What is the best lightweight thermal underwear for changeable Autumn weather?
Autumn can be an unpredictable month as one day it can be scorching hot whilst the next day you can get out of your tent and find ground frost. This swing in the weather cycle is not ideal if you are out on the hills for a prolonged period of time as you have to pack for all eventualities. But how do you fine the best lightweight thermal underwear for the job? Researching the following four properties when buying thermal underwear should help you get the best from your purchase.
To lighten your load it is a good idea to wear thermal underwear that will adapt to both hot and cold conditions. This is not an endorsement for thermal brands that claim they will keep you cool in summer and hot in winter. Anyone with a simple grasp of physics or textiles knows this is an impossibility. The best you can hope to achieve is a lightweight thermal base layer that will insulate you in the cooler weather, but not be so insulating that you start to cook when the sun comes out.
But how do you know how insulating/warm a thermal underwear brand is going to be? A simple but underused test is the TOG, which measures the thermal resistance of a fabric. You have probably seen it used on duvets, mattresses and pillows, and the lower the figure, the less insulating it is. In the Autumn you need a lightweight thermal underwear brand that has a TOG rating of between 0.35-0.50. Anything higher and you are going to be too warm, anything lower and you will chill.
What this means is basically how quick perspiration is moved from the skins surface to the outside surface of the fabric, so it can evaporate or be moved on to the next clothing layer. The best lightweight thermal underwear garments will have a hydrophilic treatment applied to them. These chemicals actively aid the movement of moisture – hydrophilic means water loving – so vastly improve the fabrics own moisture transportation system. In the cooler Autumnal weather this process keeps a dry layer of air next to the skin (air being the insulator) helping you to keep warm. In warmer weather you will probably wear the lightweight thermal underwear on its own. A quick wicking rate will pump moisture out to the surface of the thermal and allow evaporation. This will help to cool you as water has a high heat capacity and needs a lot of energy to phase change i.e. transform from a liquid to a gas. In layman’s term, your excess body heat is absorbed by the water on the fabrics surface to change to a gas, or evaporate as us mere mortals know the process.
Construction and Fit
Thirdly, the lightweight thermal underwear must be snug fitting. By snug I do not mean restricting blood to an appendage so it goes purple with loss of circulation. The base layer must be close fitting so no air can be wafted when you move. For someone who has not got the body of David Beckham this is not going to be a pretty sight as a good fitting thermal will contour to all your lumps and bumps. However, this close fit ensures that air is trapped next to the skin, insulating the wearer. It also aids hydrophilic treatments as they only work efficiently when there is a heat gradient. The best way to achieve this is by having the fabric in close contact to the skin.
The best lightweight thermal underwear garments will also be constructed sympathetically to your activity. Look for thermal underwear that has a long body length so when you bend over to pick something up or tie your shoe laces your lower back does not become exposed. The sleeves should also be of a significant length that when you lift your arms they do not ride up and expose much your skin.
Finally, your choice in colour will also play an important part in how cool and hot you feel. Black is a good absorber of light wavelengths and will attract more energy from the sun than a lighter colour. However, black is also a good radiator of heat so in overcast weather it will loose heat quicker than a lighter colour. White is obviously the best colour for reflecting heat as it absorbs very little light. If you need thermal underwear in the autumn and are not sure on what weather conditions you are going to be subjected to then choose a colour between the extremes of white and black.