Glossary
Plain-language definitions of the technical terms used across HeatSafe. Each entry leads with a one-line answer; deeper explanation follows for readers who want depth.
Acclimatisation
The body's gradual adjustment to repeated heat exposure. This slightly raises the level of heat we can tolerate.
Over 10–14 days of consistent exposure, sweating starts earlier, becomes more dilute, and is more efficient at cooling. Heart rate and core temperature stabilise at lower levels for the same heat load. Acclimatisation does not raise the body's actual safety limit — it just shifts where strain begins. People moving from cool regions to hot ones, or stepping out for the first time after a cool monsoon, are unacclimatised and at higher risk.
See also: Safety limit · Wet bulb temperature
Body temperature
The body's stable internal temperature, normally ~ 37°C / 98.6°F.
This is more or less a constant with a narrow margin of ±0.5°C (±0.9°F). The body works hard to keep it there. When core body temperature rises even a few degrees above 37°C, organs begin failing. Heat stroke is, at its core, a body that has lost the ability to hold this number steady.
The Heat Safety Index is meaningful relative to body temperature: as the index approaches 37°C / 98.6°F, the gap between the heat the body can shed and its own internal heat narrows toward zero.
See also: Heat Safety Index · Science page → "Why this matters"
Compensable heat
Heat conditions in which our bodies can still cool themselves through sweating and evaporation.
As long as the surrounding wet bulb temperature stays below the body's safety limit, the cooling mechanism keeps up: sweat forms, evaporates, draws heat away from the body, and core temperature holds around 37°C. HeatSafe's Manageable and Caution zones fall in this range for healthy adults. The opposite condition — where cooling cannot keep up — is uncompensable heat.
See also: Uncompensable heat · Safety limit
Dew point
The temperature to which the surrounding air must cool for water vapour (humidity) to begin condensing as dew or rain.
It is a measure of how much moisture is in the air in absolute terms — a high dew point means the air is already loaded with vapour, so sweat evaporates poorly.
Dew point and wet bulb temperature are related but distinct. Dew point describes the air; wet bulb describes the lowest temperature the body can reach by sweating into that air. In completely saturated air, dry bulb, wet bulb, and dew point all converge — the theoretical point at which sweat evaporation fails entirely. In practice, evaporation is already severely impaired well before this, at the high humidities (above 70%) common in tropical and coastal heat.
See also: Wet bulb temperature · FAQ → "Is wet bulb temperature similar to dew point temperature?"
Dry bulb temperature
The technical name for what an ordinary thermometer reads — what HeatSafe labels as outdoor temperature. Also known as dry air temperature, air temperature, outside temperature.
The "bulb" refers to the mercury bulb of a thermometer — the rounded reservoir at the base — not an electric bulb. "Dry" because no wet wick is wrapped around it.
It is what every weather forecast reports, measured by a thermometer shielded from sunlight and wind. HeatSafe uses outdoor temperature in its interface because that is the term users recognise. Dry bulb appears here because anyone fact-checking against the source research will see that wording.
See also: Outdoor temperature · Wet bulb temperature
Evaporation
The process by which liquid water absorbs heat to become vapour, reducing that much heat from the source.
When sweat evaporates from skin, it pulls heat off the body — this is how we cool ourselves. The cooling does not come from sweat sitting on skin; it comes from sweat leaving the skin.
This distinction matters. Sweat that doesn't evaporate — because the air is already saturated, or because it gets wiped off — does not cool. The body has still spent water and energy producing it, but the cooling job goes undone. Anchoring this in something familiar: a pot of water cools below boiling only because steam carries heat away as it rises. Water sitting in the pot, however hot, does not cool it.
See also: Sweat / sweating · Science page → "Why wiping sweat doesn't help"
"Feels like" temperature
A single number combining temperature, humidity, wind, and sometimes sun, intended to express how hot it feels to a typical adult.
