How it works

Your body cools by evaporating sweat. High humidity blocks that, so the same air temperature can be harmless or dangerous depending on the moisture in the air. So we never drive anything from dry temperature alone. Instead we compute:

• Heat index(the "feels-like") from temperature + humidity.
• Wet-bulb temperature — how much evaporative cooling is even possible.
• A WBGT estimate — a combined heat-stress measure used in sport and the military.

Heat tolerance is short-term: you build it in about 1–2 weeks of heat exposure and lose it over 2–4 weeks without. So "what your body is used to" is best captured by the heat you've actually lived through recently — not a yearly average (which ignores the season) and not a single day (too noisy).

We estimate it from your home city's last ~3 weeks of real weather, taking each day's peak and weighting recent days more heavily (a ~2-week half-life) — exactly how the adaptation itself fades.

The personalization comes from one number: the gapbetween how hot it gets where you are now (the day's peak) and that home baseline. A big gap means start gentler and ramp slower; a small gap needs little.

The adaptation stimulus is roughly 60–90 minutes of raised core temperature a day, from heat exposure with light activity. We start you at a gap-scaled fraction of that and build up over about two weeks, with most of the gains in the first week. We also pick the safest time-of-day windowso you exercise when it's coolest, and we adjust tomorrow based on how today felt — good days advance, rough days pull back, and a warning sign means a rest day.

If you stop, adaptation decays — noticeable within about a week — so the plan accounts for missed days too.

Adaptation is driven by the daily doseof heat — the timed session — not by baking all day. So once you've done it, the rest of the day is for recovering and avoiding extra heat strain: cool down, hydrate, and sleep well (a lot of the adaptation consolidates with rest). Resting somewhere cool — even air-conditioned — won't undo your progress.

So every plan includes climate-specific guidance for the rest of the day, with and without air conditioning. One detail that matters: once the air is hotter than your skin (roughly 35°C / 95°F), an electric fan stops cooling you and can speed dehydration — so in extreme heat we tell you to wet your skin so moving air still helps, take cool showers, and seek a cooler space, rather than rely on a fan alone.

A safety layer sits above everything and can never be bypassed by your goal or progress:

• A hard stop in extreme conditions (around wet-bulb 28°C, heat index 39°C / 103°F, or WBGT 31°C) — shelter and cooling only, no exposure.
• Health screening at sign-up (heart/kidney conditions, pregnancy, age extremes, and medications like diuretics, beta-blockers or anticholinergics) that caps or withholds plans — especially for risky combinations.
• Plain heat-exhaustion vs heat-strokerecognition and a clear "stop and seek help now" list on every plan.

• No machine-learning black box — the rules are transparent and auditable.
• No accounts, no tracking — everything stays in your browser (see Privacy).
• No medical claims — this is wellness guidance, not medical advice.