Arc'teryx are touting their latest addition to the massively popular Atom line as a piece of lightweight packable summer insulation, kinda like a replacement for a microfleece. This thing is actually going to be awesome for splitboarding though.
The concept behind the Atom SL Hoody is very similar to the Procline Hybrid Hoody. It also fits in nicely with today's current crop of super breathable insulation garments like the Patagonia Nano Air and similar garments that use Polartec Alpha insulation. With all these essentially the idea is to create synthetic insulation garments that are light and breathable enough to wear on the go.
The Nano Air and Polartec Alpha pieces do this by using very air permeable insulation fibres. The Atom SL Hoody is a little different. It uses 40g Coreloft - the same type of fill that the rest of the Atom line uses but in a thinner form. 40g/m2 is about as thin a weight of insulation as you can imagine. Go much thinner and lighter and you're gonna risk making the insulation so fragile that it breaks apart too easily.
Arc'teryx then place stretchy lightweight torrent fleece (the same fabric they use in the Rho LT and Stryka Hoody pieces) down the sides of the torso and under the arms. This aides in air flow and enables a nice close fit. They finish the piece off by leaving the hood un-insulated and adding a mesh lining to the sleeves - a place where condensation readily builds up during high exertion.
What you're left with is a garment that offer some insulation to the core and virtually no insulation anywhere else. Crucially though in those other areas you still get some light wind and weather protection due to the fleece side panels and outer Tyono 20 shell fabric (a 20 denier nylon with a little stretch).
The hood has the same awesome fit and adjustment as the Arc'teryx Procline Hybrid and the Squamish Hoody - perfect for use in the skin track. The Atom SL Hoody's cuffs also look spot on for those days when you regularly need to vent the forearms to let off some steam.
All in all then, despite the fact that it's targeted at summer use, I reckon the Atom SL Hoody could make for an awesome midlayer for splitboarding and touring. Stick it on over a lightweight baselayer and on cooler days it could well strike that sweet spot of weather protection vs breathability. The Atom SL Hoody will be available this spring.