Temperature isn't everything.
With good sunshine this morning, I planned on a morning hike in the snow. Yet, the winds are blowing about 17 mph, gusting every once in awhile to 20.
Says its 18 on the thermometer.
According to this chart, the chill on my body is near zero. This link will also take you to an actual calculator where you can plug in the numbers yourself. There is information on what it means in terms of frost bite, how it is determined, and much more.
This is a good indicator for full face coverage, sunglasses to protect against snow glare, and layers.
Happy Trails, and best wishes to any south bounders still out on the Appalachian Trail.