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[BananaStand]

If your'e going to reduce molecular density, don't you need to introduce some kind of energy into the equation?

how is the lack of humidity doing that?

[MadMojoMonkey]

If your'e going to reduce molecular density, don't you need to introduce some kind of energy into the equation?

how is the lack of humidity doing that?

Thermal energy is averaged out to a single temperature, but that average represents the middle of a bell curve of energies.

The tails of the curve are occupied energy / temperature states. I.e. there are some "hot" and some "cold" regions interspersed throughout the microscopic scales of anything which is considered in thermodynamic equilibrium. (Note that by "hot" and "cold" I mean relative to the mean temp)

The occasional "really hot" particle interacts with one molecule of ice, sublimating it. Now, that humidity is not left in the air, but condensed out, leaving dry air. If the air isn't dry enough, the sublimated water will condense back out like dew, leading to frost, as ong described.

The low humidity doesn't really help the situation so much as it reduces an inhibiting factor. Air can only hold so much water suspended in it.
Google relative humidity and dew point for more info.

[BananaStand]

The occasional "really hot" particle interacts with one molecule of ice, sublimating it.

Huh? How do you sublimate a single molecule? Isn't the state of matter (solid/liquid/gas) related to the configuration of many molecules? In other words, can you say that a single molecule is liquid, gas, or solid? And if so, wouldn't it always be solid?

Now, that humidity is not left in the air, but condensed out, leaving dry air.

Out where? I don't have any kind of fancy frost control gadgets on my freezer like Ong suggested.

If the air isn't dry enough, the sublimated water will condense back out like dew, leading to frost, as ong described.

Right, but why does it move first? In other words, why doesn't the sublimated particle re-condense immediately back onto the ice cube from whence it came?

[MadMojoMonkey]

Huh? How do you sublimate a single molecule?

How else could it possibly occur? All chemical processes happen on the molecular / atomic level.

Isn't the state of matter (solid/liquid/gas) related to the configuration of many molecules?

Yes and no. Each particle is in its state based on its "neighbor interactions." Specifically for water, if its movement is basically defined by hydrogen bonds, then it's an ice molecule. If it is frequently interacting with neighboring molecules, but not via stable hydrogen bonds (its specific neighbors are constantly changing), then it's liquid water. If its movement is largely unrestrained, aside from occasional (actually, well defined, here, despite the sound) interactions, then it is water vapor or steam.

Under certain conditions (not in a typical household freezer), the phases of matter can vary via a smooth transition, and not a sharp boundary.

In other words, can you say that a single molecule is liquid, gas, or solid? And if so, wouldn't it always be solid?

No, there are tons of outlier cases, but for water near STP, this is well-defined for each particle, as I briefly stated above.

Out where? I don't have any kind of fancy frost control gadgets on my freezer like Ong suggested.

You almost definitely have a frost free freezer. They haven't made the other kind in decades, AFAIK.
...
BUT
...
IDK what you've got.

You either have a freezer whose walls gather a couple of inches per year of frost, or you have a frost-free freezer.
Have you seen / do you recall those old freezers that would slowly crust over inside until they were a solid block of frost?

Right, but why does it move first? In other words, why doesn't the sublimated particle re-condense immediately back onto the ice cube from whence it came?

The air in the freezer is ~200 K and zipping about real fast like. Interactions are rare on the length scales of the particles... many hundreds or thousands of particle diameters may be covered between collisions with another particle... but they are still common in time, 'cause the particles are still moving about rapidly and there's about 1 mole of particles per liter of air, so plenty of particles in the air in the freezer to interact with.