Guides
Explanations rather than forecasts. Each guide answers one question completely — what the number means, where the published threshold actually sits, and what to do with the answer.
Two of these pages are about heat illness itself, and deliberately only two. The symptom-by-symptom
literature is written by medical bodies and there is no reason for this site to compete with it. What is
missing there, and what these pages are for, is the numbers: the thresholds, where they come from, and what
they assume about the person crossing them.
The numbers themselves
What each quantity is, how it is worked out, and where it stops being valid.
- Heat index vs feels like Why two apps show different "feels like" numbers for the same weather, and which of the four quantities answers which question.
- The heat index equation The regression in full, both official corrections, the fallback below 80°F, and the twelve chart cells where the printed table disagrees with the equation.
- Reading the chart How the grid works, where the band lines fall, and the twelve cells where the printed table and the official equation genuinely disagree.
- Wet bulb temperature The measurement, the 35°C survivability limit, why experiments suggest the real line is nearer 31°C, and where on Earth it comes closest.
- WBGT explained The three thermometers and their weights, why the sun is inside the number, and why every app-derived WBGT — including ours — is an estimate.
- The three thermometers What each of the three thermometers measures, how they relate, and where the HVAC version of this question diverges from the heat-safety one.
Rules, work and acclimatisation
What employers are actually required to do, and the published tables behind the guidance.
- OSHA and heat What is enforceable now, what the proposed federal rule would add, and why Japan already has the number the US is still debating.
- Work/rest schedules How to read the published tables, classify workload honestly, and design a break that actually lowers core temperature.
- Heat acclimatization What changes in the body and in what order, the day counts for work and for sport, and how fast the whole adaptation disappears.
- Indoor heat rules Whether a legal maximum exists, what the indoor rules require instead, and a plain statement that outdoor weather data cannot answer this.
- App or WBGT meter An honest comparison, written by someone who makes the app — including the cases where the answer is buy the meter.
Weather and heat illness
Alerts, heat domes, and the two pages on heat illness itself.
- Advisory vs warning What each alert level means, what triggers it, and why the same heat index gets an advisory in one city and nothing in another.
- Heat domes The mechanism in three steps, and why the overnight low matters more than the afternoon peak during one.
- Exhaustion vs stroke The single distinction that changes what you do — whether the body has stopped regulating — and the thresholds each becomes likely at.
- Stages of heat illness The progression from cramps to stroke, with the decision point marked — the thing symptom lists never give you.