http://www.straightdope.com/mailbag/mdryice.html
If water doesn’t sublimate until 0.006 atmospheres, then why do ice cubes get smaller in the freezer over time and how does snow accumulation lessen on a cold (freezing), but sunny day?
Because the partial pressure of water vapor in the cold air is less than that. The triple point of water is when pure ice, pure water, and water vapor are all in equilibrium. At lower temperatures, only ice and water vapor can be at equilibrium. Here is a phase diagram of water,
http://www.sbu.ac.uk/water/phase.html
But in your freezer or the snow outside is dirty ice under air, a mixture of a lot of gases, including water vapor. Treating air as an ideal gas (usually not a terrible approximation, but water is very weird stuff), ice will sublimate if the partial pressure of water in the air is less than the vapor pressure of ice at that temperature. Likewise, if the partial pressure of the water is higher, you get solidification directly from the vapor to ice, a.k.a. frost.
Which is the same reason why liquid water can slowly evaporate, even far below the boiling temperature.