I have both the Atom LT and the Atom AR. I guess this review points out what I feel are the major differences and some similarities. The Atom AR has more Coreloft insulation, and of course makes the jacket warmer. I use it for trekking or just around town on fairly cold days. Awesome warmth. I've found the LT is good to about the high 40s to around 50 degrees before I start feeling cold, just when used for casual use. I have the LT in the carbon copy color, which is a very dark gray, bordering on black and my Atom AR is a dark gray, too (The LT is much darker) I really like the stretch panels that are part of the Atom LT jacket. As the AR has a loser fit, those panels aren't as necessary, but I do wish the AR had them. Great jacket. Glad I have both for different uses and different temps. As an aside, both are size XL (I purchased before a lot of weight loss) and they fit fine, but not snug. UPDATE: While the XL still fits nice, but perhaps a bit too lose even for around town use, I wish now I'd purchased these in a Large, but had no idea I'd lose more weight.