Atom (aka LT) vs Cerium (aka LT): Temp ratings & more

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

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

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. 

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. 

Fit

Cerium comes with a trimmer cut. 

• 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: 

Packability

Cerium packs into its own stuff sack that comes attached to the jacket. Atom doesn't stuff into itself.