What is an inversion layer?

In meteorology, an inversion layer is an atmosphere layer where the vertical temperature gradient increases with the altitude, that's to say a cold air mass under a warmer air mass. It is a deviation of the normal vertical temperature gradient of the lower atmosphere.

An inversion layer usually occurs with a high pressure system in winter, when this one carries a cold air mass near the ground. A high pressure system is an area with relative higher atsmopheric pressure than surrounding regions. It's characterized by a sinking, descending air which warms up and dries as it sinks towards the ground. That's why a high pressure system is associated with a dry warm air mass in a high altitude but under the system, there may be a cold, a cool, a mild, a warm or a hot air mass. In case of there's a cold air mass, the system will produce an inversion layer, in that case called a subsidence inversion. An inversion layer is a sign that the atmosphere is very stable, and it prevent vertical lift movements such as convection. Heavy clouds can't form in an inversion layer because of the stability of the air except if the high pressure system weakens and a low pressure system comes. If the air near the ground is moist and cold enough, an inversion layer can lead to formation of fog, haze, low clouds or smog, which reduces air quality. That's why a high pressure system doesn't always bring sunny weather.