A radiation inversion forms after sunset, when heat starts to radiate away from the Earth. As the Earth begins to cool, the lowest layer of air in contact with the surface cools down. Since air is a poor conductor of heat, the higher layers of air tend to stay warm while the lower layer gets colder as the night goes on.
This creates a layer of cold air sitting below warmer air – a radiation inversion.
In an area of high pressure, the sinking air is compressed and heated because of the increasing atmospheric pressure at lower altitudes. This can lead to the subsiding, warmer air sat above a colder layer at the surface – a subsidence inversion.
High pressure systems are notorious for creating subsidence inversions that can last for days. Any particles of dust or smoke that are lifted from the surface in this time are trapped below the inversion and visibility often gets progressively worse over time.
If a cold airmass moves into an area that was previously occupied by a warm airmass, the heavier & colder air will slide underneath the warm air and force it to rise. There is now warm air sat above cold air and an inversion has formed.
The boundary between the cold and warm air is called a front (more on fronts in later lessons), so this is known as a frontal inversion.