Proton FL.
I've been publishing in-depth helpful guides to almost all Arc'teryx jackets for more than two years now (see full post list at the bottom).
Now that the summer is here, let's take a look at how the differences between Proton FL vs Atom SL play out.
Atom SL (Super Light) and Proton FL (Fast & Light) are the two most breathable, least warm alternatives you can find in Arc'teryx's insulated jacket line-up.
They're both best suited for chilly summer and/or spring mornings, followed by a high output activity (primarily rock / alpine climbing).
- #A sidenote here: Arcteryx.com's PDF guide to their jackets is GOOOLLDDD. Highly recommend you to check it out before you buy.
These jackets are more alike than they're different.
- Proton FL is slightly warmer, slightly more breathable, slightly more durable and comes with an inch longer hemline
- Atom SL is more wind resistant
Short answer is that unless you'll be using the jacket under windy conditions 90+% of the time, I'd probably recommend you to get the more versatile option, the Proton FL.
Warmth
Neither of these jackets will provide substantial warmth. Like I clarified earlier, they aren't even designed to do so anyway.
Proton FL is insulated with Octa Loft throughout the entire jacket. The Atom SL, on the other hand, is insulated with Coreloft Compact only around the core, which holds it warmth back.
- 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.
Here's the list of the lowest temperatures you can wear each Arc'teryx jacket without feeling uncomfortably cold.
Keep in mind that these are only approximate values assuming:
- worn over a shirt only and no shell over,
- during daily use (strolls, power walks etc at most - no high output activities),
- with light wind,
- and when it's dry out.
| Name | Est. Temp (F/C) | Insulation | Review | |||
|---|---|---|---|---|---|---|
| Arc'teryx Alpha Parka (Men's - Women's) | -17/-27 | Down | Down | No | M | |
| Feathered Friends Khumbu (Men's) | -17/-27 | Down | Down | No | M | |
| Arc'teryx Ceres SV (Men's - Women's) | -15/-26 | Down | Down | Review | Yes | M |
| Mountain Equipment K7 (Men's - Women's) | -13/-25 | Down | Down | No | M | |
| Rab Positron Pro (Men's - Women's) | -13/-25 | Down | Down | No | M W | |
| Patagonia Grade VII (Men's) | -11/-24 | Down | Down | No | M | |
| Arc'teryx Firebee AR (Men's - Women's) | -10/-23 | Down | Down | Review | Yes | M W |
| Mountain Equipment Exo (Men's) | -10/-23 | Down | Down | No | M | |
| Arc'teryx Thorsen (Men's) | -7/-22 | Down | ParkaD | Review | No | M |
| Patagonia Frozen Range (Men's - Women's) | -6/-21 | Down | ParkaD | Review | No | M W |
| Arc'teryx Centrale (Women's) | -5/-20 | Down | ParkaD | Review | No | W |
| Rab Neutrino Pro (Men's - Women's) | -2/-19 | Down | Down | No | M W | |
| Arc'teryx Therme (Men's) | -2/-19 | Down | ParkaD | Review | No | M |
| Arc'teryx Dually Belay (Men's) | 0/-18 | Synthetic | Synthetic | Review | Yes | M |
| Arc'teryx Camosun (Men's) | 0/-18 | Down | ParkaD | Review | No | M |
| Arc'teryx Piedmont (Men's) | 2/-17 | Down | ParkaD | No | M | |
| Arc'teryx Fission SV (Men's - Women's) | 3/-16 | Synthetic | Synthetic | Review | No | M |
| Outdoor Research Super Alpine (Men's - Women's) | 3/-16 | Down | Down | No | M W | |
| Patagonia Tres 3-in-1 (Men's - Women's) | 3/-16 | Down | ParkaD | Review | No | M W |
| Arc'teryx Radsten (Men's) | 3/-16 | Synthetic | ParkaS | No | M | |
| Patagonia Fitz Roy (Men's - Women's) | 3/-16 | Down | Down | No | M W | |
| Rab Electron Pro (Men's - Women's) | 5/-15 | Down | Down | No | M W | |
| Arc'teryx Macai (Men's | 5/-15 | Down | Down | Review | No | M W |
| Arc'teryx Nuclei SV Parka (Men's - Women's) | 5/-15 | Synthetic | Synthetic | No | M W | |
| Patagonia DAS Parka (Men's - Women's) | 5/-15 | Synthetic | ParkaS | No | M W | |
