What is meant by the term "dewpoint"?

• A. The temperature to which air must be cooled to become saturated
• B. The temperature at which air becomes saturated
• C. The temperature at which dew forms
• D. The temperature at which humidity is 100 percent

Answer: (A)

Dew point is the temperature to which air must be cooled to become saturated with water vapor at the current pressure. In other words, it’s a measure of how much moisture is in the air: if you could lower the air’s temperature (without changing how much water vapor it holds), you’d eventually reach a point where the air can no longer hold all that vapor and condensation begins.

That’s why the best description is the temperature to which air must be cooled to become saturated. It isn’t merely the current temperature at which saturation would occur, nor just the exact moment when dew starts to form on surfaces, and it isn’t simply the temperature at which humidity would be 100 percent in a general sense. The dew point specifically pins down the cooling temperature needed for saturation under the same pressure.