by edumad » Mon May 07, 2012 1:18 pm
All systems of thermal isolation work by creating a layer of low heat transfer coefficient. For clothing this is generally trapped air.
Different fabrics can have different pores and thickness and prevent heat loss to different extents.
One thing is certain, heat passes more quickly through water than through air.
If you wet your skin, you lose heat more quickly, if you wet you closes you then the trapped air is in part or in whole replaced by water (rain or sweat) and heat passes more easily through the layer.
If you are in a cold environment, then your base layer, even if especially good at wicking will take longer to lose all the moist in it than it will take you to start to feel cold. Ideally you'd replace the moist layer with dry ones (and if its windy, a wind proof), you'll keep warm. Otherwise you need to layer up to compensate for the lack of isolation your now wet clothes provide. Removing the wet base-layer might be a good idea provide your outer layer provides sufficient isolation. A thin wind proof might do wonders to keep out the cold air, but wherever it touches the skin heat is loss easily.
I usually take my DSLR with me on most hikes, so the wait of a fleece is not a problem. But I don't always change, I make sure as soon as I stop for a long period of time I layer up. I don't wait until I start to get cold. This way you make the excess heat you are generating work towards turning that trapped moisture into steam or at least warm it up, so that cold sets in later.