Final answer:
An air parcel with a relative humidity of 50% will experience a decrease in relative humidity if its temperature increases, assuming the moisture content remains constant. When reaching the dew point, the relative humidity becomes 100%, signaling saturation. If the temperature decreases, the relative humidity will increase due to the air's reduced capacity for holding moisture at lower temperatures.
Step-by-step explanation:
When discussing how temperature affects the relative humidity of an air parcel, it's essential to understand that relative humidity is the ratio of the current amount of water vapor in the air to the maximum amount of water vapor the air can hold at that temperature. If the temperature increases, the air's capacity to hold water vapor increases, thus decreasing the relative humidity if the vapor density remains constant. Conversely, when an air parcel reaches the dew point temperature, the relative humidity becomes 100%, because this is the temperature at which the air cannot hold any more water vapor and condensation begins. If instead the temperature decreases while maintaining a constant vapor density, the relative humidity increases.
For instance, suppose the relative humidity is 80% at 30.0°C. If the air cools to 25.0°C and vapor density remains constant, the relative humidity will increase, possibly beyond 100%, which is unreasonable because it implies supersaturation. The premise responsible for this unreasonable result assumes the vapor density remains constant even as temperature decreases, which does not happen in a closed system.
In another scenario, if the air's relative humidity at 20.0°C is 45% and then the temperature drops to 10.0°C, the relative humidity would increase due to the reduced capacity of cooler air to hold moisture.