For high output activities, it's a no-brainer to go for the Atom LT (see image) over the Cerium LT (which more suitable for stationary use).
Summary
The biggest difference between Atom LT and Cerium LT is in their insulators (Atom's synthetic insulation vs Cerium's down insulation).
You can see this in Arc'teryx's Naming Scheme Guide here as well - for both for "Cerium" and "Atom", as well as the meaning of modifier "LT".
In short, get the Atom LT for high output activities and/or use in damp areas. The Cerium LT would mostly be a better pick for stationary, casual use in cold and dry weather.
Warmth
In terms of warmth, they perform similarly - with the Cerium LT being slightly superior (this might not even be noticeable though - see the temperature ratings below).
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: 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. 
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
Cerium LT is down insulated. Meaning that when it's wet outside, its warmth will drop significantly, because down loses its insulating power when it gets wet (unless you have an outer layer (shell) at the top).
This is one of the disadvantages of down.
- #A sidenote here: Arcteryx.com's PDF guide to their jackets is GOOOLLDDD. Highly recommend you to check it out before you buy.
A third thing to consider for down jackets (which is the Cerium LT in our case) is that it typically takes longer for your body to warm up a down jacket compared to a synthetic jacket (Atom LT).
You don’t get that instant warm effect with down.
In wet conditions, though; that's another story (more on this a little bit later).
Atom LT fit and style. Designed for active use - such as skiing.
Another important disadvantage you get with the synthetic insulated jackets is that they lose more of their initial insulating capability over time. Down insulation, on the other hand, can provide the same warmth for decades.
This sums up the essential differences you'd better know about synthetic vs down insulated jackets.
Results
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 where an average person can wear these jackets without feeling cold can be listed as:
- Cerium LT: -8 deg C (17 deg F)
- Atom LT: -5 deg C (23 deg F)
If you think you run hotter or colder than the average person, mentally shift these temperatures accordingly.
Important:
When/if it's fairly wet out, Cerium LT's warmth will drop significantly and will end up around Atom and Proton LT's warmth range depending how wet it gets.
Breathability
Atom line is designed for active use. Atom LT's side fleece panels will breath much more than the Cerium LT.
Cerium LT, on the other hand, is mostly designed for casual uses. It breathes much less than the Atom LT. I'm 100% sure you wouldn't want to be wearing a Cerium LT when, for example, climbing or skiing.
Fragile face fabric of the Cerium LT (see image) makes it much lighter than the Atom LT.
Fit
Both the Atom LT and Cerium LT offer the same identical, trim fit.
Face Fabric Differences
This is the biggest downside of not only the LT - but also the other two (SV and SL) models of Cerium.
Since it isn't meant to be worn as a stand-alone piece when it's wet or harsh out, the face fabric is just too fragile. It isn't abrasion resistant. Not wind or weather proof either.
Atom LT, on the other hand, is much better compared to the Cerium LT in this department - although still not as good as Atom AR, Patagonia Nano Puff or Patagonia Micro Puff.
Also, both of these jackets are 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.
Although this helps Cerium with its water resistance, it just still doesn't cut it to bring it up to the Atom LT's water and weather protection level.
Conclusion
If you'll be physically active, then don't get Cerium LT. Simple as that. As explained repeatedly in this post, it just really isn't designed for high output use. Get the Atom LT instead.
If, on the other hand, you'll mostly be using the jacket in the neighborhood, then read on...
If you will be wearing an outer layer on top of these jackets...
Do consider the things I mentioned above, but in this case Atom LT and Proton LT lose most (if not all) the advantages they have over the Cerium LT.
You're highly likely to be better getting a Cerium LT. I'd recommend the one with the hoody.
Cerium LT fit and design. Not designed for active use if it will be worn as a stand-alone piece as shown here.
If you think that you might want to invest in an outer layer (a shell), then you might want to check out my guide to Arc'teryx Shells.
If you won't be wearing an outer layer on top of these jackets...
Then I'd make up my mind according to this question:
How will the weather mostly be challenging you?
- Precipitation? Then get the Atom LT or Proton LT (again, models with the hoody are recommended).
- Cold (fair amount of days under 0 deg C (30 deg F))? Then get the Cerium LT.
If it's BOTH cold and precipitation that will be challenging you, then sorry but none of these jackets would cut it for you.
In that case I'd recommend you to consider getting an Atom AR - which you can read more about here in my post. Although Atom AR won't be MUCH warmer than the Cerium LT in dry conditions, in wet conditions the difference will be day and night - you'll feel much warmer inside the Atom AR compared to Cerium LT.
Thank you for a very useful review!
Mayby you can help me.. 🙂 I have a Cerium hoody jacket, but how do I see whether it’s a SL or LT model?
Inside the jacket some numbers appear, but I’m not able see which model it is when I search the internet.
The numbers: 13238-89453-11/15
Fingers crossed you know how to spot the model.
Thank you in advance,
Julie