Final answer:
When the temperature drops and the mass of water in the air stays the same, the relative humidity increases because the air's capacity to hold water vapor decreases.
Step-by-step explanation:
Relative humidity is the measure of the amount of water vapor present in the air compared to the maximum amount the air could hold at a specific temperature. When the temperature drops and the mass of water in the air stays the same, the relative humidity increases. This is because as the temperature decreases, the air's capacity to hold water vapor decreases, so the same amount of water vapor in the air becomes a higher percentage of the maximum amount it can hold.
For example, if the relative humidity is 50% and the temperature drops, the air's capacity to hold water decreases. So the same amount of water vapor will represent a higher percentage of the maximum amount the air can hold, resulting in an increased relative humidity.
To illustrate this further, let's say the mass of water in a certain volume of air is 10 grams and the air can hold a maximum of 20 grams at that temperature. If the temperature drops and the air can now only hold a maximum of 15 grams, the 10 grams of water vapor in the air will be 66.7% of the maximum capacity, resulting in a relative humidity of approximately 67%.