What does it mean when a sample of air has reached 100% relative humidity?

When a sample of air has reached 100% relative humidity, it means that the air has reached its dew point. At this point, the air can no longer hold additional moisture, and any further cooling or addition of moisture will result in condensation. The dew point is the temperature at which air becomes saturated with moisture, leading to the formation of dew or condensation on surfaces. This is a critical concept in understanding moisture behavior in the atmosphere, as 100% relative humidity indicates the maximum capacity of the air to hold water vapor at a specific temperature. Thus, recognizing this condition as synonymous with reaching the dew point helps in various applications, including weather forecasting and HVAC design, where moisture management is crucial.