A Review No M Arc'teryx Dually Belay (Men's) 0/-18 Synthetic ThermaTek 92 g/m² Review Yes M Arc'teryx Camosun (Men's) 0/-18 ParkaD N/A N/A Review No M Arc'teryx Piedmont (Men's) 2/-17 ParkaD N/A N/A No M Arc'teryx Fission SV (Men's - Women's) 3/-16 Synthetic Coreloft Continuous 90 + 65 g Review No M Outdoor Research Super Alpine (Men's - Women's) 3/-16 Down N/A N/A No M W Patagonia Tres 3-in-1 (Men's - Women's) 3/-16 ParkaD N/A N/A Review No M W Arc'teryx Radsten (Men's) 3/-16 ParkaS Coreloft Compact 120 g/m² No M Patagonia Fitz Roy (Men's - Women's) 3/-16 Down N/A N/A No M W Rab Electron Pro (Men's - Women's) 5/-15 Down N/A N/A No M W Arc'teryx Macai (Men's 5/-15 Down N/A N/A Review No M W Arc'teryx Nuclei SV Parka (Men's - Women's) 5/-15 Synthetic Coreloft Continuous 190 g/m2 + 90 g/m2 No M W Patagonia DAS Parka (Men's - Women's) 5/-15 ParkaS PrimaLoft Gold Insulation Eco 133 + 40 g No M W Arc'teryx Patera (Women's) 5/-15 ParkaD N/A N/A Review No W Arc'teryx Cerium SV (Men's - Women's) 7/-14 Down N/A N/A Review Yes M W Black Diamond Access Down (Men's - Women's) 7/-14 Down N/A N/A No M W Arc'teryx Kappa (Men's - Women's) 7/-14 Synthetic Coreloft Continuous 140 g/m² Review Yes M Arc'teryx Prema (Women's) 7/-14 ParkaD No W Arc'teryx Radsten Insulated Men's) 7/-14 ParkaS Coreloft Compact 120 g/m² No M Rab Microlight Alpine (Men's - Women's) 7/-14 Down N/A N/A No M W Patagonia Macro Puff (Men's - Women's) 8/-13 Synthetic PlumaFill 135 + 90 g Review No M W Feathered Friends Eos (Men's - Women's) 8/-13 Down N/A N/A No M W Arc'teryx Macai LT (Men's) 8/-13 Down N/A N/A No M Arc'teryx Seyla (Women's) 8/-13 ParkaD No W Arc'teryx Koda (Men's) 8/-13 ParkaS Coreloft 200 g/m² No M Arc'teryx Thorium AR (Men's - Women's) 8/-13 Down N/A N/A Review No M W Arc'teryx Kole (Men's - Women's) 10/-12 ParkaD No W Patagonia Vosque 3-in-1 (Women's) 10/-12 ParkaS ThermoLite 100 g Review No W Arc'teryx Alpha IS (Men's - Women's) 12/-11 Synthetic Coreloft Continuous 65 g/m² Yes M Patagonia Jackson Glacier (Men's - Women's) 12/-11 ParkaD N/A N/A Review No M W Arc'teryx Sensa (Women's) 14/-10 ParkaS Coreloft Compact 120 g/m² No W Patagonia Hi Loft Down (Men's - Women's ) 14/-10 Down N/A N/A Review No M REI Magma 850 Hoodie 2.0 (Men's - Women's) 14/-10 Down N/A N/A No M W Arc'teryx Atom AR (Men's - Women's) 18/-8 Synthetic Coreloft Continuous 120 g/m2 + 80 g/m2 + 60 g/m2 Review No M W Patagonia Downdrift (Men's - Women's) 18/-8 Down N/A N/A No M W Patagonia Down Sweater (Men's - Women's) 19/-7 Down N/A N/A Review No M W Arc'teryx Magnus (Men's) 19/-7 ParkaS Coreloft 80 g No M Arc'teryx Agrium (Men's - Women's) 21/-6 Down N/A N/A No M W Patagonia AlpLight Down (Men's - Women's) 21/-6 Down N/A N/A No M W Patagonia Topley (Men's) 23/-5 Down N/A N/A No M Rab Xenon 2.0 (Men's - Women's) 23/-5 Synthetic PrimaLoft Silver 60 g No M W Patagonia Silent Down (Men's - Women's) 26/-3 Down N/A N/A Review No M W Mountain Hardwear Ghost Whisp/2 (Men's - Women's) 28/-2 Down N/A N/A No M W Arc'teryx Cerium LT (Men's - Women's) 28/-2 Down N/A N/A Review No M W Arc'teryx Nuclei FL (Men's - Women's) 32/0 Synthetic Coreloft Continuous 65 g/m² Review No M W Patagonia City Storm Down (Men's) 32/0 ParkaD N/A N/A No M Arc'teryx Atom LT (Men's - Women's) 36/2 Synthetic Coreloft Continuous 60 g/m² Review No M W