UKC

Warmer and more weatherproof alternatives are better for winter belaying duty, but the Proton Heavyweight is notable chiefly for its balance between insulation and breathability, says John McKenna, and there can't be many synthetic jackets this thick that are so comfortably worn on the go.

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In reply to UKC/UKH Gear: So a thicker version of the Patagonia Nano air (400g or so), where high breathability =  not so much windproof? 

But still useful - I wore my nano air at the weekend to climb in as the insulation layer.  

In reply to wbo2:

Yes, exactly, which is similar to what the Arc'teryx Proton (non-heavyweight version) is like.

The breathability for the likes of climbing in a piece like this is excellent (especially since you probably have a much less breathable - and totally windproof - shell on over the top).

I've used the Proton Heavyweight on colder days climbing and I'm glad I had it. Although not sure I'd want it climbing during close-to-freezing level on long leads. Although if I wasn't the one leading and standing at a belay for a long while, then it is perfect for that.

Standing about with it as a outer layer on a windy day though, not so much.