For those individuals who work outside or for anyone planning on an outdoor activity, it is worth considering wearing some thermal underwear underneath your outer layers.
Thermal underwear comes in a variety of shapes and sizes these days and the materials being used have changed over the years, with advancements being made in the materials sciences. However, the theory behind thermal underwear remains pretty much the same.
Thermal underwear was invented by Sir Benjamin Thompson, who was later to be called Count Rumford. Sir Benjamin Thompson was an Anglo-American Physicist and someone who enjoyed inventing and he had a real interest in heat. Thompson would lead experiments into the thermal properties of different materials and found that trapped air in fabrics was the most important measure of heat insulation. He decided that military uniforms should be cotton in summer, wool in winter and he went on to invent thermal underwear.
Thermal underwear is generally made by a material that is 2 ply, meaning that there are 2 layers to the underwear. Thermal underwear works by trapping in heat, through an in-built insulation system in the clothing. The human body can lose heat by convection, conduction, radiation and evaporation. Thermal underwear can help to combat these physical processes. When your body releases heat in cold surroundings, the underwear confines the expelled heat, keeping it close to your body and therefore keeping you warm.
The modern day thermal underwear can have the added ability to remove moisture developed by the body and yet still keep you warm at the same time. If your body remains moist, then this moisture will serve to cool the body down. If your body remains dry, then you will remain warmer. The moisture generated by the body can be drawn between the 2 layers of the materials and then dried by the process of evaporation. Dispersing this moisture away from the body is an important quality to have in thermal underwear, especially when you are taking part in a physical outdoor pursuit where sweating will occur.
With thermal underwear remaining lightweight and by not restricting the movement of the wearer, thermal underwear has been a large contributing factor to the heights mountaineers can climb or how far skiers can go. There is a whole science behind the use of thermal underwear, with newer, lightweight materials being invented all the time. The Long Johns have moved on from the uncomfortable, unfashionable looking items of clothing you may remember your granddad wearing and have now become an integral piece of equipment for practitioners of outdoor pursuits.