Relative humidity is influenced by both temperature and pressure, as the same amount of water vapor results in higher relative humidity in cool air than warm air. While the provided reference doesn't explicitly detail the direct relationship between pressure and relative humidity, we can infer and explain the connection based on established scientific principles. Here's how:
Understanding Relative Humidity
Relative humidity (RH) is the amount of water vapor present in air expressed as a percentage of the amount needed for saturation at the same temperature. In simpler terms, it tells us how close the air is to being completely saturated with moisture.
The Role of Pressure
Pressure impacts relative humidity indirectly, primarily by affecting the amount of water vapor the air can hold. The capacity of air to hold water vapor increases with increasing pressure, and temperature.
Here's how pressure affects relative humidity:
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Increased Pressure: Increasing the pressure of air at a constant temperature will initially not change the mass of water vapor in the air. However, it will increase the saturation vapor pressure (the maximum amount of water vapor the air can hold). If the actual amount of water vapor stays the same, the relative humidity will decrease. This is because the air can potentially hold more water vapor than it currently is.
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Decreased Pressure: Decreasing the pressure of air at a constant temperature will lower the saturation vapor pressure. If the actual amount of water vapor is constant, the relative humidity will increase. If the pressure drops low enough, the air will reach saturation (100% relative humidity), and condensation can occur.
Example
Imagine a sealed container containing air with a certain amount of water vapor.
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Increasing Pressure: If you compress the container (increase the pressure), the air's capacity to hold water vapor increases. If the actual amount of water vapor stays the same, the relative humidity goes down.
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Decreasing Pressure: If you expand the container (decrease the pressure), the air's capacity to hold water vapor decreases. If the actual amount of water vapor stays the same, the relative humidity goes up.
Key Takeaway
While temperature has a more pronounced effect on relative humidity, pressure does play a role by influencing the air's capacity to hold moisture. Increased pressure generally leads to lower relative humidity (if the amount of water vapor remains constant), and decreased pressure generally leads to higher relative humidity (if the amount of water vapor remains constant).
