Click your city, then click another to compare. We translate: "to feel what they're feeling today, your city would need to hit X°C". The translation factors in air conditioning, heat-retaining buildings, acclimatisation, and infrastructure — so a 25°C London day can equate to a 43°C New York day. The feels-like number is the real NOAA Heat Index; the endurance model is a transparent heuristic, never folded into the physics.
Feels-like (NOAA Heat Index) — the physics number: The real apparent temperature on human skin. When it's warm we use the NOAA Heat Index (Rothfusz regression on Steadman's biometeorological model) from live temperature and relative humidity. Otherwise the apparent temperature is essentially the air temperature. Same equations weather services use to issue heat advisories — no invented numbers.
Endurance — a transparent heuristic, not science: The Heat Index plus a small per-factor penalty for how a city actually handles heat. We start from the physics number and add: +2°C per grade of AC scarcity (indoor exposure dominates total exposure time — without AC, indoor temps track outdoor and exceed it overnight via building thermal mass; Lancet Countdown 2023 shows ~3–4× heat mortality in low-AC populations at equivalent outdoor temperatures); +1.5°C per grade of unacclimatisation (short-term acclimatisation studies show populations adapt over 7–14 days to tolerate ~3–5°C higher operative temperatures); +1.5°C per grade of heat-retaining building stock (heavy masonry retains overnight heat 2–4°C above lightweight construction in heatwaves — LSHTM, UCL Bartlett); +1.5°C per grade of cooler-norm infrastructure (no shaded streets, transit not built for heat, no cooling centres). Each factor is scored 0–3 from published data; the maximum possible total uplift is ~19.5°C. This is a model we constructed — defensible per-component but not a published formula. It's the number that argues "London 25°C is more insufferable than Dallas 35°C", and is shown alongside the Heat Index, never replacing it.
Why two numbers, not one: The Heat Index is physics — what a thermometer-on-skin reads. Endurance is about lived experience and depends on infrastructure and bodies, not air. Folding them into a single number would hide the trade-off you actually care about. The endurance factor scores (AC, mass, acclim, infra) are approximate and drawn from published estimates (IEA, EIA, national statistics, public health research).
Data: Live temperature and humidity from OpenWeatherMap, refreshed daily at 06:00 UTC via a Cloudflare Worker cron. If live data is unavailable, the site falls back to a seasonal snapshot and the badge shows SNAPSHOT.