Atom SL vs LT

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".

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.

Warmth

Let's take a look at Arc'teryx's warmth ratings first. 

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.

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.

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. 

I'd recommend both jackets with the hood. SL isn't even available without the hood anyway.

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