A layer of warmer air above the cooler air layer on the ground is called an

• A. Inversion layer
• B. Thermal cap
• C. Mixing layer
• D. Wind shear layer

Answer: (A)

When a layer of warmer air sits above cooler air at the surface, the temperature actually rises with height in that layer. This is an inversion, a stable stratification that acts like a lid on vertical air movement. Because the air cannot mix upward easily, smoke, heat, and pollutants tend to stay near the ground. That’s why this layer is called the inversion layer—the temperature profile is inverted from the usual pattern.

This term precisely describes the situation in which warmer air traps cooler air beneath it. The other terms refer to different atmospheric features: a mixing layer is the lower, actively mixed portion of the atmosphere; a wind shear layer involves rapid changes in wind with height; a thermal cap isn’t the standard term for this phenomenon.