Below 70: add a thin base layer
Below 60: Add knee warmers and maybe a l/s jersey
Below 50: Gloves, light jacket, fleece knee and arm warmers, skullcap
Below 40: Add heated insoles to the shoes and heat packs to the gloves, fleece tights and L/S base layer
Below 30: Add Windproof jacket
Below 20: Windproof tights, jacket, full shoe covers, heated gloves (as in battery heated), heated insoles... at this point, I just have to make it the 10 miles to work. It'll be in the 30's by afternoon

Where I am it is not uncommon to see a 30-degree differential between night and day, especially in late fall or early spring. It might be 20 at night and 50 during the day. It makes packing for work a pain in the butt.