Inversion

Normally it gets colder with increasing altitude, but during an inversion this relationship reverses: a warmer layer of air lies above colder air near the ground. This stratification is extremely stable, because cold air is heavier and cannot rise. The warm layer acts like a lid.

Inversions occur particularly frequently during the winter half-year. On clear nights, the ground radiates its heat and cools down significantly. As a result, the air near the ground becomes colder than the air above it. This is favoured by high-pressure systems, in which air masses sink over a large area and warm up in the process. In the Swiss Plateau, cold air then collects as if in a bowl, while mild temperatures prevail in the mountains. This situation can persist for days and produces the typical high fog, which traps moisture and pollutants in the layer near the ground.

Types

• Surface inversion (radiation nights)
• Subsidence inversion (high pressure)
• Frontal inversion