Choose a lightweight layer, such as the Fjällräven Bergtagen Thinwool or a pair of synthetic bottoms, for moderate to cool temperatures––about five to ten degrees; cold temperatures that range between five and zero degrees call for a midweight layer like a Burton Multipath Lifestyle Crew or Smartwool’s 1/4 Zip Baselayer; finally, if you’re heading out into below-freezing weather, opt for a heavyweight layer from SAXX.