What is relative humidity balance

What is relative humidity?
        The ratio of water vapor content to saturated water vapor content in the air at the same temperature.

        Detailed explanation:
        The relative humidity of humid air with pressure P and temperature T refers to the ratio of the molar fraction of water vapor in a given humid air to the molar fraction of saturated water vapor on the surface of pure water at the same temperature T and pressure P, expressed as a percentage.

        Relative humidity is the ratio of two pressure values:% RH=100 x p/ps

        Here p is the actual partial pressure value of water vapor in the surrounding environment; PS is the saturation pressure value of water in the surrounding environment

        Relative humidity sensors are usually calibrated at standard room temperature conditions (above 0 degrees Celsius), and accordingly, it is generally believed that these sensors can indicate relative humidity under all temperature conditions (including those below 0 degrees Celsius)

        The vapor pressure generated by ice is lower than that of liquid water. Therefore, when liquid water appears in the form of ice, condensation occurs when the relative humidity is below 100%.

Relative humidity balance
        Hygroscopic substances strive to maintain a balance between their own humidity and the humidity of the surrounding environment. Water in a substance generates water vapor pressure (PM) on its surface, while water in the surrounding atmosphere also generates water vapor pressure (P). If PM and P are the same, the substance achieves relative humidity balance with its environment. Any difference between PM and P will result in humidity exchange, leading to changes in the moisture content of the substance until relative humidity equilibrium is reached. Therefore, the relative humidity balance of a substance is defined as the relative humidity in the surrounding atmosphere that does not cause moisture exchange. (The humidity of the atmosphere must be greater than the humidity of the substance)

Water vapor pressure and relative humidity
        Although the content of water vapor in the atmosphere is not high, it is an extremely active component that plays an important role in weather and climate. There are many methods for measuring the water vapor content in the atmosphere, and in daily life, people are most concerned about water vapor pressure, absolute humidity, and relative humidity.
Water vapor pressure (e) is the partial pressure of water vapor in atmospheric pressure, measured in hundreds of pascals, just like atmospheric pressure. Previously, atmospheric pressure and vapor pressure were often measured in millimeters of mercury, with 1 hPa=0.75008 millimeters of mercury. The partial pressure at which water vapor in the air reaches saturation at a certain temperature is called the saturated water vapor pressure (E). The saturated water vapor pressure rapidly increases with the   increase of temperature. 

        Absolute humidity (a) refers to the mass of water vapor contained in a unit volume of moist air, which is the density of water vapor in the air, expressed in grams per centimeter or kilograms per meter. Absolute humidity is not easily measured directly and is rarely used in practice. If the unit of water vapor pressure is hPa and the unit of absolute humidity is kg/m3, then the relationship between the two is:

        Where T is the temperature.
        Relative humidity (f) refers to the ratio of the water vapor pressure e of the air to the saturated water vapor pressure E at the same temperature, expressed as a percentage:

        The magnitude of relative humidity indicates the degree to which the air is approaching saturation. When f=100%, it indicates that the air has reached saturation; When not saturated, f<100%; When oversaturated, f>100%. The magnitude of relative humidity is not only related to the water vapor content in the atmosphere, but also decreases with increasing temperature.