Patagonia Micro Puff (Men's - Women's) 36/2 Synthetic PlumaFill 65 g Review No M W Patagonia Isthmus (Men's 36/2 ParkaS ThermoGreen 60 g No M Arc'teryx Proton LT (Men's - Women's) 37/3 Synthetic Coreloft Compact 80 g/m2 + 60 g/m2 Review No M W Patagonia Nano Puff (Men's - Women's) 37/3 Synthetic PrimaLoft Gold Insulation Eco 60 g Review No M W Arc'teryx Norvan SL Ins. (Men's - Women's) 39/4 Synthetic Coreloft Compact 40 g/m² Yes M W Patagonia Nano Air (Men's - Women's) 41/5 Synthetic FullRange 60 g Review No M W Arc'teryx Cerium SL (Men's - Women's) 43/6 Down N/A N/A Review No M W Arc'teryx Proton FL (Men's - Women's) 48/9 Synthetic Octa Loft 98 g/m² Review No M W Arc'teryx Atom SL (Men's - Women's) 52/11 Synthetic Coreloft Compact 40 g/m² Review Yes M W The North Face Summit Down (Men's - Women's) Testing Down N/A N/A No M W Mountain Equipment Lightline Echo (Men's) Testing Down N/A N/A No M Mountain Equipment Kryos (Men's - Women's) Testing Down N/A N/A No M W Mountain Equipment Xeros (Men's - Women's) Testing Down N/A N/A No M Arc'teryx Andra LT IS (Women's) Testing ParkaS Coreloft 60 g/m² No W Arc'teryx Phoria IS (Women's) Testing ParkaS Coreloft 100 g/m² No W Feathered Friends Helios (Men's) Testing Down N/A N/A No M Feathered Friends Volant (Men's) Testing Down N/A N/A No M Rab Axion Pro (Men's - Women's) Testing Down N/A N/A No M W Feathered Friends Rock & Ice (Men's) Testing Down N/A N/A No M Rab Valiance (Men's - Women's) Testing Down N/A N/A No M W Rab Expedition 8000 (Men's) Testing Down N/A N/A No M Rab Zero G (Men's) Testing Down N/A N/A No M Rab Generator Alpine (Men's) Testing Synthetic PrimaLoft Gold 133 g/m2 + 80 g/m2 No M UBR Regulator Parka II (Men's) Testing ParkaS PrimaLoft Innovation 120 g No M Helly Hansen LifaLoft (Men's - Women's) Testing Synthetic LifaLoft 100 g No M Mammut Eigerjoch (Men's - Women's) Testing Synthetic Polartec Alpha Direct 95 g No M W Helly Hansen Lumines (Men's - Women's) Unknown Synthetic PrimaLoft ??? No M Haglofs V Series Mimic (Men's - Women's) Unknown Synthetic Mimic Platinum Ultracluster Graphene ??? No M W Black Diamond First Light (Men's - Women's) Unknown Synthetic PrimaLoft Silver Active 60 g No M W
Packing
Air is packable whereas the Atom and Proton LT aren't.

This makes Nano Air also a great choice for traveling.
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.
Which is why the Nano Air our top choice for that type of use.
Of all the Patagonia and Arcteryx jackets we reviewed, Nano Air is the most breathable and certainly lives up to the hype.
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.
DWR is a coating. It's basically sprayed on to the surface of these jackets as a finish. It stands for Durable Water Repellent - which means that water will be repelled under wet conditions instead of soaking into the fabric. DWR also decreases dry time. It's a good thing these jackets are coated in DWR, but pay attention to the fact that DWR is a coating - meaning that it will wear off over time. Which is why you might want to invest in a DWR spray such as this one for after treatment. As you see, there's a strong correlation between FP x FW and temperature ratings, but not more than, say, 80%. I explained why this happens here in this post of mine.
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.