Frequently Asked Questions
Questions readers and friends have asked since launch. Answers in plain language, with links to deeper material on the Science and Tips pages.
Why does HeatSafe show 28°C when the outdoor temperature is 38°C?
They measure two different things. 38°C is the air. 28°C is the cooling limit your body has against that air.
The 38°C is dry-bulb temperature — what a thermometer in the shade reads. Standard, familiar, what every weather app shows.
The 28°C is wet-bulb temperature — the lowest temperature your skin can reach through sweat evaporation, given the current heat and humidity. It is always lower than the air temperature, because evaporation cools.
The gap between the two depends on humidity. In drier air, sweat evaporates fast and wet-bulb sits well below the air temperature — the body cools well. In humid air, sweat barely evaporates and wet-bulb is close to the air temperature — the body cools poorly.
The same 38°C outdoor reading produces very different effects on the body. At 30% humidity, wet-bulb is around 25°C — Manageable. At 50% humidity, around 29°C — Danger. At 70% humidity, around 33°C — Critical.
The smaller number is not wrong. It is the more important one.
Why one more tool when weather apps already exist?
Weather apps tell you the temperature. HeatSafe tells you whether your body can handle it.
The temperature shown in any weather app is one part of the picture. Our bodies' ability to cool depends on temperature and humidity together. A 32°C day at 70% humidity is more dangerous than a 42°C day at 20% humidity — but every weather app shows them as if 42°C is "worse."
HeatSafe combines the two into a single number — the wet bulb temperature, displayed as the Heat Safety Index — that tells you whether the body's cooling mechanism (sweat evaporation) can keep up. It is not a replacement for weather apps. It is the missing layer that turns weather data into safety information.
When is it actually safe to step out?
"Manageable" is the answer. There is no fully unconditional state in summer.
HeatSafe's lowest zone is "Manageable," not "Safe." This is deliberate. In Indian summer, even on the lowest reading, our bodies are working harder than in cooler months — sweating more, losing more water, regulating temperature actively. So normal hydration always matters.
"Manageable" means the body's cooling mechanism is working efficiently. You don't need to take special precautions beyond normal summer awareness — drink water, take breaks if you're outside for hours, listen to your body.
Labelling this zone "Safe" would mislead readers on hot dry days. A 42°C dry-air reading shows as Manageable on HeatSafe (because sweat evaporates well in dry air), but calling that "Safe" feels wrong. "Manageable" is honest: our bodies can handle it, with normal care.
How long is "too long" to be outside?
Heat injury is dose-dependent. The same reading may be fine for 10 minutes and dangerous for 2 hours.
HeatSafe's zones describe the conditions, not your cumulative exposure. The same Caution reading carries different actual risk for someone walking 15 minutes to a shop versus someone playing cricket for 2 hours.
General principles: shorter exposures are safer than longer ones. Shade is safer than sun. Hydration extends safe time. Heavy exertion shortens it. Vulnerable groups (elderly, children, ill) reach the limit sooner than healthy adults.
HeatSafe shows the baseline. You add your context — what you're doing, how long, in what conditions, who you are. Specific time guidance for each zone, anchored in heat-stress research, is being added to the Tips page in a future update.
Why doesn't sweating itself cool me?
Sweating is necessary for cooling, but not sufficient. Evaporation of sweat using body heat is what cools.
Our bodies produce sweat in anticipation of evaporation. The actual cooling happens when sweat evaporates from the skin — converting liquid sweat to vapour draws heat energy from the body, which leaves with the vapour. If sweat doesn't evaporate (because the air is saturated, or because you wipe it off), no cooling happens. The sweat just drips off.
This is also why wiping sweat off with a towel feels right but actually hurts cooling — you're removing the water that was about to do the cooling work.
More on this in the Science section.
What does "humidity 70%" actually mean?
It's a ratio, not an amount. 70% humidity means the air holds 70% of the maximum water vapour it can hold at the current temperature — not that the air is 70% water vapour.
The maximum changes sharply with heat:
| Outdoor temp | Water vapour % in air | |
|---|---|---|
| Humidity 100% | Humidity 70% | |
| 5°C | ~0.9% | ~0.6% |
| 30°C | ~4% | ~3% |
| 45°C | ~9.5% | ~6.5% |
The remaining 96–99% of the air is the usual mix of nitrogen and oxygen, regardless of temperature.
