The World's Thermometer

What a heatwave is

A heatwave is not an absolute temperature; it is a run of days that sit well above what a place normally endures for the time of year. This series detects one wherever the daily maximum clears the local threshold for at least three days in a row.

• Local threshold: the day-of-year 90th percentile of daily maximum temperature, measured over a 15-day window against the 1991 to 2020 baseline.
• Spell: at least three consecutive days above that threshold. Duration matters as much as the peak.
• Severity: the degree-days a spell accumulates, how far over the line it runs, added up across its days.

Why 35 degrees means different things

The same reading lands differently in different places. Thirty-five degrees is an ordinary afternoon in Delhi and a national emergency in London, because each place has adapted to its own climate. Ranking heat against the local normal, not a single global bar, is what makes a heatwave comparable across the map.

An anomaly is that gap: how far today's maximum sits above the local threshold. A city glowing on this map is having an unusual day for itself, wherever it is in the world.

Dry heat and humid heat

Air temperature is only half the danger. When humidity is high the body cannot cool by sweating, and the wet-bulb temperature, which combines heat and moisture, is the measure that matters.

• Wet-bulb: sustained values near 35 degrees are the theoretical survivability limit for a healthy person, even at rest in the shade.
• The silent disaster: heat breaks nothing and photographs poorly, so its toll surfaces in mortality data days later, not on the street.