Heat health warnings are based on more than the maximum temperature.
Humidity
Reducing the effectiveness of sweating.
Which is why heat index measures combine the two.
Overnight temperatures
Bodies recovering during cool nights.
Which is why consecutive hot nights are dangerous.
Duration
Multi-day events producing cumulative strain.
Vulnerability
Older people, infants and those with conditions at higher risk.
Which warning systems are designed around.
Why night-time temperature matters most
The body recovers from heat stress when conditions cool, and if they do not, strain accumulates across days.
Which is why heatwaves with warm nights produce far higher mortality than hot days followed by cool nights.
Warning systems in several countries now weight overnight minima heavily for this reason, and the research behind it is well established.
Heat index and equivalents
Combining temperature and humidity into a felt-temperature measure.
Which is what warnings are usually based on.
Urban heat islands
Built areas retaining heat overnight.
Which raises risk in cities specifically.
Practical measures
Hydration, shade, cooling and checking on vulnerable people.
A general note
Health services publish heat guidance during warning periods.
Why heat is underestimated
It kills more people in many countries than any other weather hazard, and it does so quietly, in homes, over days.
Which means it produces no dramatic images and receives a fraction of the attention that storms do.
Excess mortality studies consistently find heatwave death tolls far higher than contemporary reporting suggested, because the deaths are recorded as cardiac or respiratory rather than as heat.
Who is most at risk
Older people living alone, those with chronic conditions, outdoor workers and infants.
Which is who warning systems are designed to reach.
Cooling centres
Public spaces opened during warnings.
Which local authorities publicise.
Medication interactions
Some medicines affecting heat regulation.
Which pharmacists advise on.
Checking on people
The single most effective community measure.
Why understanding the system matters
Weather warnings are the output of a large, expensive, internationally coordinated system that most people interact with only through an app icon. It is one of the clearest examples anywhere of publicly funded science producing something everyone uses daily without noticing.
The practical value of understanding it is in reading warnings correctly. A cone is not a map of the storm, a watch is not a warning, a hundred-year flood is not once a century, and a radar app is not a warning service. Each of those misunderstandings has cost lives, and each is corrected in a sentence.
Where the authoritative information is
National meteorological services issue the warnings, publish the forecasts, explain the products and archive the data, all freely. Local emergency management agencies publish evacuation zones, shelter locations and preparedness guidance specific to the hazards where you live.
Those two sources answer almost every question anyone has about weather risk, and both are considerably more reliable than aggregated coverage, which frequently reproduces the misunderstandings described above.
A general note
Warning terminology, alert thresholds and emergency procedures differ between countries and between agencies. Anything with safety consequences should be checked against the service responsible for the area you are actually in.
How the warning system is actually organised
National meteorological services observe, model and forecast; they issue watches, warnings and advisories against defined criteria; and local emergency management agencies decide what action to take in response, including evacuations, shelter openings and road closures.
Those are two different organisations with two different jobs, which is why a national forecast office issues a hurricane warning and a county decides who evacuates. Knowing which one to follow for which question saves a great deal of confusion during an event.
What is worth doing before a season starts
Find out which hazards actually affect where you live, check whether your address sits in a flood or surge zone, identify where you would shelter, confirm that emergency alerts are enabled on your phone, and know how you would get information if the power and the network both failed.
None of that takes long, all of it is free, and every element of it becomes considerably harder to arrange once a warning has been issued. Emergency managers say the same thing after every major event: the households that coped were the ones who had decided in advance.
Where the authoritative information is
National meteorological services issue the warnings, publish the forecasts, explain what each product means and archive the underlying data, all freely. Local emergency management agencies publish evacuation zones, shelter locations and preparedness guidance specific to the hazards where you actually live.
Between them those two sources answer almost every question anyone has about weather risk, and both are considerably more reliable than aggregated coverage, which routinely reproduces the misunderstandings that cause harm.
A general note
Warning terminology, alert thresholds and emergency procedures differ between countries and between agencies, and they are revised as understanding improves. Anything with safety consequences should be checked against the service responsible for the area you are in rather than against a general description.