A temperature that triggers an emergency warning in one city passes without comment in another. The thresholds are set locally and deliberately, and the reasoning is based on health outcomes rather than on the number itself.
Thresholds are derived from local health data
Agencies examine the relationship between daily temperature and excess deaths or hospital admissions in a specific population over many years.
That relationship produces a curve with an inflection point, above which harm rises sharply, and the alert threshold is placed near that point.
Because the curve differs between populations, the resulting threshold differs too, even where the underlying method is identical.
Acclimatisation shifts the whole curve
People living in consistently hot regions adapt physiologically and behaviourally, through changes in sweating response, activity timing and housing design.
The same absolute temperature therefore imposes less strain in a place accustomed to it than in one where it is unusual.
This is why a heat emergency in a temperate city can be declared at a temperature that is an ordinary summer day elsewhere.
Humidity and night-time temperature matter more than the peak
The body sheds heat primarily by evaporating sweat, and high humidity reduces how effectively that works regardless of the air temperature.
Composite measures combining temperature and humidity are therefore used in place of a plain thermometer reading in many alert systems.
Overnight minimum temperature is often the stronger predictor of harm, because a night that does not cool prevents the recovery that makes the next day survivable.
Duration is built into the criteria
A single hot day produces less harm than the same temperature sustained across several days, as heat accumulates in buildings and in people.
Alert criteria commonly require consecutive days above the threshold, or a forecast of them, rather than triggering on one reading.
This is also why warnings are sometimes issued at the start of a spell and escalated later, as the accumulated exposure grows.
The alert is a trigger for action
Thresholds are set at the level where a defined response is worth activating, including cooling centres, checks on vulnerable residents and adjusted working rules.
Setting the threshold too low causes alerts to be ignored, while setting it too high delays the response until harm has already occurred.
Criteria and response arrangements vary by jurisdiction and are revised as local climate records and health data are updated.