How heatwaves are measured

A heatwave is local. The global trend explains why the local line keeps moving.

The question this tool answers

Heatwaves are deceptive because a single number on a weather app does not say whether a place is unusually hot for that week, whether the heat is lasting long enough to stress people, or whether the local baseline has shifted. This dashboard asks three things at once: how hot the planet has become, whether monitored US cities are in active heatwaves now, and how each city's summer heat threshold is changing.

Why Phoenix and Boston are not judged by the same number

A heatwave in Phoenix would be ordinary summer weather in Death Valley. A heatwave in Boston can be a cool day by Phoenix standards. That is why this dashboard uses a local-relative definition: a city is flagged when daily maximum temperature stays at or above its own weekly 95th-percentile high for at least three days. The threshold is built from the station's 1991-2020 climate for that week of the year.

This means the line moves with season and place. A July threshold in Miami is different from a May threshold in Chicago. The point is not to say where the absolute hottest air is; it is to say when a community is experiencing heat unusual enough, and persistent enough, to become a health and infrastructure risk.

How we know

The global stripe comes from NOAA Climate at a Glance, which publishes global land-and-ocean temperature departures from the twentieth-century average. It is the background: the planet's heat baseline, year by year, since 1880.

The local panels use NOAA NCEI GHCN-Daily station observations for a fixed set of fifteen US cities. For each station, we download daily maximum temperature, build the weekly 95th-percentile threshold from 1991-2020, and look for runs of at least three days above that line. The station stripes average June-August highs by year and show how summer has changed since the early 1990s.

The choices we made

We lead with the global trend even when no city is in heatwave. Off-season does not mean the risk disappeared; it means the local weather has not crossed the heatwave rule today. The global stripe is the reason the same local threshold is easier to cross over time.

We show only fifteen US stations because daily current-month station access is more reliable than trying to build a global live map from mixed national feeds. The dashboard is a monitor and explainer, not a complete global heat-alert system. If you need operational warnings, use your national weather service.

What heat does to a body

The body survives heat by moving heat outward and evaporating sweat. Humidity blocks that evaporation. Warm nights remove recovery time. Age, illness, pregnancy, outdoor work, medications, housing, and access to cooling all change risk. That is why the dashboard includes body-threshold context alongside dry-bulb station temperatures.

Wet-bulb temperature combines heat and humidity into a measure of evaporative cooling potential. Around 32°C wet bulb, sustained work becomes dangerous without shade, water, rest, and cooling. Around 35°C wet bulb is often cited as a theoretical survival limit for even healthy adults in shade with unlimited water. Real-world harm can happen well below those thresholds, especially for vulnerable people.

What this tool cannot tell you

It does not calculate heat index or wet-bulb temperature for the stations. It does not know indoor conditions, urban shade, power outages, or whether someone is working outside. It does not cover every US city or any international station in live mode. And the nearest-city panel may be far from you, because it chooses among a fixed monitoring set.

The value is consistency: the same station, the same local percentile rule, the same global context, updated on a daily pipeline.

Credits

This dashboard depends on NOAA station observers, NCEI data systems, Climate at a Glance, and public-health researchers who turned “hot day” into measurable human risk. It is not a warning product. During extreme heat, follow official heat alerts and local public-health guidance.

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