| Arc'teryx Patera (Women's) | 5/-15 | Down | ParkaD | Review | No | W |
| Arc'teryx Cerium SV (Men's - Women's) | 7/-14 | Down | Down | Review | Yes | M W |
| Black Diamond Access Down (Men's - Women's) | 7/-14 | Down | Down | No | M W | |
| Arc'teryx Kappa (Men's - Women's) | 7/-14 | Synthetic | Synthetic | Review | Yes | M |
| Arc'teryx Prema (Women's) | 7/-14 | Down | ParkaD | No | W | |
| Arc'teryx Radsten Insulated Men's) | 7/-14 | Synthetic | ParkaS | No | M | |
| Rab Microlight Alpine (Men's - Women's) | 7/-14 | Down | Down | No | M W | |
| Patagonia Macro Puff (Men's - Women's) | 8/-13 | Synthetic | Synthetic | Review | No | M W |
| Feathered Friends Eos (Men's - Women's) | 8/-13 | Down | Down | No | M W | |
| Arc'teryx Macai LT (Men's) | 8/-13 | Down | Down | No | M | |
| Arc'teryx Seyla (Women's) | 8/-13 | Down | ParkaD | No | W | |
| Arc'teryx Koda (Men's) | 8/-13 | Synthetic | ParkaS | No | M | |
| Arc'teryx Thorium AR (Men's - Women's) | 8/-13 | Down | Down | Review | No | M W |
| Arc'teryx Kole (Men's - Women's) | 10/-12 | Down | ParkaD | No | W | |
| Patagonia Vosque 3-in-1 (Women's) | 10/-12 | Synthetic | ParkaS | Review | No | W |
| Arc'teryx Alpha IS (Men's - Women's) | 12/-11 | Synthetic | Synthetic | Yes | M | |
| Patagonia Jackson Glacier (Men's - Women's) | 12/-11 | Down | ParkaD | Review | No | M W |
| Arc'teryx Sensa (Women's) | 14/-10 | Synthetic | ParkaS | No | W | |
| Patagonia Hi Loft Down (Men's - Women's ) | 14/-10 | Down | Down | Review | No | M |
| REI Magma 850 Hoodie 2.0 (Men's - Women's) | 14/-10 | Down | Down | No | M W | |
| Arc'teryx Atom AR (Men's - Women's) | 18/-8 | Synthetic | Synthetic | Review | No | M W |
| Patagonia Downdrift (Men's - Women's) | 18/-8 | Down | Down | No | M W | |
| Patagonia Down Sweater (Men's - Women's) | 19/-7 | Down | Down | Review | No | M W |
| Arc'teryx Magnus (Men's) | 19/-7 | Synthetic | ParkaS | No | M | |
| Arc'teryx Agrium (Men's - Women's) | 21/-6 | Down | Down | No | M W | |
| Patagonia AlpLight Down (Men's - Women's) | 21/-6 | Down | Down | No | M W | |
| Patagonia Topley (Men's) | 23/-5 | Down | Down | No | M | |
| Rab Xenon 2.0 (Men's - Women's) | 23/-5 | Synthetic | Synthetic | No | M W | |
| Patagonia Silent Down (Men's - Women's) | 26/-3 | Down | Down | Review | No | M W |
| Mountain Hardwear Ghost Whisp/2 (Men's - Women's) | 28/-2 | Down | Down | No | M W | |
| Arc'teryx Cerium LT (Men's - Women's) | 28/-2 | Down | Down | Review | No | M W |
| Arc'teryx Nuclei FL (Men's - Women's) | 32/0 | Synthetic | Synthetic | Review | No | M W |
| Patagonia City Storm Down (Men's) | 32/0 | Down | ParkaD | No | M | |
| Arc'teryx Atom LT (Men's - Women's) | 36/2 | Synthetic | Synthetic | Review | No | M W |
| Patagonia Micro Puff (Men's - Women's) | 36/2 | Synthetic | Synthetic | Review | No | M W |
| Patagonia Isthmus (Men's | 36/2 | Synthetic | ParkaS | No | M | |
| Arc'teryx Proton LT (Men's - Women's) | 37/3 | Synthetic | Synthetic | Review | No | M W |
| Patagonia Nano Puff (Men's - Women's) | 37/3 | Synthetic | Synthetic | Review | No | M W |
| Arc'teryx Norvan SL Ins. (Men's - Women's) | 39/4 | Synthetic | Synthetic | Yes | M W | |
| Patagonia Nano Air (Men's - Women's) | 41/5 | Synthetic | Synthetic | Review | No | M W |
| Arc'teryx Cerium SL (Men's - Women's) | 43/6 | Down | Down | Review | No | M W |
| Arc'teryx Proton FL (Men's - Women's) | 48/9 | Synthetic | Synthetic | Review | No | M W |
| Arc'teryx Atom SL (Men's - Women's) | 52/11 | Synthetic | Synthetic | Review | Yes | M W |
| The North Face Summit Down (Men's - Women's) | Testing | Down | Down | No | M W | |
| Mountain Equipment Lightline Echo (Men's) | Testing | Down | Down | No | M | |
| Mountain Equipment Kryos (Men's - Women's) | Testing | Down | Down | No | M W | |
| Mountain Equipment Xeros (Men's - Women's) | Testing | Down | Down | No | M | |
