Which conditions are indicators of an impending inversion?

The correct answer highlights that still air and cool evening temperatures often precede an inversion. An inversion occurs when a layer of warm air traps cooler air closer to the ground, preventing it from rising. In conditions where the air is still and cool—particularly during the nighttime—heat from the earth’s surface can dissipate rapidly. This cooling, combined with the lack of wind, allows the cooler air to settle in low-lying areas, creating an environment that is conducive to the development of an inversion layer.

In contrast, the other conditions listed would not typically signify the onset of an inversion. For instance, strong winds and sudden temperature changes can lead to a mixing of air layers, which disrupts the formation of an inversion. High humidity and increased evaporation may contribute to fog or cloud formation but do not directly indicate the stable, calm conditions needed for an inversion. Dry air and elevated wind speeds also suggest a turbulent atmosphere, which is antithetical to the stillness required for an inversion to form.