Water Damage Restoration Technician (WRT) Certification Practice Test

When air cools, its ability to hold moisture decreases, which leads to an increase in relative humidity as the temperature drops. Relative humidity is defined as the current amount of moisture in the air compared to the maximum it can hold at that temperature, expressed as a percentage.

At 100% relative humidity, the air is fully saturated with water vapor. This point indicates that the air can no longer hold additional moisture, which often leads to the formation of dew, fog, or precipitation. Thus, when the air cools to the point where it reaches 100% relative humidity, any additional cooling will result in condensation, as the capacity of the air to retain that moisture has been exceeded.

The other percentages listed reflect air that is still capable of holding more water vapor. At values like 70%, 80%, or 90%, the air is not yet saturated, meaning it can still absorb additional moisture without resulting in condensation. Therefore, the situation being described in the question is accurately answered by stating that relative humidity reaches 100% when air cools to its maximum moisture-holding capacity.