During the rainy season, there is excess moisture in the air, which gets absorbed by the fibrous pulp of the wood., even when it has been dried. That is the nature of wood, it absorbs water.
Only wood that has been treated with chemicals and wood that has been varnished or painted does not absorb water.
Once the wood absorbs water it expands. And when the rains stop the water gradually evaporates, making the wood shrink back into its original dry state.
This is the best lay man's explanation that I can provide.
If you want to prevent the doors from expanding, use a good quality varnish on them, on all sides that are exposed.
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So,in rainy season, wood enlarges a bit more and gets stuck to the door frame.
The stronger a light shines the darker are the shadows around it.
'Idhuvum Kadandhu Pogum "
Even this challenging situation would ease
In winter and rainy season, wood absorb moisture from the air and hence expands and this can be noticed easily as the door and windows made of wood tightens in these two season.
During summer the same piece of wood loses all its moisture and hence it contracts a little bit and becomes loose.
The main reason behind this contraction and expansion of wood is moisture whose loss and gain affects its dimension.
Another good example of contraction and expansion because of weather effect is railway lines. In summer railway lines expands after gaining temperature and hence the distance between both the rails increases from its normal difference. But during winter, reverse of it take place. The distance between the rail decreases a little bit because iron contracts due to loss in temperature.
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