A heat dome is a mass of very hot air that settles for several days.

This phenomenon appears when a high pressure system stays in one place and blocks the hot air underneath. 

The air is thus imprisoned and moves downward, where it warms up even more.

The wind or the disturbances which arrive from outside are pushed back making it impossible for the heat to dissipate. 

For some time now, heat waves have been occurring more frequently and for longer periods.