| Arc'teryx Andra LT IS (Women's) | Testing | Synthetic | ParkaS | No | W | |
| Arc'teryx Phoria IS (Women's) | Testing | Synthetic | ParkaS | No | W | |
| Feathered Friends Helios (Men's) | Testing | Down | Down | No | M | |
| Feathered Friends Volant (Men's) | Testing | Down | Down | No | M | |
| Rab Axion Pro (Men's - Women's) | Testing | Down | Down | No | M W | |
| Feathered Friends Rock & Ice (Men's) | Testing | Down | Down | No | M | |
| Rab Valiance (Men's - Women's) | Testing | Down | Down | No | M W | |
| Rab Expedition 8000 (Men's) | Testing | Down | Down | No | M | |
| Rab Zero G (Men's) | Testing | Down | Down | No | M | |
| Rab Generator Alpine (Men's) | Testing | Synthetic | Synthetic | No | M | |
| UBR Regulator Parka II (Men's) | Testing | Synthetic | ParkaS | No | M | |
| Helly Hansen LifaLoft (Men's - Women's) | Testing | Synthetic | Synthetic | No | M | |
| Mammut Eigerjoch (Men's - Women's) | Testing | Synthetic | Synthetic | No | M W | |
| Helly Hansen Lumines (Men's - Women's) | Unknown | Synthetic | Synthetic | No | M | |
| Haglofs V Series Mimic (Men's - Women's) | Unknown | Synthetic | Synthetic | No | M W | |
| Black Diamond First Light (Men's - Women's) | Unknown | Synthetic | Synthetic | No | M W |
Liked this table?
If you did, check out this post of mine to see more Arc'teryx tables like this one, such as fill details for all down jackets.
Breathability
Going from top to bottom (Atom SL, LT and AR respectively), the warmth increases and the breathability decreases.
Although Atom SL is the most breathable unit in the Atom line, Proton FL still breathes more.
Actually, as I explained before, this is also the case between Proton LT and Atom LT as well.
Water, wind and abrasion resistances
While neither are comparable to outer layers (shell or belay jackets) at all, they'll fill the bill for spring and summer activities.
They're comparable in terms of water resistances, but Atom SL (Tyono 20) offers a slightly more wind resistance and slightly less abrasion resistance compared to Proton FL (Fortius Air 20).
They're both 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.
Fit and size measurements
For a small size Atom SL:
- Chest width: 107 cm
- Shoulder length: 16.4 cm
- Waist width: 98.4 cm
- Front length: 67.3 cm
- Center back length: 71 cm
For a small size Proton FL:
- Chest width: 109 cm
- Shoulder length: 14.8 cm
- Waist width: 101.0 cm
- Front length: 75.7 cm
- Center back length: 75.7 cm
Both offer a trim cut.
Conclusion
I'd probably get the Proton FL. It's more versatile.
A helpful video review:
I hope this post was helpful for you to learn more about the jacket(s) in your mind.
Feel free to ask me any questions or provide feedback and I'll be happy to respond in the comment section below.
Guide: Insulated & Shell Jackets
Start here:
Complete list:
Also see:
Thank you for reading!
Hello,
What ti wear over the proton fl in winter to protect from wind and little more heat?
here is my current setup for winter ice climbing;
Base Layer : OR echo t-shirt (Synthetic moisture wicking tee shirt)
Mid/insultation layer : Patagonia R1 fleece
Shell : Arcteryx Alpha FL
This setup works great when combine it all together when climbing, but i often overheat on the approach. If i take the shell off the wind cuts right through the R1. I am hesitating between the Proton FL and the new atom sl anork. Any suggestion?
Thanks for the review. I have one Proton FL and am trying to purchase another. They seem to have disappeared or are no longer in stock. Supply problems or has this great model been discontinued by Arcteryx? This jacket is my go-to mid-layer when it’s too warm (or I’m too warm) for my Atom LT, Proton LT, etc. Fantastic Arcteryx product. I have learned a lot (and purchased a lot) after reading reviews and comparisons on your website. Really appreciate your analysis!