The London Weather Station

Warm coats and weatherproof jackets protect from wind, rain and cold. With new technologies such as Gore-Tex, the rain cannot go through the jacket unto your skin, but if you are hot, your sweat can go through the jacket and evaporate. This is because Gore-Tex is a fabric that is very closely woven and almost nothing can pass throughy it, for example no normal water drops. But sweat is warm and therefore like steam, and the little drops in the steam are small enough to pass through the fabric.