The meaning of dew point
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Dew point is a temperature value used to indicate humidity; it represents the temperature at which air becomes saturated when cooled, under conditions where both the water vapor content and air pressure remain unchanged – in other words, it is the temperature at which the water vapor in the air turns into dew drops. Why is it used to represent humidity? Because, when the water vapor in the air is saturated, the air temperature is equal to the dew point temperature ； When the water vapor is not saturated, the air temperature is necessarily higher than the dew point temperature; therefore, the difference between the dew point and the air temperature indicates the degree to which the water vapor in the air is close to saturation. At 100% relative humidity, the temperature of the surrounding environment is the dew point temperature. The lower the dew point temperature compared to the temperature of the surrounding environment, the less likely condensation will occur. This means that the air is drier. The dew point is not affected by temperature, but rather by pressure. When the temperature drops sharply below the dew point, the moisture in the air rapidly condenses into tiny water droplets, forming fog.
(1) Definition or explanation: ① The temperature at which the unsaturated water vapor already present in the air becomes saturated is called the dew point. ②The temperature at which the relative humidity of the air becomes 100%, that is, when the actual water vapor pressure equals the saturated water vapor pressure, is called the dew point. (2) Unit *Conventionally, Celsius temperature is commonly used. (3) Explanation: ① People often determine the absolute and relative humidity of air by measuring the dew point; therefore, the dew point is also a way of expressing air humidity. For example, if the temperature of the air at a certain air pressure is 20°C and the dew point is 12°C, then the table can be used to find that the saturated vapor pressure at 20°C is 17.54 mmHg, while the saturated vapor pressure at 12°C is 10.52 mmHg. Then at this time: the absolute humidity of the air is p=10.52 mmHg, and the relative humidity of the air is B=(10.52/17.54)×100% = 60%. Using this method to determine air humidity has great practical value. But a crucial point here is that students are required to learn the method for determining dew point. ②The determination of dew point is of great significance in agriculture. When the humidity of the air drops to the dew point, the water vapor in the air condenses into dew. If the dew point is below 0°C, then when the temperature drops to that level, water vapor will directly condense into frost. By knowing the dew point, it is possible to predict whether frost will occur, thus protecting crops from damage. ⑨The smaller the difference between the air temperature and the dew point, the more saturated the air is. The temperature and dew point are close to each other, which means the relative humidity is high at this time, making people feel that the climate is humid ； A large difference between temperature and dew point means that the relative humidity is low, and people perceive the climate as dry. The moderate relative humidity that is comfortable for the human body is 60–70%. ④Strictly speaking, the saturated vapor pressure at the dew point is not equal to the water vapor pressure of the air at that time. Since the pressure of unsaturated vapor varies with temperature according to the following rule: Pt = P0(1 + t/273).
What about the hydrocarbon dew point? It seems different from the water dew point curve.