Final answer:
A thermal inversion occurs when the temperature increases with altitude, which is a reversal of the normal temperature pattern where it usually decreases with altitude.
Step-by-step explanation:
During a thermal inversion, the normal pattern of temperature change with altitude is reversed. Instead of the temperature decreasing as altitude increases, it increases with altitude. This phenomenon occurs when warmer air resides above cooler air, preventing the cooler air near the surface from moving upwards. Since warmer air is on top of cooler air, the normal lapse rate is inverted, leading to potential issues such as trapping pollutants near the ground and causing poor air quality.