What phenomenon occurs when a stable layer of warmer air lies above cooler air?

The phenomenon where a stable layer of warmer air lies above cooler air is known as inversions. This temperature inversion creates a stable atmospheric condition, where the cooler air is trapped beneath the warmer air. As a result, this can lead to various effects on air quality and weather patterns, such as the trapping of pollutants and limited vertical mixing of the atmosphere.

During an inversion, the warm air acts as a lid, preventing the cooler air beneath from rising. This situation can lead to the formation of fog and smog in urban areas, as pollutants become concentrated in the lower layer of cool air. Understanding inversions is crucial for comprehending air quality issues, particularly in regions susceptible to pollution.

The other options describe different atmospheric processes: convection currents involve the movement of air caused by temperature differences, thermal gradients refer to variations in temperature over a distance, and weather fronts represent the boundary between different air masses, typically leading to changes in weather. These concepts are related to atmospheric dynamics, but they do not specifically describe the phenomenon of a warm layer sitting above cooler air.