What phenomenon occurs when a pocket of warm air sits above cooler air at the surface, preventing circulation?

The phenomenon being described is known as an inversion. An inversion occurs when warm air is trapped above cooler air at the surface, creating a stable atmospheric layer that inhibits vertical mixing and circulation of air. This can lead to an accumulation of pollutants and a reduction in air quality, as the warm layer acts as a lid, preventing the cooler, denser air from rising.

Inversions are often observed in urban areas where industrial pollution and vehicle emissions can become trapped beneath the warm air layer. It is a critical concept in understanding weather patterns and air quality, particularly in meteorology.

The other choices refer to different phenomena: a cyclone is a large-scale air mass that rotates around a center of low atmospheric pressure, a high-pressure system is characterized by sinking air that leads to clear skies and stable weather, and a front represents a boundary between two different air masses. While these terms relate to atmospheric conditions, they do not describe the specific situation of warm air sitting above cooler air, which is what distinguishes an inversion.