Dry Friction and Triboelectric Drag

image from the Physics of Skiing

At very cold temperatures, skis begin to slide less easily. This is because the coldness of the snow prevents the easy formation of the customary film of water. It turns out, even in extremely cold condition there is a extremely thin film of water.

Oxygen molecules at the surfaces of snow and ice crystals vibrate faster than those on the interior. This vibration creates an extremely thin layer of molecules, that, because of this vibration, behave like a fluid and provide for the basic slipperiness of snow and ice.
Even with a thicker layer of water lubricant, there is still some dry friction. Here is a graph of the total coefficient of friction compared to some of its components:

image from the Physics of Skiing

Skis rub constantly with the ground. Like with other substances, your hair and a pillow for example, the skis develop a static charge. This Static charge picks up any dust, pollen or other contaminants on the trail and adhere them to the base of the ski. This dirt on the base of the ski can create creates much more drag and abrasion. This static charge also attracts the ski base to the snow for greater friction. This is termed Triboelectric Drag