Real weather, if you want it
Everything below works from the two sliders alone. Fetching hourly data adds three things the sliders cannot know: which hour of today is actually the worst, how many nights in a row have given no relief, and whether this heat is new to you.
Where this air puts you
Drag the point, or use the fields. Every line is computed from the same physics as the numbers beside it.
Who is in this air
The published limits describe young, healthy, acclimatised people at rest — which is not who dies in heat waves. Each of these moves the threshold by a stated number of degrees.
What to do, in order
Ranked by what each one is worth in these conditions. The first line is the one that matters most.
The day, hour by hour
Two questions on one time axis: when this air is at its worst, and when opening a window stops importing heat. Without a fetched forecast both curves are a model built from a high and a low, drawn dashed for that reason.
Where the heat lives
Not today's weather — the shape of the problem. Each dot is the wet-bulb temperature a place reaches in a normal hot season, from three years of reanalysis. Drag to turn, scroll to zoom. Look up a place above and the globe goes there.
What actually cools a room
Most heat advice is a flat list in which "close the curtains" sits beside "shade the window from outside", as if they were comparable. They differ by about five times.
What this cannot do
not a doctor
Nothing here is medical advice, and the thresholds are calibrated to published physiology, not to you. If someone is confused, agitated, or has stopped sweating in heat, that is an emergency — call, do not calculate.
not a forecast
It does not know what the weather is unless you ask it to look. Official heat warnings know things this does not, including how long the heat has already lasted.
two limits, not one
35 °C wet bulb is the theoretical limit. Around 31 °C is where strain has actually been measured on young healthy subjects. Quoting only the 35 makes the danger look four degrees further away than it is.
shade, and standing still
Every number here assumes shade and rest. Direct sun adds the equivalent of several degrees; physical work can multiply your heat production tenfold.