I know that density of moist air is less than density of dry air becuase water molecules replace air molecules, and hence as the average molecular mass of water is less than that of air, the density decreases.
Now my doubt is why do the water molecules replace air molecules, why don't they just get mixed up with air molecules without replacing the already existing ones?
I assume the answer might be due to atmospheric pressure( to maintain it almost constant). But I am not able to find a logic to it.
Please explain. Also suggest if I have to add any extra tags relating to the topic.