Y'all know this is just my opinion... right?

I own both. Winter '24 Cerium (aka LT) on the left in Black Sapphire and Winter '24 Atom (aka LT) on the right. Both are size M. I'm 5'11, 165 lbs, athletic build.
Summary
The Arc'teryx Atom (aka LT) and Arc'teryx Cerium (aka LT) jackets are directed toward completely different uses.
The Atom is a synthetic insulated jacket. It breathes a lot through its side fleece panels. This makes it a perfect pick for high-output activities.
The Cerium, on the other hand, is a down jacket. It's noticeably warmer and much lighter. It's perfect for casual use. But it doesn't breathe at all.
One of the most important factors you should consider when it comes to Atom vs Cerium is that the Cerium doesn't resist precipitation at all. Keep in mind that the insulating power of down is completely lost once it becomes wet.
This is why I recommend the Cerium Jacket (hoodless) and the Atom Hoody. I'll explain why in a moment ...
In summary, the only scenario where the Cerium would be a better item is casual use in cold, dry areas. For any other uses I'd go with the Atom.
Cerium vs Atom on arcteryx.com: https://arcteryx.com/us/en/compare/X000007313/X000007487
Warmth
Performance data axis of synthetic jackets (table above) and shell jackets (this table) are different. Don't compare them.
Name Activity Temp (F/C) Breathability Weather Res. Durability Bottom line Review Pit zips? Two way zipper? Stuffs into itself? Face Fabric Fit Weight Length Alpha (aka AR) (Men's - Women's) Climbing N/A 8/10 9/10 8/10 4/10 Go-to climbing harshell. Review Yes No No N40d & N80d Most Rugged 3L Gore-Tex Pro Regular 430 g / 15.2 oz 79.5 cm / 31.25 in No Alpha SV (Men's - Women's) Climbing N/A 8/10 10/10 10/10 3/10 Strongest climbing hardshell. Review Yes No No N100d Most Rugged 3L Gore-Tex Pro Regular 510 g / 1 lb 2 oz 78 cm / 30.75 in No Beta (Men's - Women's) All Round N/A 6/10 7/10 7/10 8/10 Streamlined all-doer shell. Review No No No N30p-X 3L Gore-Tex Trim 300 g / 10.6 oz 76 cm / 30 in No Beta AR (Men's - Women's) All Round N/A 8/10 9/10 9/10 4/10 Go-to all-round harshell. Review Yes No No N40d & N80d Most Rugged 3L Gore-Tex Pro Regular 460 g / 1 lb 76 cm / 30 in No Beta AR StormHood (Men's - Women's) All Round N/A 8/10 9/10 9/10 4/10 Review Yes No No N40d & N80d Most Rugged 3L Gore-Tex Pro Regular 460 g / 1 lb 76 cm / 30 in No Beta Coat (Women's) All Round N/A 6/10 5/10 No data 5/10 No Yes No 3L Gore-Tex Regular 320 g / 11.3 oz 88 cm / 34.5 in No Beta Down Insulated (Men's) All Round 0/-18 9/10 7/10 8/10 No data Yes Yes No 2L 40D Gore-Tex Regular 760 g / 1 lb 10.8 oz 79 cm / 31 in No Beta Insulated (Men's - Women's) All Round 21/-6 9/10 7/10 8/10 No data Go-to hybrid jacket. Review Yes Yes No N40d 2L Gore-Tex Regular 680 g / 1 lb 8 oz 78.5 cm / 31 in No Beta Insulated Coat (Women's) All Round 16/-9 7/10 No data No data No data No Yes No N40d 2L Gore-Tex Regular 600 g / 1 lb 5.2 oz 86 cm / 34 in No Beta Lightweight (Men's - Women's) All Round N/A 8/10 7/10 7/10 6/10 Review Yes No