What is the definition of dew point?

The dew point is defined as the temperature at which a parcel of air reaches saturation under constant pressure. This means that at the dew point, the air can no longer hold all of its moisture in the form of vapor, leading to condensation. When air cools to the dew point temperature, it will begin to form water droplets, resulting in phenomena such as dew, fog, or clouds. This fundamental concept is crucial in meteorology as it helps in understanding humidity levels, cloud formation, and forecasting weather conditions.

In contrast, other options do not accurately define the dew point. While the idea of humidity reaching 100% is somewhat related, it's not the most precise description as it does not consider the nuances of air parcels. The temperature at which air can support no more moisture implies a complete state of saturation but doesn’t capture the specific nature of the dew point itself. Moreover, the temperature of the air at sea level does not pertain to the dew point and is unrelated to the moisture content in the air. Therefore, identifying the dew point as the saturation temperature of an air parcel provides a clear and accurate framework for understanding moisture in the atmosphere.