Cold is relative, and you would be suprised how well a quality shell, with a longsleeve liner is overall. Light althletic pants work for me until about 29 or less. I use some cc ski/bike pants after that until it hits single digits. Low 20s means a thicker shell, with perhaps a fleece on top of a ls athletic shirt. I swear by neck gaiters, wool, and moisture wick. Hat thickness depends on temp, but i bought a rain snow cover for my helmet which helps. Wool hiking socks are a must. I live in CO, so i have tons of layering options though including different glove/mitt combos. It is more about bejng mentally willing to start the ride, after pedaling a bit, i never regret it.