What is defined as the temperature at which water vapor condenses into water?

The dew point is defined as the temperature at which water vapor in the air begins to condense into liquid water, marking the point where the air becomes saturated. When the air cools to its dew point temperature, the water vapor present in the air condenses, forming dew, fog, or clouds. This concept is essential in meteorology and aviation because it helps determine humidity levels and can influence weather conditions and visibility.

The heat index relates to how hot it feels when humidity is factored in, which is different from the physical process of condensation. The saturation point refers to the condition when a gas is at its maximum capacity to hold water vapor, leading to precipitation but is not a specific temperature. The condensation level is the altitude at which clouds form due to cooling and condensation but does not define the temperature at which this occurs. Thus, the dew point is the correct choice as it uniquely describes the temperature threshold for condensation.