UKC

The WHISPER® harness is perfect for climbers and mountaineers seeking minimal weight and maximum mobility. Its MATRYX® fabric ensures comfort, durability and compact design. A slim waist belt and leg loops provide freedom of movement, while five gear loops carry all your essentials. Built tough with reinforced tie-in points, it's ready for your most demanding adventures.

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What's the difference between this and the Petzl Altitude? (Apart from 20g). I'm struggling to differentiate the two products 

Competing with the Blue Ice Addax, light trad harness 

The Addax (140-150g) has limitations, gear loops aren't great but relatively cheap £70 

Whisper slightly heavier and presumably slightly more room in pack but if the gear loops are good might be a winner, expensive currently £150

Presumably the lower weight is the difference. It's a bigger market now than in the past. There is room for similar but not identical products. It does look a very nice harness, but not cheap!

Only two caritool slots, shame. Looks really nice otherwise. 

The altitude has a lot slimmer waist band which (i presume, I haven't compared them) will be far less comfortable. It also had two perfunctory gear loops, that are plenty for skiing but not sufficient for climbing.

Thanks, so the thicker waistband probably accounts for the extra 20g the Whisper has over the altitude. So maybe this could be viewed as a more comfortable Altitude? Pretty similar products though!

quite a lot really in the design:

the Petzl WHISPER is essentially a very light weight conventional fixed leg loop harness, (even lighter than their Sitter), The Blue Ice Addax is a similar design in style. The Whisper is lightweight but also designed to be used for technical winter climbing or even as super light sport harness.

the Petzl Altitude is also very light weight harness but can be put on nappy style (without taking skies off etc) as the belay loop is not stitched into the waist band at the top and the leg lops are held in with the plastic buckles. The Blue Ice Chouas is a similar design. These are both more likely to be used for ski touring and mountaineering rather than technical winter climbing.

A FLY and also a SITTA are closer in comparison than an ALTITUDE. But as others have mentioned they're specifically designed for different use.

FLY - Ultra-lightweight and adjustable harness for ski mountaineering and technical mountaineering
WHISPER - Ultra-lightweight and technical harness for high-performance climbing and mountaineering

There's always the option of adding more if your use needs them with CARITOOL EVO