Synthetic filled clothing fulfills a similar role to down filled clothing, that is as an insulator.
Even in the coldest conditions, your body produces heat, and you feel cold because this heat is removed quickly in a cold environment. The job of insulation layers is to trap air next to the body, allowing your body to warm up this air and then stop losing heat to the environment.
How well the item of cloting does its job depends on a few things; how much air it can trap, how windproof it is (i.e. how much it can stop the trapped air being stripped away by the wind) and how well it does this job if it gets wet.
Synthetic insulation emulates down in that it is made from a tangle of tiny fibres, which trap air and keep warmth in. It is not as good at insulating as down - you will need more synthetic insulation to give the same warmth, but it is getting close! The outer of a synthetic jacket will usually be made of Pertex - a wind resistant, breathable material which stops the warm air in the fibres from being replaced by cold air in a breeze.
Where Synthetic insulation has a big advantage over down is that it doesn't absorb water very well and is quite strong. This means it drys quicky, and doesn't clump together when it becomes wet, unlike down which loses its insulative properties when wet (if the down has clumped, there is nowhere for air to be trapped). For the UK, or anywhere where you are likely to encounter cold, wet conditions, this property of synthetic insulation makes it much more appropriate than down.
If you are heading to altitude and conditions are likely to become so cold that all water is frozen, a synthetic jacket is likely to be too bulky to be practical if it is warm enough and down is a much better choice.
A light, full-zip insulated jacket regularly used by military personnel, rescue teams and outdoor professionals, the Buffalo Belay Jacket is a pile lined Pertex Quantum outer jacket for cold weather and winter mountaineering adventures.
Blending Pertex Quantum with Polartec Alpha Direct and Pontetorto Tecnostretch, this jacket offers great weather repellency, warmth and fit for a light weight.Weight: 340g Cut: Athletic Fit Lightweight, vapor-permeable, wind- and water-repellent Pertex® Quantum Air outer material with Diamond Fuse technology for an ideal balance of weather protection, breathability and abrasion resistance Lining-free Polartec® Alpha® Direct insulating material (95 g) offering heat retention without heat build-up Enhanced breathability and absolute freedom of movement thanks to elasticated Pontetorto® Stretch fleece inserts on the sides, arms and back MAMMUT Georganic 3D Technology Y Shape Pattern for an optimized fit and for maximum freedom of movement MAMMUT High Reach Technology™ for unrestricted freedom of movement when climbing MAMMUT Thermo Management System™ for optimum weather protection and comfort 2 climbing-harness-compatible front pockets with cross-over access Elastic cuffs with flat thumb loops Partially elastic hem for a better fit MAMMUT CONNECT inside
A brilliant versitile insulation piece for mountainous environments where weather is unreliablePertex Quantum AIR© 20 CFM Face Fabric with 4-Way Stretch Polartec® Alpha® Insulation Attached Hood with Elastic Binding Zippered Handwarmer Pockets Zippered Chest Pocket