Theory
Let me remind you that a substance can be in a solid, liquid or gaseous state (we are talking about living quarters with a temperature of 15-30 o C at normal - 720-770 mm Hg - atmospheric pressure). Liquids are characterized by the volatility of vapors - at a given temperature, a certain number of liquid molecules pass into a gaseous state. The number of such molecules in the air depends on the properties of the liquid (there are extremely volatile, like ether, and there are not at all volatile, like silicone oil), temperature and pressure.
How long and how many molecules will it take to leave the liquid phase? It depends on the amount already present in the air (gas phase). This is a direct analogy with market competition. If the market is empty and profitable, many manufacturers "burst" there. But the more there are, the higher the competition and the lower the income. As a result, a kind of equilibrium is established in the market. The same thing happens with air. Water molecules will pass into the gas phase either until the liquid is exhausted or to a state of equilibrium (at a given temperature and pressure). Equilibrium in this case is expressed in the achievement of equality between the number of molecules from the liquid phase leaving with the number of molecules returning there.
If we are talking about the ocean, then water molecules obviously cannot run out, there is a lot of water. But there is usually no open water mirror in the room from where evaporation occurs (except for our favorite cup of tea or coffee). Water is contained in hygroscopic materials in the room (wood, fabric, etc.). Those. equilibrium in this room will not be achieved due to the saturation of the air with water vapor to the maximum value, but because the water has run out.
It is important to understand that the number of water molecules in the air depends on the temperature. When growth temperature is 10 to C number grows approximately doubled. With its decrease, the equilibrium content also decreases rapidly. When the temperature becomes negative, there is very little water in the air. Let me remind you that the Celsius scale is based on two reference points associated with the properties of water. Water (especially pure, I will make a reservation) freezes at zero degrees, and boils at 100 o C. That is. at negative temperatures, the air dries quickly.
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I would especially like to emphasize an important point: in everyday life there is no need to measure the relative humidity with a very high accuracy, an integer is enough. Perfectionism in this matter only leads to ineffective budget spending. When manually controlling the humidity in the house, a two-digit value is sufficient. It is only important to make sure that the purchased hygrometer, in principle, works and does not lie by tens of percent. The purchase of various devices (especially expensive ones) makes sense only if you are consciously and deeply fond of this topic.