Occasionally, a layer of cool air forms under a layer of warmer air in the troposphere, which is different from the normal conditions. this change from the normal temperature profile in the troposphere is called a

    Thermal inversion

Explanation:  Thermal inversion or change the position of the air layer, when the warmer layer of air is above the cold layer, which is usually not the case.  It happens due to a change in the normal temperature gradient, i.e. when the gradient becomes inverted. Such thermal inversions can occur in oceanic areas with a lot of upwelling near the coasts. Thermal inversion also occurs when sun radiation is less than radiation from the earth, which is the case during the night. The significance of these inversions is great, especially for air quality in polluted environments such as urban. The strongest inversions are during the winter and can then be dangerous due to poor air quality, which is measured by the AQI (Air Quality Index). The higher the AQI index, the greater the risk of bad air,  that is, the worse air.