Plain English Meaning
A slice of the sky where the air actually gets warmer as you climb instead of cooler. Because warm air is sitting on top of cooler air, nothing wants to rise through it, so haze, smoke, and moisture get trapped underneath.
Full Definition
A layer of the atmosphere in which temperature increases with altitude rather than decreasing, reversing the normal vertical temperature pattern. Inversion layers are very stable, suppress vertical air movement, and often trap moisture, smoke, haze, and pollutants beneath them. They commonly form near the surface on clear, calm nights as the ground cools, or aloft where a warm air mass overrides cooler air below.