So "70% humidity" on a cool morning is barely any water vapour in the air. "70% humidity" on a hot afternoon is more than ten times that amount, enough to seriously block sweat evaporation. The same percentage means very different things to the body.
Why is the word "humidity" easy to associate with cooling, when it actually makes heat dangerous?
The word feels cool. The effect on the body is the opposite.
In everyday language, "humidity" and its synonyms — moisture, dampness, ஈரப்பதம் (Tamil), wetness — carry cool, refreshing associations. Moist soil feels cool. A damp cloth feels cooling. Rain brings relief.
But for our bodies in already-humid air, the relationship inverts. The dangerous effect of humidity isn't the moisture itself — it's that humid air is already full of water vapour and cannot accept more. So sweat sitting on the skin has nowhere to evaporate to. The cooling mechanism stops working.
This is why a 32°C humid day can be more dangerous than a 42°C dry day. The dry day still allows evaporation; the humid day does not. The word "humidity" doesn't warn us of this — our intuition fights against it. HeatSafe makes the actual effect visible.
Is wet bulb temperature similar to dew point temperature?
Related, but different. Dew point is about condensation. Wet bulb is about cooling.
Dew point is the temperature to which the current temperature must fall for moisture to condense. Pilots and weather forecasters use it to predict fog, cloud formation, and dew.
Wet bulb temperature is the lowest temperature our bodies can reach through sweat evaporation. It tells us the cooling limit the body has against the air. It is what HeatSafe calculates and displays as the Heat Safety Index.
Both involve humidity, but they answer different questions. Dew point predicts when moisture will condense. Wet bulb predicts the body's cooling limit.
One useful rule: wet bulb is always between dry bulb (air temperature) and dew point. In dry air, the gap is wide (sweat cools well). In saturated air, all three converge — and that is when the body's cooling fails.
What does the Chennai Air Show example tell us?
Real-world conditions push the actual safety limit lower than the experimental baseline.
In October 2024, an Air Show on Chennai's Marina Beach drew 1.5 million attendees. By afternoon, 5 people had died from heat-related causes and around 200 needed hospital care. The wet bulb temperature that day was approximately 28°C — which falls in HeatSafe's "Caution" zone, not "Critical."
At first read, this can sound like the tool understated the danger. It didn't. The limits HeatSafe displays are based on Vecellio et al. (2022), who measured wet-bulb temperature limits that cause heat-stress in healthy young adults at rest in shade with adequate hydration. The Air Show conditions broke every one of those baseline assumptions: full sun for hours, prolonged standing, dense crowds, mixed-age population, varying fitness and hydration.
HeatSafe shows the baseline. Real-world conditions add stressors on top. A "Caution" reading under ideal conditions can carry "Critical" risk for someone in full sun, or someone elderly, or a child playing outdoors. The reading is the starting point — your conditions add to it.
More on this in the Science section.
Why do HeatSafe readings at times differ from my thermometer reading or other weather apps?
Real microclimate variation, not error. Both readings can be correct.
HeatSafe uses Open-Meteo, which aggregates data from official weather stations worldwide (including the IMD network in India). The reading you see is from the station nearest to your search location — not from your exact spot.
Three reasons HeatSafe may differ from another reading:
- Station distance: The nearest official weather station may be 5-30 km from where you are. A 1-2°C difference between the station and a personal thermometer is normal.
- Different stations within a metropolitan area: Coastal and inland parts of the same city can differ by 2-3°C. Coastal areas are typically cooler and more humid (sea breeze); inland is hotter and drier. When you search a city name, HeatSafe picks one station; other apps may pick a different one.
- Update interval: Weather data updates roughly hourly. Real conditions can shift within the hour, especially during weather transitions.
HeatSafe shows what the nearest official station reports. This is the same data foundation IMD and most public health tools rely on.
Can I get a live reading from my phone for my exact location?
Phones do not have outdoor temperature or humidity sensors.
It would be useful — but the hardware doesn't exist. Phone temperature sensors measure phone-internal heat (battery, processor), not the ambient outdoor air.
Apps that appear to give "live" weather readings are using the same approach HeatSafe uses: pulling data from the nearest official weather station via an API. Different apps may use different data providers (AccuWeather, OpenWeather, Weather Channel, Open-Meteo, IMD), which is why their numbers can differ slightly. None of them are reading "live" from your phone.
The only way to get truly live local readings is a personal weather station mounted outside your home or office.
Last updated: May 18, 2026