What is the term for the temperature at which water vapor in the air begins to condense?

The correct term for the temperature at which water vapor in the air begins to condense is the dew point. The dew point is a crucial concept in HVAC as it indicates the threshold temperature where air becomes saturated with moisture, resulting in condensation. When the air temperature drops to this level, the water vapor present in the air can no longer remain in gaseous form and starts to transform into liquid water, which is often observed as dew on surfaces.

Other terms mentioned in the choices relate to different concepts in the study of air and moisture. The humidity ratio refers to the amount of water vapor present in the air compared to the total mass of the air, providing insight into the moisture content but not directly indicating the temperature at which condensation occurs. Wet bulb temperature is a measure of the lowest temperature that can be attained by evaporating water into the air, while dry bulb temperature is simply the measure of air temperature without accounting for moisture content. Each of these plays a role in understanding HVAC principles, but none directly defines the temperature of condensation like the dew point does.