What causes ice to stick together

[Pengwuino]

I think everyone sees this happen. You buy a pack of ice from the store, it's all nice and loose and easy to use and stays that way in the storage bin. You bring it home and if you don't use it, you put it in the freezer. For a couple of days the ice stays nice and detached and usable. However, as time goes on, the ice starts getting stuck together up to the point it all gets attached into one giant piece.

What causes this? I've been trying to figure it out but I'm clueless. The freezers are well below 0... maybe moisture in the air starts filling the gap between the ice cubes to bond them together?

 

[Fewmet]

... maybe moisture in the air starts filling the gap between the ice cubes to bond them together?

That's my guess. The amount of build-up you get in older freezers (perhaps before the "frost-free" systems were well-engineered) makes clear there is a substantial source of water vapor.

Or maybe it is regelation?

I know from earlier work that the temperature cycles above and below the temperature you set it at. The temperature will likely rise above 0 C when the door is opened, but I would think too little energy gets into cause significant melting.

A few tests comes to mind:
1) Does the effect correlate to how often or how long the door is opened?
2) Does it correlate to what the temperature is set at?
3) Does it happen more to ice at the more of the bag than at the top? (I'm thinking about regelation here).
4) Does it happen more if weights on put on top of the ice bag?
5) What are the effects storing the ice in a reasonably air tight container?

I think I see a great testable question in this for my students...