No N40p 3L Gore-Tex Fitted 380 g / 13.4 oz 75 cm / 29.5 in No Beta LT (Men's - Women's) All Round N/A 7/10 7/10 8/10 6/10 Review Yes No No N40d 3L Gore-Tex Fitted 395 g / 13.9 oz 76 cm / 30 in No Gamma (aka LT) (Men's - Women's) All Round N/A 7/10 3/10 7/10 3/10 Go-to softshell. Review No No No Wee Burly Double Weave Regular 545 g / 1 lb 3.2 oz 76 cm / 30 in No Gamma MX (Men's) All Round N/A 6/10 4/10 8/10 3/10 Strongest softshell. Review No No No Fortius 2.0 Regular 555 g / 1 lb 3.6 oz 74 cm / 29.1 in No Norvan Shell (Men's) Running N/A No data No data No data No data No No No 13D 3L Gore-Tex Trim 190 g / 6.7 oz 73 cm / 28.75 in No Rush (Women's) Skiing N/A 8/10 9/10 9/10 3/10 Go-to ski jacket. Review Yes No No N80p-X & N100p-X 3L Most Rugged Gore-Tex Pro Regular 590 g / 1 lb 4.8 oz 79.5 cm / 31.25 in No Rush Insulated (Women's) Skiing 16/-9 3/10 5/10 5/10 9/10 Review No No Yes Hadron 2L Gore-Tex Infinium Regular 515 g / 1 lb 2.2 oz 79 cm / 31 in Yes Sabre (Men's) Skiing N/A 8/10 8/10 7/10 2/10 Review Yes No No N80p-X 3L Gore-Tex Regular 705 g / 1 lb 8.9 oz 79 cm / 31 in No Sentinel (Women's) Skiing N/A 8/10 7/10 7/10 2/10 Go-to ski jacket. Review Yes No No N70p 3L Gore-Tex Regular 640 g / 1 lb 6.6 oz 73.7 cm / 29 in No Sentinel Insulated (Women's) Skiing 21/-6 8/10 No data No data No data Review Yes No No N80p 2L Gore-Tex Regular 600 g / 1 lb 5.2 oz 79.8 cm / 31.5 in No Sentinel Relaxed (Women's) Skiing N/A No data No data No data No data Yes No No N80p 3L Gore-Tex Regular 655 g / 1 lb 7.1 oz ? No Sidewinder (Women's) Skiing N/A 8/10 8/10 7/10 2/10 Yes No No N80p 3L Gore-Tex Regular 570 g / 1 lb 4.1 oz ? No Solano (Men's - Women's) Around Town N/A No data No data No data No data Review No No No Gore-Tex Infinium Relaxed 355 g / 12.5 oz 72 cm / 28.3 in No Squamish (Men's - Women's) Running N/A 5/10 4/10 7/10 9/10 Go-to running jacket. Review No No Yes 30D Tyono Trim 140 g / 4.9 oz 77.5 cm / 30.5 in No Theriss Down (Women's) Skiing No data 7/10 6/10 7/10 2/10 Review Yes No No N80p 2L Gore-Tex Regular 709 g / 1 lb 9 oz 63 cm / 24.8 in No Alpha FL (Men's - Women's) Climbing N/A 7/10 No data No data 10/10 No No Yes Hadron 3L Gore-Tex Trim 209 g / 7.4 oz 76.5 cm / 30 in Yes Alpha IS (Men's) Climbing 23/-5 7/10 5/10 No data 8/10 No Yes Yes N40p-X 2L Gore-Tex Regular 610 g / 1 lb 5.5 oz 78 cm / 30.75 in Yes Alpha SL (Men's - Women's) Climbing N/A 5/10 7/10 No data 9/10 No No Yes N40p 3L Gore-Tex Trim 326 g / 11.5 oz 77.5 cm / 30.5 in Yes Beta SV (Men's - Women's) All Round N/A 8/10 10/10 10/10 3/10 Strongest all-round harshell. Review Yes No No N80p-X Most Rugged 3L Gore-Tex Pro Regular 495 g / 1 lb 1.5 oz 79 cm / 31 in Yes Gamma SL (aka lightweight) (Men's - Women's) All Round N/A 7/10 2/10 6/10 5/10 Review No No No Fortius 1.0 Trim 310 g / 10.9 oz 74 cm / 29.1 in Yes Incendia (Women's) Skiing N/A 8/10 9/10 9/10 3/10 Review Yes No No NP100d 3L Gore-Tex Trim 610 g / 1 lb 5.5 oz 79 cm / 31 in Yes Incendo (Men's) Running N/A 7/10 4/10 5/10 10/10 Review No No Yes Lumin Trim 125 g / 4.4 oz ? Yes