A Central Valley heat wave is measured by its nights, not its afternoons

Sacramento reaching 105°F is ordinary. Sacramento failing to drop below 75°F overnight is the dangerous version, and it means the Delta breeze did not arrive.

The Central Valley is habitable in August because of a gap. When the valley heats up, the pressure difference between it and a 55°F ocean draws marine air through the Golden Gate, up Carquinez Strait and out into the Delta — and a 105°F Sacramento afternoon becomes a 60°F night.

That nightly recovery is what bodies and buildings rely on. Overnight cooling is when heat stress resets, when houses without air conditioning become survivable, and when the grid gets its break.

The failure mode

In the heat events that actually cause harm, the breeze does not come. Typically a ridge has built far enough west to shut the gradient down, so the marine air stops moving inland and the valley holds its own heat through the night. The afternoon maximum may be no higher than a routine hot day. The minimum is what changes, and it is the minimum that accumulates.

This is why heat advisories increasingly quote overnight lows, and why comparing two heat waves by their peak temperature alone will rank them in the wrong order.

Where to watch it

The Delta is the throat of the system — the only sea-level break in the Coast Ranges after the Golden Gate itself. Antioch sits where the breeze leaves the strait and the valley heat begins, which makes it the place the mechanism is easiest to see.