The dangerous number in a heat wave is not the one on the afternoon forecast. It is the one at four in the morning.

Phoenix is under an Extreme Heat Warning through July 9, with highs of 110 to 117°F. [1] But the figure that matters medically is the overnight low, which is running in the 80s and approaching 90°F. [2] A body sheds the day's heat load at night. When the low never drops far enough, it doesn't fully recover, and the deficit compounds across consecutive nights until the cardiovascular system is carrying heat it cannot offload. That is the mechanism behind most heat deaths — not a single scorching afternoon, but a string of nights that never cool.

The paper's July 7 account of Phoenix overnight lows held that the failure unit is the overnight low that prevents bodies from cooling, not the daytime high. Today that frame carries a geography. This is the second dome of the July heat. The first sat over the eastern United States and Canada from June 28 through July 5; the western and central dome built from July 6. [3] The eastern dome is still tallying its dead: New Jersey now attributes at least 29 deaths to it, many of them found indoors in homes without air conditioning, and Atlantic City hit 106°F on July 4, breaking a record set in 1966. [4] Those indoor deaths are the clearest statement of what heat actually is — a care problem that kills inside, at night, where the forecast high never reaches.

The divergence is between a temperature and a system. On X, the reaction splits between climate-attribution alarm and seasonal-normalcy dismissal — it's the desert, it's always hot. Mainstream coverage maps the dome and reports the warning population, leading with highs and the size of the affected area. [3] Both are organized around the daytime number. The paper's gap is that heat is measured by the overnight low, the indoor death, and the exposure that comes when power or a medical device fails — a care problem, not a headline temperature.

The reader tasks follow from that. Find the nearest cooling center and its hours before the hottest part of the day, not during it. Treat consecutive warm nights as the real threat and check on neighbors who are older, live alone, or lack working air conditioning — the population that dies indoors. And know the exposure that turns a heat wave into an emergency: anyone dependent on refrigerated medication or a powered medical device is one outage away from crisis, and the plan for that has to exist before the grid strains.

The July 6 edition put named counts on the eastern dome's care burden — outages in the hundreds of thousands, roughly three million home-medical-equipment users exposed, an ER heat-visit curve the CDC called "extremely high." Those totals belong to the eastern event; fresh named counts for the western dome had not been published as this went to press, and the paper flags them as a carry rather than reprinting eastern figures as if they were Phoenix's. What holds across both is the unit. The heat is not the high. It is the night that won't let go.

-- DARA OSEI, London