Incendo Airshell (Men's Running N/A No data No data No data No data No No Yes ? Trim 105 g / 3.7 oz ? Yes Incendo SL (Men's) Running N/A 7/10 4/10 4/10 10/10 Review No No Yes Permair 20 & Canim Trim 80 g / 2.8 oz 73 cm / 28.75 in Yes Macai (Men's) Skiing 5/-15 7/10 7/10 6/10 1/10 Review Yes No No N40p-X 3L Gore-Tex Regular 990 g / 2 lb 2.9 oz 75 cm / 29.5 in Yes Macai Lightweight (aka LT) (Men's) Skiing 8/-13 7/10 7/10 6/10 2/10 Review Yes No No N40p-X 3L Gore-Tex Regular 855 g / 1 lb 14.2 oz ? Yes Macai Shell (Men's) Skiing N/A No data No data No data No data Review Yes No No N80p-X 3L Gore-Tex Regular 775 g / 1 lb 11.3 oz ? Yes Micon (Men's) Skiing N/A No data No data No data No data Yes No No N80p 3L Gore-Tex Regular 685 g / 1 lb 8.2 oz 83 cm / 32.5 in Yes Micon Heather Skiing N/A No data No data No data No data Yes No No NP100-d 3L Gore-Tex Regular 685 g / 1 lb 8.2 oz 83 cm / 32.5 in Yes Nodin (Men's - Women's) Running N/A 8/10 5/10 5/10 10/10 No No Yes 20D Tyono Trim 130 g / 4.6 oz 75 cm / 29.5 in Yes Norvan LT (Men's - Women's) Running N/A 5/10 5/10 8/10 6/10 No No No N13p 3L Gore-Tex Trim 190 g / 6.7 oz ? Yes Norvan SL (Men's - Women's) Running N/A 7/10 6/10 8/10 8/10 Review No No Yes Gore-Tex Infinium Trim 120 g / 4.2 oz 77 cm / 30.4 in Yes Norvan SL Ins. (Women's) Running 39/4 No data No data No data No data No No Yes Gore-Tex ShakeDry Trim 270 g / 9.5 oz 71.5 cm / 28.1 in Yes Norvan Windshell (Men's Running N/A No data No data No data No data No No Yes Permeair 20 Trim 90 g / 3.2 oz 73 cm / 28.75 in Yes Procline (Men's) Skiing N/A No data No data No data No data Yes No No N70p Gore-Tex Infinium Trim 720 g / 1 lb 9.4 oz 76 cm / 30 in Yes Ravenna (Women's) Skiing N/A 8/10 7/10 8/10 3/10 Review Yes No No N70s-4 3L Gore-Tex Trim 725 g / 1 lb 9.6 oz 69.5 cm / 27.4 in Yes Ravenna LT (Women's) Skiing N/A 6/10 6/10 6/10 5/10 Review Yes No No N70p 3L Gore-Tex Trim 540 g / 1 lb 3 oz 69 cm / 27.2 in Yes Sabre AR (Men's) Skiing N/A 7/10 8/10 7/10 7/10 Review Yes No No N70p 3L Gore-Tex Regular 700 g / 1 lb 8.7 oz 79 cm / 31 in Yes Sabre Insulated (See related products) Skiing No data 7/10 No data 6/10 No data Review Yes No No N80p 2L Gore-Tex Regular 815 g / 1 lb 12.7 oz 76 cm / 30 in Yes Sabre LT (Men's) Skiing N/A 7/10 8/10 7/10 8/10 Review Yes No No N80p 3L Gore-Tex Regular 660 g / 1 lb 7.3 oz 83 cm / 32.5 in Yes Sabre SV (Men's) Skiing N/A 10/10 10/10 10/10 1/10 Strongest ski jacket. Review Yes Yes No N100d Most Rugged 3L Gore-Tex Pro Regular 595 g / 1 lb 5 oz 85 cm / 33.5 in No Sentinel AR (Women's) Skiing N/A 7/10 8/10 7/10 7/10 Review Yes No No N70p 3L Gore-Tex Regular 590 g / 1 lb 4.8 oz 72.5 cm / 28.5 in Yes Sentinel LT (Women's) Skiing N/A 7/10 8/10 7/10 8/10 Review Yes No No N80p 3L Gore-Tex Regular 565 g / 1 lb 3.9 oz 73.5 cm / 29 in Yes Shashka Stretch (Women's) Skiing N/A No data No data No data No data Yes No No N70p Gore-Tex Infinium Trim 670 g / 1 lb 7.6 oz 71 cm / 28 in Yes Trino SL (Men's) Running N/A 8/10 5/10 7/10 6/10 Review No No No Gore-Tex Infinium Trim 340 g / 