The one answer I’ve found from long ago is “Sometimes, especially after rain, the dense cold air gets moist and becomes denser. If it gets dense enough, it falls and compresses hot air beneath it, disallowing it from rising and trapping it there.”
But I don’t understand how that can work. Gasses and liquids cannot trap other gasses or liquids below them that is why is you breath out underwater the bubbles move to the surface.
In: Physics
A heat dome happens when a high-pressure system traps a mass of hot air beneath it. Think of it like a lid on a pot: the high-pressure system acts as the lid, preventing the hot air from escaping.
In more detail, the high-pressure system pushes down on the air below it. This compresses the air, which heats it up even more and keeps it from rising and dissipating. As the sun continues to heat the ground, the trapped air gets hotter, leading to a build-up of extreme heat over several days.
So, unlike water where bubbles rise to the surface, a heat dome works because the high-pressure system effectively seals in the heat, creating a “dome” of hot air that can’t escape.
Basically with a heat dome, you get a high pressure system sitting over a large area of land, like the middle of a continent. This high pressure system doesn’t move for an unusually long time, and it simply traps the hot air beneath it while preventing both convection – which would normally dissipate the heat into other parts of the atmosphere, especially the upper atmosphere – and it also prevents precipitation, which would normally help move heat around through the atmosphere. Formally a heat dome also normally involves surface temperature irregularities, like high sea surface temperatures offshore that lead to increased ground level heating onshore, and the upper pressure in the high atmosphere prevents that air from dissipating.
The phrase “heat dome” is also used in the popular media – like what we’re seeing right now – to simply describe a protracted heat event. A heat dome is just a trapped heat wave. What you’re describing above is not a heat dome; I’m honestly not sure what it is, and have not heard of that process, though that doesn’t mean it doesn’t exist. When rain condenses, it gives up heat to the atmosphere as part of the process of condensation. This is the latent heat of a phase change. Liquid at the surface, or in any place, that dissolves into the air also involves a latent heat of phase change process, but will cool the air because it absorbs heat from the air (and surfaces) in order to evaporate. At the same time, falling dense air will compress air beneath it but if it keeps falling, it will displace the air beneath it.
There are circumstances where you have things like microbursts that happen because of air temperature, density and humidity level changes in a thunderstorm. There are also heat bursts, which happen when moisture evaporates into a parcel of air high in the atmosphere (normally in the storm system), a situation similar to virga. This makes the air denser, and it rapidly begins to descend. When it descends, it is compressed, and it very, very rapidly warms up. It hits the surface in a situation similar to a hot microburst, where the air grows very hot and hits the ground with rapid speed. It can cause 20+ degree temperature changes at the surface.
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