From top to bottom: Atom SL, LT and AR.
Atom SL and LT are both breathable insulated jacket options.
In summary, their main differences are:
- Atom LT is A LOT warmer, and also a little bit more weather resistant. It's designed for both during high output activities and casual outgoings
- Atom SL is A LOT lighter, and also a little bit more breathable and packable. Designed more for warm weather (during the summer and/or spring) use in the upper alpines.
You can see this in Arc'teryx's Naming Scheme Page here as well - for both "LT" and "SL", as well as "Atom".
- #A sidenote here: Arcteryx.com's PDF guide to their jackets is GOOOLLDDD. Highly recommend you to check it out before you buy.
The only scenarios I'd recommend the SL over LT would be professional climbing (or alpinism), ultralight summer hiking and/or traveling since SL is more breathable and packable.
For all the other folks, (will be out in the town when it's chilly and also during various high output outdoor activities such as climbing, skiing, fat biking, etc.), then my recommendation would be Atom LT - although I'd check out Proton LT as well.
- 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.
Warmth
Let's take a look at Arc'teryx's warmth ratings first.
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.
Now there are 4 main things that make Atom LT warmer than the SL:
1) 40 g/m2 (Atom SL) vs 60 g/m2 (Atom LT) insulation density
Coreloft Compact 40 g/m2 of Atom SL is Arc'teryx's lowest density (1.2 oz/yd2) Coreloft insulation.
This is also one of the things that make SL exceptionally light - especially compared to the Atom LT {9.2 ounces (260 grams) vs 14.6 ounces (415 grams)}.
Atom LT's Coreloft 60 g/m2, on the other hand, obviously makes it more insulated but also naturally heavier.
2) Atom SL is only core-insulated!
The Atom SL is only insulated in core area of the jacket (kind of like a vest).
The sleeves and hood of the jacket AREN'T INSULATED. They only cut the wind.
Atom LT is much warmer than the SL.
Whereas Atom LT is insulated everywhere.
This also causes a big warmth difference.
3) Coreloft (Atom LT) vs Coreloft Compact (Atom SL)
Besides these differences, another difference here is that SL comes with Coreloft Compact whereas the LT with the basic Coreloft.
Coreloft of Atom LT is more thermally efficient and less air-permeable. Which means that it's warmer but also breathes less.
4) Atom LT is a little baggier than the SL
This helps the LT trap more air between the jacket and your body. That's why, in the end, more air gets heated by your body and works as insulation.
But the fit difference isn't big, and hence it doesn't cause a big warmth difference.
Combine all these and you get...
I'd say, generally speaking, when it's dry out, with only a long sleeved shirt under and without a shell on top, with a mild wind and very low to no activity level...
The approximate lowest temperatures an average person can wear these jackets without feeling cold would be around:
- Atom SL: 50 deg F (10 deg C)
- Atom LT: 23 deg F (-5 deg C)
If you think you run hotter or colder than the average person, then shift these temperatures accordingly.
Atom SL is slightly more breathable than the Atom LT.
Breathability
Now for the 3 main reasons listed under the warmth heading, you'd expect the SL to be breathable, and that's not incorrect, but there's a caveat here.
Unlike Proton LT or Patagonia's Nano Air, Atom line doesn't use highly breathable advanced insulation technology.
Most of its breathability comes from the side fleece panels.
That's why, the insulation differences laid out above actually make up the smaller chunk of this breathability department.
That's why, in the end, SL really IS more breathable, but it's by a much, much smaller amount than it's colder than the LT.
Weather Resistance
Atom SL isn't designed to be waterproof. Its Nu water repellent treatment does a mediocre job at best - can only protect you from very light rain and moisture - nothing more.
LT's DWR coating noticeably does a better job than the SL, but it still isn't anywhere near others such as the Atom AR or even the Proton LT.
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.
I'd recommend both jackets with the hood. SL isn't even available without the hood anyway.
No products found.
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!
Would you recommend the LT as a mid layer under a hard shell for active sports such as resort skiing?
You seem to have little understanding of the products or the materials used. (1) The Atom LT uses Coreloft Compact, too. (2) The difference between Coreloft and Coreloft Compact is about the nature of the fibers, not the density at which they are found in the jackets. Either material can be packed at any density. What worse than having no information is having very misleading information as is found here.