Final answer:
Upon cooling an air parcel, vapor pressure decreases, relative humidity increases due to a fall in saturated vapor pressure, and entropy decreases due to a reduced degree of disorder.
Step-by-step explanation:
When an air parcel is cooled, several physical properties change. The vapor pressure (VP) decreases as the capacity of the air to hold water vapor is directly related to temperature. This leads to an increase in the relative humidity (RH), since RH is the ratio of the current vapor pressure to the saturated vapor pressure (SVP), and SVP declines with a drop in temperature. It is important to note that if the temperature drops enough, the air becomes saturated with water vapor, which can result in dew, fog, or precipitation if the conditions allow.
The entropy (S) of the air parcel also decreases, as cooler temperatures correspond to a lower degree of disorder in the system. Additionally, in the context of Temperature Curves, as temperature decreases during processes like freezing, condensation, and deposition, these are exothermic reactions, meaning heat is released into the surroundings, as described in phase change scenarios.