An inversion occurs when the normal temperature gradient in the atmosphere is reversed. Typically, air temperature decreases with altitude, but during an inversion, a layer of warmer air traps cooler air near the ground. This can lead to poor air quality and increased pollution, as the trapped air prevents the dispersion of pollutants. Inversions are common in valleys and during certain weather conditions, such as high-pressure systems or clear nights. They can also contribute to the formation of fog and low clouds, as moisture in the cooler air condenses. Understanding inversions is important for meteorology and environmental science.