Thermal and winter socks

Warmth comes from trapped air, not from thickness.

A thermal sock is knitted at a heavier gauge, usually in wool or a brushed blend, so the structure holds a layer of still air against the foot. Thickness is a side effect of that, not the mechanism.

The distinction matters commercially. A sock made thick with cheap acrylic feels warm in the hand and cold on a mountain, because it compresses flat inside a boot and the air is gone.

Which wool, and how much. Merino is fine, soft against skin and expensive. Coarser wools are warmer per gram and scratchier. A blend percentage on its own does not describe the sock; the wool type has to come with it.

Brushed interior, or not. Brushing raises the fibres inside the sock and adds real warmth. It also sheds slightly in the first washes, which needs saying on the packaging.

The boot it goes in. Thermal socks are worn inside footwear that is already snug. A sock that adds two millimetres of loft can make a well-fitting boot cold by restricting circulation, which is the opposite of the intent.

Wash instructions. Wool blends need a stated wash temperature. Getting this wrong on the label is the most common reason a good thermal sock comes back shrunk.