Atom SL vs Proton FL: Key Differences [Jul 2020 Guide]

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).

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.

I'd recommend you to take a look at the lowest temperatures they're expected to be worn comfortably by an average person (both are positioned at the bottom):

Lowest temperatures you can wear these jackets without feeling uncomfortably cold are...

	°F / °C	Insulation
Arc'teryx LEAF Cold WX Parka SVX (Men's)	-40/-40	Down
Arc'teryx Ceres SV (Men's)	-15/-26	Down	Read Review
Arc'teryx Firebee AR (Men's - Women's)	-10/-23	Down	Read Review
Arc'teryx Dually Belay Parka (Men's)	-7/-22	Synthetic
Arc'teryx Thorsen Parka (Men's)	-7/-22	Down	Read Review
Patagonia Frozen Range Parka (Men's - Women's)	-6/-21	Down	Read Review
Arc'teryx Centrale (Women's)	-5/-20	Down	Read Review
Arc'teryx Cerium SV (Men's - Women's)	-5/-20	Down	Read Review
Arc'teryx Therme Parka (Men's)	-2/-19	Down	Read Review
Patagonia City Storm Parka (Men's - Women's)	0/-18	Down	Read Review
Arc'teryx Camosun Parka (Men's)	0/-18	Down	Read Review
Arc'teryx Fission SV (Men's - Women's)	3/-16	Synthetic
Patagonia Tres 3-in-1 Parka (Men's - Women's)	3/-16	Down	Read Review
Arc'teryx Patera Parka (Women's)	5/-15	Down	Read Review
Arc'teryx Kappa (Men's)	7/-14	Synthetic
Patagonia Macro Puff (Men's - Women's)	8/-13	Synthetic
Arc'teryx Thorium AR (Men's - Women's)	8/-13	Down	Read Review
Arc'teryx Atom AR (Men's - Women's)	10/-12	Synthetic	Read Review
Patagonia Vosque 3-in-1 Parka (Women's)	10/-12	Down	Read Review
Patagonia Jackson Glacier (Men's - Women's)	12/-11	Down	Read Review
Patagonia Hi Loft Down (Men's - Women's)	14/-10	Down	Read Review
Arc'teryx Magnus Coat (Men's)	14/-10	Down
Arc'teryx Cerium LT (Men's - Women's)	17/-8	Down	Read Review
Patagonia Down Sweater (Men's - Women's)	19/-7	Down	Read Review
Arc'teryx Nuclei FL (Men's - Women's)	19/-7	Synthetic	Read Review
Arc'teryx Atom LT (Men's - Women's)	23/-5	Synthetic	Read Review
Arc'teryx Proton LT (Men's - Women's)	24/-4	Synthetic	Read Review
Patagonia Micro Puff (Men's - Women's)	24/-4	Synthetic	Read Review
Patagonia Silent Down (Men's - Women's)	26/-3	Down
Patagonia Nano Puff (Men's - Women's)	28/-2	Synthetic	Read Review
Patagonia Nano Air (Men's - Women's)	32/0	Synthetic	Read Review
Arc'teryx Cerium SL (Men's - Women's)	40/5	Down	Read Review
Arc'teryx Proton FL (Men's - Women's)	43/6	Synthetic	Read Review
Arc'teryx Atom SL (Men's - Women's)	50/10	Synthetic	Read Review

Read more about this chart here in my Insulated Outerwear Temperature Ratings Guide.

These are only approximate values assuming:

you only have a shirt under and no shell over,
during daily use (strolls, power walks etc at most - no high output activities),
with enough wind to cause an umbrella to twist in your hand,
and when it's dry out.

If you think you run warmer or colder than the average person, then shift temperature ranges accordingly.

Also check out my Arc'teryx Favorites and my Patagonia Favorites.

If you're interested in down jackets only, see my Best Down Jackets post.

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.

What is DWR by the way?

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.