What is a temperature inversion?
A temperature inversion is where temperature increases with height.
How do they form?
The lowest part of our atmosphere is the troposphere, which can extend to heights of 16 km and is where most of our weather happens. It is also a section of the atmosphere where the temperature typically gets lower the higher up you go, for example, when you climb a mountain it is often colder at the top. However, sometimes a small layer can form where the temperature increases with height. This layer is called an inversion.
This often happens in areas of high pressure, where the air high up often sinks towards the ground. As it falls, it dries out and warms up. This warm layer of air can act as a lid and trap cooler air near the surface (this is because warm air is more buoyant than cold air, and so it will tend to 'float' above the colder air, trapping it). This gives us the inversion, because if you were now to climb the mountain, it would get warmer as you got to the top. This is inverted compared to what you would normally expect, hence the term 'inversion'.
Inversions are most common in winter when mist and fog become trapped in the cooler air low down, but inversions can happen all year round.