The key to keeping a layering system light for alpine climbing is keeping the ratio of shell weight/insulation low, in other words, the maximum amount of insulation with the least amount of shell.
One way this can be achieved is by having fewer layers - a big down jacket and a small down jacket have a similar shell weight but one has far more insulation. Wearing a lot of layers with shells also reduces breathability.
However, having fewer layers comes at the cost of versatility - you are always either cold, or sweating.
Until now, insulation that did not require a shell to hold it together (such as polar fleece) was not particularly light or compressible for its warmth.
I had some questions about functionality of this layer, so I bought one for a winter/spring trip to Chamonix. I used the Nitro on a number of routes in the Mont Blanc Massif, including the North Face of Grandes Jorasses via the Walker Spur and Croz Spur, where the temperatures ranged from -5C to –15C, and also many ski tours including a large portion of the Haute Route, and also on a ski descent of Mont Blanc.
Lightweight. At 140g for a medium size, you will not find a mid-layer of the same warmth for equal weight.
Fitted. This layer does not feel bulky when worn with a prothermal and shell but is a very warm combination. Note, try the layer on before buying – I am usually a L in Macpac mid-layers but I definitely wanted a XL).
Compressibility. This is a big one for me. I like skitouring with a small backpack. Because of the high energy output of touring I often never wear my warmer layer but I always have something in case I have to stop. I don’t notice the Nitro at the bottom of my backpack.
Changing. Because this can’t be an outer layer, since it would soak up snow and probably rip, you have to strip down to your baselayer to put it on.
No full zip. This is great for weight, but means the helmet has to come off too. I definitely found that on the Walker Spur I delayed putting this layer on until I was pretty cold.
Durability. I can’t really say much on this front but most ultralight gear is not ultra durable - I kept this layer for climbs and skitours when weight mattered. For my normal days skiing and climbing I used my Macpac Pisa jacket.
Outdoor activities that have continuous movement but still warrant carrying a warm layer. For example- mountain running, multisport, skitouring.
Paired with a prothermal and a lightweight shell this layer will definitely be my go to for walking/running missions like Western Arthurs in Tasmania in a day or the Overland run as well as big ski-touring or ski-mountaineering trips.
Cold alpine climbing where you will have a fairly consistent temperature. Think shady faces that you will mostly be either pitching or moving continuously but not constantly changing from one to the other.
Not so great for
Activities that don’t have such a requirement for light weight clothing. For example skiing at the resort, travelling etc. There are better layers for this, for example, the Pisa jacket.