12 oz 74 cm / 29.1 in Yes Trino SL Anorak (Women's) Running N/A 8/10 7/10 8/10 7/10 Review No No No Gore-Tex Infinium Trim 290 g / 10.2 oz 65 cm / 25.5 in Yes Venda Anorak (Women's) Around Town N/A No data No data No data No data No No No Gore-Tex Relaxed 255 g / 9 oz 75.5 cm / 29.7 in Yes Zeta AR (Men's) Hiking N/A 7/10 8/10 7/10 4/10 Yes No No 3L Gore-Tex Regular 425 g / 15 oz 80 cm / 31.5 in Yes Zeta FL (Men's - Women's) Hiking N/A 4/10 6/10 4/10 6/10 No No No N20r 2L Gore-Tex Trim 221 g / 7.8 oz 76 cm / 30 in Yes Zeta LT (Men's - Women's) Hiking N/A 5/10 7/10 8/10 5/10 No No No N40P-X 3L Gore-Tex Trim 335 g / 11.8 oz 78 cm / 30.75 in Yes Zeta SL (Men's - Women's) Hiking N/A 3/10 5/10 4/10 5/10 Review No No No 2L Gore-Tex Trim 310 g / 10.9 oz 77.8 cm / 30.6 in Yes
The fill that take place inside the Cerium is the highest quality Arc'teryx uses (850 FP). The "850" here indicates the volume in cubic inches. It's the volume that one unit mass (ounce) of a fill occupies.
The higher fill power, the greater the insulating properties per unit mass.
One easier way to think of fill power is to think of it as a measure of loft or "fluffiness". 850 fill power is as high as the fill powers get in down jackets.
Ranking Arc'teryx down jackets from the warmest to the least warm:
- Alpha line: 850 FP.
- Thorium line: 750 FP.
- Cerium line: 850 FP.
Cerium has a higher fill power than the Thorium, but it's less warm. Because it's equipped with less fill inside.
Arc'teryx increased the fill inside the Cerium by half an ounce (10 grams) in recent years. But it's still behind the Thorium line.
In comparison to the Atom (aka LT), however, Cerium is noticeably warmer AND lighter.
Breathability
Breathability of Atom (aka LT) is so good that it even tops other Arc'teryx synthetics like the Proton (aka LT).
Breathability of Cerium (aka LT) is horrible, just like any other down jacket.

Close up: Side fleece panels of Atom (aka LT).
It's impossible to compete Atom in terms of breathability.
Face fabrics
Arc'teryx upped the outer fabric thickness of Cerium from Arato 10 to Arato 15 in recent years. That's 50% thicker face fabric. But it's still fragile compared to Atom. I'd like to treat it more gently.
For water resistance, Atom tops Cerium, as I explained earlier.
For wind resistance, both are mediocre. When your arms are up (climbing, cycling, etc), Atom might fall behind the Cerium in this regard. Otherwise the Atom is more likely to better resist the wind.
Both come with DWR coating.
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 at arcteryx.com 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
Cerium comes with a trimmer cut.
Canberk is 5'11" and 165 pounds, with an athletic build. Size Medium fits him perfectly across almost all brands and product lines.
Packability
Cerium packs into its own stuff sack that comes attached to the jacket. Atom doesn't stuff into itself.
Check out our other content on jackets:
Guide: Insulated & Shell Jackets
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