So you can control the cooling effect of evaporation. Sitting on the ground, snow, touching cold equipment, or being rained upon are all examples of how heat can be lost through conduction. If you become wet a large amount of body heat is lost rapidly. Perspiration or rain should never be allowed to saturate your clothing which can reduce their insulating values. Sit or sleep on a closed-cell insulating pad.
Radiation causes the largest heat loss from uncovered skin, particularly the head, neck, and hands.
It is important to cover these areas in keeping warm and preventing further heat loss. The body continually warms this layer of air close to the body. A wet suit uses this same theory, but when a person falls into the water you are chilled for a few moments before the water next to your skin is warmed by your body. A dry suit has less initial shock because water does not get inside to start with so the clothing you wear under the dry suit captures the air to retain your warmth. Heat is lost rapidly with the slightest breeze unless you wear a nylon or gortex shell over your clothing to prevent the warm air from being lost.