Why can air humidity be reduced by ventilation in cold winter?

To answer this question, we need to start with the concept of air humidity.

air humidity is often expressed as relative humidity or absolute humidity. Absolute humidity refers to the density of water vapor in the air, which is expressed by the weight (grams) of water vapor contained in 1 cubic meter of air. The more water vapor content, the higher the absolute humidity of air. There is a certain limit to the water content in the air. When it reaches the maximum capacity, it is called the saturated water vapor content. When the temperature of air rises, its saturated water vapor content also increases accordingly; When the temperature decreases, the saturated water vapor content of the air decreases accordingly (Table 1). Therefore, the absolute humidity of cold air is lower than that of hot air, so seedlings are easy to lose water and shrivel when they are blown into cold air (of course, this shrivel is also related to freezing).

Relative humidity refers to the ratio of the water vapor content in the air to the saturated water vapor content at the same temperature, and is expressed as a percentage. The relative humidity of air is determined by the water content and temperature of air. Under the condition that the water content of air is constant, with the increase of temperature, its relative humidity will decrease accordingly. When the temperature decreases, the relative humidity of the air increases (Table 2). When the seedbed is closed, the air humidity in the bed increases at night, which is mainly the reason for the decrease of bed temperature.

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Table 1 1 Saturated water content in cubic meters of air

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Table 2 Under the condition of constant water content, the relative humidity of air decreases with the increase of temperature

When the seedbed is ventilated, the warm air with high humidity in the bed is discharged, and the cold air outside is introduced into the seedbed. Because the absolute humidity of cold air is low, and after it enters the seedbed, its relative humidity decreases due to the increase of temperature, so it absorbs water from the surrounding. With the constant exchange of air inside and outside the bed, the humidity in the seedbed gradually decreased. This is why the humidity of seedbed is reduced by ventilation. Therefore, under the premise of not making the temperature of the seedbed too low, even in cold weather, if the humidity in the seedbed is high, it is still necessary to reduce the humidity by ventilation.