What occurs when you cool a parcel of air while keeping the water vapor content constant?

When you cool a parcel of air while keeping the water vapor content constant, the relative humidity increases. This is because relative humidity is a measure of how much water vapor is present in the air compared to the maximum amount of water vapor the air can hold at a given temperature.

As the temperature decreases, the capacity of the air to hold moisture also decreases. Therefore, with the same absolute amount of water vapor present, a reduction in temperature results in a higher relative humidity. This situation is particularly important in meteorology because it can lead to conditions that may cause condensation, cloud formation, and precipitation if the air becomes saturated.

The other choices do not directly apply in the same manner to the scenario described. For instance, while it is true that the temperature of the air decreases, this does not directly indicate a change in relative humidity without considering the water vapor content. Relative pressure is not influenced by this cooling process in the way that relative humidity is, and evaporation typically requires heat to occur; hence, lower temperatures generally do not promote evaporation under constant moisture conditions.