A thermal inversion, which can lead to serious pollution events, occurs when?

A thermal inversion occurs when cool air is trapped near the surface by a layer of warmer air above it. This phenomenon creates a stable atmospheric condition where the warmer air acts like a lid, preventing the cooler, denser air below from rising. As a result, any pollutants that are released into the cool air are unable to disperse upward and can accumulate near the ground, leading to increased concentrations of harmful substances and potential pollution events.

In contrast, the other options do not accurately describe the conditions of a thermal inversion. Warm air rising and cool air settling is a description of normal atmospheric movement, while humidity changes do not specifically relate to the inversion condition. The statement about cold air trapping pollutants near the ground is misleading since it is the warm air that causes the pollution to be trapped, not the cold air itself. Therefore, the correct understanding of a thermal inversion focuses on the role of the warm air layer inhibiting vertical mixing, which is pivotal in pollution events in urban areas and other environments.