A temperature inversion is a layer of the atmosphere where temperature increases with altitude instead of following the normal decrease (~6.5 °C/km). This reversal creates an extremely stable layer that acts as a lid, suppressing vertical mixing and trapping whatever is below it — including fog, pollution, smoke, and moisture. Inversions are among the most important phenomena in applied meteorology.
Several mechanisms produce inversions: radiative inversions (the ground cools rapidly on clear nights, chilling the air near the surface while air above remains warmer — the most common type), subsidence inversions (air descending within an anticyclone compresses and warms aloft, creating a warm layer above cooler surface air — very common in subtropical highs), frontal inversions (warm air riding over cold air along a warm front), and advection inversions (warm air flowing over a cold surface).
The consequences of inversions are far-reaching. Persistent winter inversions in valleys and basins trap cold air and pollutants for days or weeks, creating hazardous air quality episodes — Madrid, Barcelona, and cities in the Ebro valley regularly experience this. The infamous London Smog of 1952 (which killed ~4,000 people) was caused by a persistent inversion trapping coal smoke. In mountainous terrain, inversions create cold-air pools in valleys, where frost forms while slopes above remain mild — the reverse of the normal altitude-temperature relationship. For aviation, inversions are significant because they can trap low cloud and fog beneath them, creating persistent low-visibility conditions. For weather forecasting, identifying inversion layers on radiosonde soundings is essential for predicting fog, pollution episodes, and convective potential.