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I'm preparing some lunch and freezing it in packages. I store the packages in the freezer in a stacked manner:

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After a long time in the freezer, I noticed that the top package lid gets foggy with frozen water and, in the means that it goes down in the stack, the lids get less foggy:

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The packed lunches are prepared in this way: first, the food is normally cooked on the stove. Once the food is done, it's kept inside the kitchen pot till it reaches room temperature. When the food is at room temperature, it is stored in the package, the package is closed, then it's stored in the freezer.

All the lunch packages contain 300g, have the same food and were stored in the freezer at the same time.

So the question is, why the top lid gets foggy? How to avoid it?

Would storing all the packages in a closed plastic bag inside the freezer solve this problem (didn't try yet)?

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Almost every modern refrigerator or freezer runs a defrost cycle to remove ice from the evaporator (part that gets cold). Ice accumulates as water vapor in the air comes into contact with cold surfaces, and if it weren't dealt with automatically, it would accumulate and have to be removed manually by shutting off the refrigerator and letting it partially melt, then removing the loose chunks. (A chore I remember from my childhood!)

So, for a short period, instead of the freezer compartment being cooled, it is being warmed. This doesn't just melt ice on the coils, it also melts the surface of the food a little bit. This releases liquid water and some water vapor, and some of the water condenses onto the underside of the lid — just like you see droplets of water on the inside of the lid of a pot or a microwaved container. But these are cooled all the way back down to frozen, rather than running off when you open the lid.

As to the containers on top being most affected: During the defrost cycle, to keep the heat mostly in the evaporator and not in the freezer compartment, the refrigerator's air-circulating fan is not running. Still, there will be some hot air, and that hot air will rise, so it's plausible that more heat is transferred to the higher containers. Also, the topmost container does not have the cold mass of another container's bottom side against it.

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