What term describes a layer of warm, moist air trapped under a cold layer above?

The term that describes a layer of warm, moist air trapped under a colder layer above is known as an inversion layer. In this scenario, the typical temperature gradient of the atmosphere is reversed; instead of air temperature decreasing with altitude, the air temperature increases with height in the inversion layer. This situation can cause significant weather effects, such as suppressing vertical mixing, which keeps pollutants and moisture trapped near the surface, potentially leading to fog or smog and affecting air quality.

The other options do not capture this phenomenon. A pressure ridge refers to an area of relatively high atmospheric pressure, which can influence weather patterns but is not specifically related to the layering of warm air under cold air. A cumulonimbus lull does not exist as a meteorological term; cumulonimbus clouds are associated with thunderstorms and severe weather. The heat island effect is a phenomenon related to urban areas being warmer than their rural surroundings due to human activities and surface changes, not related to the stratification of air layers. Thus, the inversion layer is the most accurate and relevant term for the described condition.