Patagonia Capilene 2 Lightweight Zip Neck
This comfy long-sleeved wicking base layer ($55) is perfect over a wicking sports bra and tank, especially if merino wool tends to irritate your skin. The smooth and stretchy double-knit fabric moves with you, keeps you dry, and breathes well so you'll never feel cold when you start to get a little sweaty. The zip-up neck keeps your neck protected, but can easily zip down to cool you off when things heat up. The thumb holes are a nice feature, keeping your shirt tucked underneath your mittens.