The lid over a heat dome is sinking air, not the stratosphere — and it is what kills the Delta breeze

In June we published a piece here attributing the intensity of recent heat domes to what we called stratospheric inversion. That is not the mechanism and it is not a term meteorologists use for this. The lid over a heat dome forms in the troposphere, well below the stratosphere, and it is made by air sinking rather than by anything happening above it.

What actually makes the lid

A heat dome is a blocking anticyclone: a very tall, very persistent ridge of high pressure that parks and refuses to move. Inside it air descends. Descending air is compressed by the increasing pressure beneath it and warms without any heat being added — adiabatic warming, the same physics that makes a bicycle pump hot.

That sinking warm air creates a subsidence inversion: a layer where temperature rises with height instead of falling. Rising air stops when it reaches air warmer than itself, so the inversion behaves as a ceiling. Nothing convects through it. Clouds do not form, so sunlight arrives at the ground unfiltered, and the heat it deposits has nowhere to go but into the surface and the shallow layer above it. Each day starts hotter than the last.

The stratosphere sits above the tropopause, roughly ten kilometres up in the mid-latitudes, and it is not what caps this. Our original piece put the mechanism in the wrong layer of the atmosphere.

What made the 2021 event

The clearest case on record is the Pacific Northwest in late June 2021. The blocking pattern took the shape meteorologists call an omega block, and the 500-hectopascal geopotential height reached about 5,980 metres — a value with no precedent for western North America in the record running from 1948 to that month.

The ridge did not appear from nowhere. Convection associated with the East Asian monsoon around 20 June seeded a Rossby wave train that travelled east along the mid-latitude wave guide, and latent heat released south of Alaska fed wave activity that converged over western Canada. The dome was assembled several thousand kilometres upwind, days before anyone underneath it felt anything.

How unusual it was

A rapid attribution analysis published in Earth System Dynamics in 2022 treated the event as a roughly 1-in-1000-year occurrence in the current climate. Against a world without human-induced warming, the authors found such an event would have been at least 150 times rarer. And a heat wave of the same rarity in 1850 to 1900 would have been about 2 °C cooler than what actually happened.

Those are three different statements and they are worth keeping apart. The event was rare even now. Warming did not create the block — blocks have always happened — it raised the temperature the block was working with, and shifted the odds by more than two orders of magnitude.

What it means in this valley

Sacramento's summer defence is the Delta breeze: marine air pulled in through the one gap in the Coast Range as the Valley heats and rises. It is the reason a 40-degree afternoon here can end in a tolerable evening.

A blocking ridge is the thing that takes it away. Subsiding air suppresses the pressure gradient that draws the breeze inland, so the nights stop cooling — and overnight temperature is what most of the epidemiology on heat deaths actually turns on, because a body that does not get a break does not recover. The forecast number to watch during a ridge is not the afternoon high. It is the overnight low, and whether the breeze arrives at all.

What we are not saying

Attribution figures are for a defined event in a defined region, and they do not transfer to the next hot week somewhere else. Whether blocking itself is becoming more frequent or more persistent in a warming climate is an open research question with a genuine disagreement in the literature — we are not settling it here, and the 2021 numbers do not settle it either.