The Rab Nimbus Jacket is a lightweight synthetic jacket designed as a lightweight insulating layer for damp conditions. The Pertex Quantum outer is lightweight, breathable and windproof whilst the Cirrus insulation is a synthetic insulator which attempts to emulate down, but without the drawbacks when it gets wet. It's contained in stitch through mini baffles which provide good insulation coverage throughout the jacket. The hood is able to be used under or over a helmet and has a lycra bound opening.
Cirrus insulation combines the benefits of down with synthetic fill. It is in fact a synthetic insulation that unlike others has not been formed into a sheet. Instead it remains a loose water resistant down like fibre that expands to form a "cloud of warmth". It will deliver the same warmth for its weight as 600 fill power duck down. As well as lofting properties Cirrus is very compressible as well as robust. It's also very fast drying allowing Cirrus to get back up to its full insulation potential a lot faster than Hydrophobic down or synthetic insulation that is formed into a sheet.
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I bought this jacket for mountaineering - as a mid-layer underneath a hard-shell - and as a warm outer layer while belaying at the crag. I can't praise this jacket enough: it's super light and warm enough for a mild winter. Where I live it usually doesn't go below 5C and there were winter nights that I would feel comfortable with nothing but a long-sleeve shirt underneath the jacket.
I haven't tried it on in the mountain or in near-zero temperatures, but I'm fairly confident that it will do the trick, with maybe a t-shirt and a fleece underneath. The fabric seems to be super-sturdy notwithstanding the lightweight. I also admit that I picked it out for its design: it looks casual enough to be an around-town jacket and I've used if more than once outside the crag.
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