What phenomenon occurs when air contacts a surface at or below the dew point?

When air comes into contact with a surface at or below the dew point, condensation occurs. This phenomenon is the result of the air being unable to hold all the moisture in vapor form as it cools down. Once the air temperature drops to the dew point, the water vapor in the air begins to transition into liquid water, which forms on surfaces.

Understanding this concept is crucial in building performance and energy efficiency because condensation can lead to issues such as mold growth, structural damage, and unwanted heat loss if not properly managed. Recognizing the conditions under which condensation occurs helps professionals implement strategies to control moisture levels, thereby improving indoor air quality and energy efficiency.

The other options represent different processes which do not apply to the situation described. For example, evaporation is the process by which liquid turns into vapor, which occurs when water warms up. Precipitation refers to any form of water, such as rain or snow, falling from the atmosphere to the earth's surface, while transpiration is the process through which plants release water vapor into the air from their leaves. All of these processes are distinct from the phenomenon of condensation.