Description

A heat dome is a mass of abnormally warm air that extends from the surface to well into the atmosphere. It is associated with high pressure both at the surface and aloft. Heat domes develop when the atmosphere traps hot air, as if bounded by a lid or cap. The upper air weather patterns are slow to move, referred to by meteorologists as an Omega block. 

Creation of heat domes

In still, dry summer conditions, a mass of warm air builds up. Sinking air with the high pressure pushes the warm air down. The air is compressed, and as its net heat is now in a smaller volume, so it must get hotter. As the ever warmer near the surface air attempts to rise, the high pressure above it forces it down, to get hotter, and its pressure grows higher.