This Arc’teryx top is the best synthetic baselayer tested for temperature control. The wicking properties are genuinely impressive, so good I found it immediately noticeable. It kept me dry on a long scrambling day, as well as on runs. The Arc’teryx website has lots of complicated graphics and words like ‘Phi Yarns’ explaining why it’s so effective, but in short it uses two yarns, one great at moving the water and another great at wicking, and I believe it (I usually avoid this stuff, but I was impressed). As for the layer itself, it’s tight but stretchy and moves well with the body. the seams are flatlocked and the zip is easy to move and comfortable when zipped fully up. Arc’teryx makes three weights, the SL being the lightest. It’s a good price too.