You can compare warmth by checking the weight of insulation. The Arc'teryx Atom is designed to work as a mid-layer, as well as a light, stand-alone jacket. It's got stretch panels fleece under the arm to reduce bulk, and add some breathability. This compliments the fairly close fit, so as not to be constricting. I wear a medium in this, just like I do in nearly every other Arc'teryx item.

I emailed Arc'teryx about the Atom LT and this is the response they sent me.........

Coreloft is a fabric that we have made for us. It's similar to Primaloft, but we have tweaked a few things to try to get better performance.

Core Loft is constructed of a double strand of continuous polyester filaments. The finer yarns (1 denier) are "crimped" to help trap air molecules which in turn help trap body heat while the larger yarns (3.5 Denier) provide loft and resilience from compression. The fibers are siliconized to help add resistance to moisture and decrease drying time.

The Atom is a lightweight insulated shell for more active pursuits in cold/wet weather that offers more breathability than a belay jacket while still offering enough wind/water protection.

Let me know if you have any other questions,

The Coreloft is used because it has large and small diameter insulation fibers. This adds durability, and also allows using a face fabric that breathes better, since it isn't as prone to penetrating the fabric because of the larger fibers. This compliments the Luminara which is a 20denier ripstop that has good weather resistance but breathes much better than most other nylon fabrics.

Arc Teryx searches the globe to find new materials that distinguishes them above other outerwear companies. This vigilance also costs more money, but more often than not the result is a product that is the best of class in the category.

Although I don't own an Atom Jacket, I've checked it out pretty thoroughly and am quite impressed. The outer material (Luminara Nylon) is simply a high-tenacity, tightly woven 20D material that has a great hand. It felt like it had enough durability to withstand some backcountry use, although it still needs to be treated fairly nice.

The Coreloft Insulation is something I had never heard of, but I don't think it's proprietary to ArcTeryx. There was a hang tag listing what it's comprised of, but I don't know who is making it? It's essentially a combination of Primaloft (short staple fibers) and Polarguard or Climashield (Continuous Filaments) thus giving it both the softness of down (Primaloft) but giving it the added durability that plagues Primaloft (Polarguard). The jacket also had stretch panels and had an excellent fit including articulated, curved sleeves. For an ultra-light synthetic piece, I haven't seen it's equal (the closest being Montbell's ThermaWrap Jackets).

thanks for your replies. It's an awesome jacket!

I agree, I think that this would be too warm for biking, the R1 would be a good option or a Cloudveil Run don't walk would be a little warmer. I just picked up a Mountain Hard Wear Power Stretch Zip T and have been using that, so far so good.

I would think it is much to warm for that. I would suggest the Patagonia R1 or R1 hoody, or the Backcountry.com Prime Power Stretch mid layer

I've been wearing the Medium (5'11,170) and it fits close enough to layer under shells, but loose enough to be comfy. If you want a more casual fit the Lge would also work.

I am 6'3" an went with an XL for the arm length. So you would probably be able to get by with a L.

This should be the perfect jacket for that. It will also work great as a jacket to layer on days that it is really cold. This jacket uses a synthetic insulation opposed to down so it can take a beating as far as weather is concerned.