In normal atmosphere, temperature decreases with increasing altitude ... it gets colder as you go up.
During relatively unusual conditions, a mass of warmer air sits on top of the cooler air below. The temperature increases with increasing altitude ... it gets warmer as you go up.
That's the unusual, abnormal situation, called a "temperature inversion".
A thermal inversion, or temperature inversion, is when the normal pattern of cooler air at higher altitudes is inverted, resulting in warmer air above cooler air at the Earth's surface. This phenomenon can lead to poor air quality due to the trapping of pollutants. Thermal inversions typically occur under specific conditions, such as clear nights or high-pressure systems.
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