The same afternoon temperature can produce a heat warning in one American city and no alert at all in another. The thresholds are deliberately local.
Heat harm is relative to what a population is used to
Illness and death from heat rise most sharply when conditions depart from what a place normally experiences, not when they cross a single national number.
A northern city where summers are mild has fewer air conditioners, less shaded infrastructure and residents with less physiological acclimatization to prolonged heat.
A desert city with the same reading has housing, habits and cooling capacity built around routinely hot summers, so the identical temperature carries lower risk.
Thresholds are derived from local distributions
Weather Forecast Offices set criteria against the climatology of their own service area, typically anchored to how unusual a value is for that location and time of year.
That is why criteria differ between neighboring offices and shift through the season. Early-summer heat draws warnings at lower readings than the same heat in August.
The approach treats the alert as a statement about danger rather than a statement about temperature, which is the only version that helps a local audience.
Humidity and night-time relief enter the calculation
Heat stress depends on whether the body can shed heat by evaporating sweat, which humid air slows considerably. Dry heat and humid heat are not interchangeable.
The heat index combines temperature and humidity into a single apparent value, which is why humid regions can reach warning criteria at lower actual temperatures.
Overnight minimums matter as much as afternoon peaks. A night that fails to cool denies the body recovery time and drives multi-day heat events toward danger.
Watches, advisories and warnings mark escalating confidence
A watch signals that conditions favorable for dangerous heat are possible in the coming days, giving agencies and employers time to prepare.
An advisory covers heat that is hazardous but below warning criteria, while a warning indicates conditions expected to be dangerous to the general population.
The tiering exists so that messaging can escalate without every hot spell sounding identical, which is what erodes public response over a long summer.
Cities are increasingly treated as separate microclimates
Dense construction, dark surfaces and low tree cover keep built-up neighborhoods warmer than surrounding countryside, particularly after sunset.
That contrast means one metropolitan area can contain conditions that differ by a meaningful margin between neighborhoods on the same afternoon.
Alerting still operates on county-scale geography in most cases, so local heat response plans increasingly add neighborhood detail that a regional warning cannot carry.