Which phenomenon occurs when warm air traps cooler air near the surface?

The phenomenon that occurs when warm air traps cooler air near the surface is thermal inversion. During a thermal inversion, the normal temperature gradient in the atmosphere is reversed. Typically, air temperature decreases with altitude, but in an inversion, a layer of warmer air overlays cooler air near the ground. This situation can lead to various atmospheric effects, including the stabilization of the cooler air, which can trap pollutants and moisture, resulting in fog or smog.

In contrast, a pressure system refers to areas of high or low atmospheric pressure that typically drive weather patterns but does not specifically describe the behavior of warm and cool air. Cumulonimbus development involves the formation of tall, dense clouds associated with thunderstorms and does not relate to the trapping of air layers. A cold front passage occurs when a cold air mass moves in and replaces a warm air mass, causing a vertical displacement of the air rather than trapping it. Therefore, thermal inversion is the precise term for the phenomenon described in the question.