Arc’teryx Thorium vs Cerium: Do you want warmer or lighter?

Thorium vs Cerium: Overview 

Names of Thorium and Cerium were previously Thorium AR and Cerium LT. 

When it comes to Cerium vs Thorium, first thing to note is that both are down jackets. 

Their warmth (particularly spectacular warmth-to-weight ratio) make them perfect for casual use and travel (they're packable, i.e they come with stuff sacks). 

But you can't use them in damp areas because they soak water like crazy. 

That's why I like them hoodless, but a survey I conducted here revealed that only 30% of people are on board with me on this. 

Lastly, active use isn't recommended with down jackets either because they're not breathable and resistant against abrasion at all. 

If literally any of these is a dealbreaker to you, then stop reading this post here and check out my Arc'teryx Proton vs Atom guide. 

The difference between Thorium vs Cerium is rather smaller. 

Here's how Arc'teryx compares the Cerium vs Thorium: 

https://arcteryx.com/us/en/compare/X000007250/X000007467

I'd recommend you to check out their warmth ratings below and make your purchase accordingly: 

Try out the jacket finder tool at arcteryx.com too. 

Before we begin, let me clear this out of our way. I wouldn't recommend either of these jackets if: 

• You'll be wearing them as a stand alone piece when there's either heavy rain, snow or wind a fair amount of days; 
• You won't be treating them very kindly and/or its outer shell will be in contact with rough surfaces. 

This is all because both jackets have quite fragile face fabric with Thorium being a little superior to the Cerium (more on this later). 

You can see this in Arc'teryx's Naming Scheme Page here as well - for both "Cerium" and "Thorium". 

That said, although they make quite vulnerable outer layers out against these harsh conditions, they are perfect midlayers. If the above two bullets don't apply to your situation, they'd make perfect stand-alone pieces as well. 

• Fit

Canberk is 5'11" and 165 pounds, with an athletic build. Size Medium fits him perfectly across almost all brands and product lines. 

• Arc'teryx's 2024 men's fit revision

Here's what Arc'teryx says about its most recent fit revision: 

Source: https://arcteryx.com/us/en/help/sizing/mens/tops/new

Not all jackets come with the revised fit - only the ones with following statement: 

Warmth

Cerium uses a 850 fill power European grey goose down, whereas Thorium uses 750.

This value is in cubic inches, and represents the end volume the down eventually fully expands to when it's at rest. 

The more the volume, the more air will be > trapped inside the down >  heated by your body > and ultimately work as an insulation. 

That's why the higher the fill power, the warmer you'll feel inside the jacket provided that both jackets are filled with the same amount down. 

What does that mean?

It means that a down jacket with a 750 FP (fill power) down can provide more warmth than a 850 FP down by using more of 750 FP down.

And this is exactly what's happening in our case. Thorium provides more warmth than Cerium by using this strategy. 

• Thorium: Equipped with 140 grams (≈5 ounces) of 750 down fill
• Cerium: Equipped with 102 grams (≈4 ounces) of 850 down fill

The downside of this (you guessed that right) is the added weight.

That's why, in the end, this trade-off makes the Thorium nearly 200 grams heavier than Cerium {(517 grams (1 lb 2.2 oz) vs 335 grams (11.8 oz)} in return. 

But I'd give relatively less importance to this weight difference, because as you can see here on Arc'teryx's own Naming Scheme Page, both Cerium and Thorium lines are on the lighter side. 

One more thing to consider for warmth: Fit

Thorium fits baggier. 

This doesn't only make under layering easy and sweet, but it also increases warmth even when you wear the same under layer beneath these jackets.

How?

Due to the baggier cut, there's more air trapped between your body and the jacket, which works as a perfect extra insulation.

Baggier cut doesn't always equal more warmth. There's a fine balance and you can often go too baggy and eventually and let too much air circulate & end up providing even less warmth than with the trimmer case to begin with.

But Thorium is at the warmer side of this line and hence provides extra warmth leveraging its baggier cut.

Lastly, both offer the same hem length at 72 cm (28.3 inches).

Weather resistance

Cerium uses a thinner, lighter fabric on the outer shell (Arato 15) compared to Thorium AR (Arato 30). 

Cerium's previous model, Cerium LT, had equipped only Arato 10. It was insanely fragile so Arc'teryx must have decided an upgrade in this regard. 

Thorium's Arato 30 is absolutely more wind resistant, water resistant and durable, although still not nearly as much as synthetic insulated jackets like Atom Heavyweight (Men's - Women's - read review). 

In dry conditions, Atom Heavyweight (aka AR) would be less warm than Thorium. 

But in wet conditions, being equipped with synthetic insulation rather than down, (also having a super robust outer shell) Atom Hw would be warmer. 

Thorium isn't windproof by any means either. It's only lightly wind-resistant. 

That said, both Thorium and Cerium are DWR coated. 

DWR will be enough to repel some moisture and light rain. But after, say, 15-30 mins, they both will start soaking.