Every year, numerous people die in New Hampshire's White Mountains die from hypothermia. It's already snowing on Mt. Washington.
To understand how to keep warm when the weather outside is frightful, you first have to understand how the human body loses heat - conduction, convection, evaporation, radiation and respiration.
Conduction is heat loss when the body comes into direct contact with a cold object - snow, a metal trekking pole/ice ax, or a cold rock.
Convection is heat loss when air or water passes by the body such as a brisk wind passing by the surface of the skin.
Evaporation is heat loss when moisture on the body becomes airborne - either sweat or rain water on wet clothing
Radiation is heat loss that occurs when heat escapes directly into the still air
Respiration is heat loss that occurs when breathing air that is colder than body temperature that is warmed by the body then exhaled.
Here's my strategy for avoiding hypothermia from any of these causes.
During Fall/Winter/Spring I wear:
A torso base layer of thin polyester (Arcteryx Rho LT)
A torso shell layer of Gortex Pro Shell (Arcteryx Alpha LT)
A lower extremity combination of thin insulation and shell (Arcteryx Gamma MX)
A head base layer of thin polyester (OR Ninjaclava)
A warm, windproof hat (OR Windpro Hat)
A hand base layer of thin polyester (OR PL100 Gloves)
A waterproof/windproof shell layer (OR Cornice Mitts)
A belay jacket on my upper extremity when I stop moving (Arcteryx Solo)
How do these work?
Conduction - the lower extremity insulation layer slows conduction if I sit on the snow or on a rock. I do not lie down on snow, so my upper extremity does not contact anything cold directly.
Convection - the upper extremity Gortex layer and the lower extremity wind proof softshell minimizes an wind related heat loss
Evaporation - you'll notice that I do no wear any upper body insulation - just a wicking layer of thin polyester. This ensures I do not sweat, even when climbing thousands of feet. I may be a bit cold when I start, but as I climb, the exertion keeps me warm without sweat, minimizing evaporative heat loss
Radiation - my multilayered hats prevent significant radiant heat loss since greater than 50% of the body's radiation occurs through the head. The Belay jacket, which I put on over the Gortex layer when I stop moving, minimizes radiant heat loss.
Respiration - there's really nothing I can do to eliminate heat loss due to respiration. However, I carry 1 liter of boiling water and 3 Lara Bars to keep myself fed and hydrated. I carry the hot water in nalgene polyethyelene container (BPA free) wrapped in an OR water bottle parka Adding warm liquids to my body during an ice climb or hike maintains my body core temperature.
Dress right, eat right, keep moving and no matter what the temperature, you'll keep warm.
Remember, in New England there is no bad weather, just poor clothing choices!