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

Relative humidity refers to the ratio of water vapor content in the air to saturated water vapor content at the same temperature, expressed as a percentage. The relative humidity of air is determined by the water content and temperature of air. When the water content of air is constant, its relative humidity will decrease with the increase of temperature. 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 m3 saturated water content in 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, warm air with high humidity in the bed is discharged, and cold air from outside is introduced into the seedbed. Because the absolute humidity of cold air is low, and after entering the seedbed, the 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 ventilation reduces the humidity of seedbed. 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, ventilation should be used to reduce the humidity.