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 — the US National Weather Service Rothfusz regression.
• Wet-bulb temperature — how much evaporative cooling is even possible (Stull 2011 approximation).
• 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 weeksof 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. Whether a fan helps depends on both temperature and humidity: in humid heat, moving air keeps cooling you well past skin temperature because it drives evaporation from wet skin — but in hot, dryair (very roughly above 35°C / 95°F, and higher when it's humid) a fan alone can stop cooling and even speed dehydration. So we tell you when to wet your skin so moving air still helps, take cool showers, and seek a cooler space, rather than rely on a fan alone.

Nights matter too.Adapting well depends on shedding the day's heat overnight and sleeping — and humidity is what quietly prevents that: a high dew point keeps nights warm and sticky, so the air never really cools and sweat can't evaporate at rest. When tonight won't cool off we flag it and nudge you toward a cooler sleeping space; and if you sleep badly in the heat, the next morning's check-in eases your plan automatically.

A heat session is outdoor exertion, and exercise makes you breathe harder and stay in the sun longer — so two other outdoor hazards bracket it, and we surface both alongside the heat. Neither changes the heat plan; they inform it.

Air quality. We read the live US EPA Air Quality Indexfor where you are. Because harder breathing means inhaling more pollution, the advice tightens a step earlier than passive "just being outside" guidance: once air is unhealthy for sensitive groups (AQI 101+) we suggest keeping the session shorter or moving it indoors — a warm room still gives the heat stimulus — and higher still we say skip outdoor exertion.

UV. We show today's peak WHO Global Solar UV Indexwith the sun-protection level that goes with it. There's a happy overlap here: the cooler morning and evening windows the plan already steers you toward are also the low-UV hours, so timing your exposure for the heat tends to reduce your sun load at the same time. Air-quality and UV data come from Open-Meteo, fetched in your browser like the weather — nothing extra is sent anywhere.

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.

For the curious, here is the exact pipeline that turns today's weather and your profile into a number of minutes. Every value below is computed in your browser from fixed formulas and constants — no server, no live AI.

1. Read the real heat (three numbers)

From the hourly air temperature and humidity we compute a heat index (feels-like), a wet-bulb temperature (how much evaporative cooling is possible), and a WBGT heat-stress estimate:

2. Your baseline and the gap

Your baseline is a decay-weighted average of your home city's daily-peak heat index over the last ~21 days (a ~2-week half-life, mirroring how acclimatization fades). The whole plan is driven by one number:

3. The base dose (minutes)

Each goal has a full daily dose and a ramp length:

• Adapt to a hotter place — 75 min, ramped over ~8 days.
• Restore my sweat response — 60 min, ramped over ~14 days.
• Salvage a hot trip — 35 min, ramped over ~4 days.

A bigger gap starts you lower and climbs more slowly. A "gap penalty" slides from 0 to 1 as the gap grows from 2°C to 18°C; it sets the starting dose (60% of the full dose at a tiny gap, down to 30% at a big one), stretches the ramp by up to 40%, and makes the early climb more gradual:

4. Yesterday's check-in nudges it

• Warning signs (or dizziness + nausea together) → a rest day, and we ease the ramp back.
• Session not completed → hold at the same target.
• A "strain" tally of low mood, poor sleep, headache, high thirst, hard effort or dizziness — 2 or more → reduce today by about 40%.
• A clean, good day → advance a little; anything in between → hold.

5. Safety caps — the overlay always wins

These gates sit above the whole calculation and can only ever lower the dose, never raise it:

• Caution (wet-bulb ≥ 25°C, or feels-like ≥ 32°C / 90°F, or WBGT ≥ 28°C) → capped to ~20–25 min of light activity only.
• Hard stop (wet-bulb ≥ 28°C, or feels-like ≥ 39°C / 103°F, or WBGT ≥ 31°C) → no exposure; shelter and cool.
• Your health screening tightens every threshold (by 2–3°C) and caps the minutes (elevated risk ×0.7; higher risk ≤ 20 min and passive only).

The thresholds are deliberately conservative: the critical wet-bulb limit for young, healthy adults is already ~30–31°C, so ours sit below it — and exposure is always timed to the coolest safe window.

Worked example.You arrive somewhere that peaks at feels-like ~38°C from a home baseline of ~24°C → a 14°C gap. For "Adapt to a hotter place" (full 75 min), that gap sets the starting fraction near 38%, so the base target is about 28 minutes. Because feels-like 38°C is in the caution band, the overlay trims it to 25 minutes of light activity in the coolest window — and if the wet-bulb reads 28°C it becomes a rest-and-cool day instead, whatever your goal.

• No AI or machine-learning model runs in the app — the recommendations were built with AI during development, then fixed as transparent, auditable rules.
• No accounts, no personal tracking — everything stays in your browser (see Privacy).
• No medical claims — this is wellbeing guidance, not a medical device or medical advice.