Different services calculate it differently — heat index, apparent temperature, RealFeel — and the underlying formulas are not standardised across providers.
"Feels like" describes a sensation. The Heat Safety Index describes a physical limit: the level of heat the human body is designed to still cool itself through. The two are not the same number and should not be compared directly.
See also: Heat Safety Index · FAQ → "Why do HeatSafe readings differ from other weather apps?"
Heat exhaustion
An early warning state in which the body is straining hard to cool itself but has not yet failed.
Symptoms include heavy sweating, weakness, nausea, headache, light-headedness. Core body temperature is elevated but still under control.
Heat exhaustion is the body asking you to stop. Moving to shade, drinking water, and resting reverses it. Ignored, it can progress to heat stroke — at which point the body has lost control of its own cooling.
See also: Heat stroke
Heat Safety Index (HSI)
A single number that tells you whether your body can still cool itself in current outdoor conditions.
The HSI combines outdoor temperature and humidity into one reading, then maps it onto a colour-coded zone — Manageable, Caution, Danger, or Critical — that signals risk at a glance.
Behind the scenes, the HSI is the wet bulb temperature: the lowest temperature skin can reach by sweating in the surrounding air. Most users do not need to know this. The colour and the verdict carry the message.
The HSI works alongside familiar indices like the UV Index and Air Quality Index — one number, an actionable zone, a basis for everyday decisions.
See also: Wet bulb temperature · Zones · Science page → "How HeatSafe works"
Heat stroke
A medical emergency in which the body has lost the ability to cool itself and core temperature is rising.
Sweating may stop. Skin may become hot and dry. Confusion, collapse, and loss of consciousness follow. Without rapid cooling and medical attention, heat stroke causes organ failure and death within minutes to hours.
Heat stroke is what HeatSafe is built to help you avoid — by signalling, in advance, when conditions are crossing the level that pushes the body toward this state.
See also: Heat exhaustion · Uncompensable heat
Humidity
The amount of water vapour in the air, expressed as a percentage of the maximum the air can hold at the current temperature.
What weather reports call "humidity" is technically relative humidity — a ratio, not an amount. 70% humidity does not mean the air is 70% water vapour; it means the air holds 70% of the maximum it could carry at the current temperature. That maximum changes sharply with heat:
| Outdoor temperature | 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, oxygen, and trace gases. That mix doesn't change. On a hot day, "70% humidity" represents over ten times the water vapour of "70% humidity" on a cool morning.
The word humidity is also emotionally miscoded. Across English, Tamil, and most languages, "moisture / ஈரப்பதம் / dampness / wetness" carries cool, refreshing, relief associations — moist soil, damp cloth, post-rain coolness, monsoon air. The word feels cooling. The effect on the body is the opposite: high humidity is precisely what shuts down sweat evaporation, which is the body's only real cooling mechanism in heat.
Two layers of misconception — what the word means and what the word feels — are part of why humidity numbers in weather reports are so easily ignored.
See also: Science page → "Humidity is a ratio, not an amount" · FAQ → "What does humidity 70% actually mean?"
Microclimate
A small geographic area whose weather differs measurably from the surrounding region.
A coastal neighbourhood is more humid than an inland one a few kilometres away. A street with dense tree cover is several degrees cooler than the open road next to it. A concrete plaza in afternoon sun is hotter than the park across the street.
HeatSafe pulls weather data tied to a city or station. Your specific block, courtyard, or rooftop may differ. Take the reading as a baseline for the area, not a direct measurement of where you are standing.
See also: FAQ → "Why does HeatSafe sometimes differ from my own thermometer?"
Outdoor temperature
The temperature of the air around you, measured in shade, away from direct sunlight.
This is what every weather forecast reports and what an ordinary thermometer reads. It tells you what the air is doing. It does not, on its own, tell you whether the body can cope with that air — that is what humidity and the Heat Safety Index add.
Technically known as the dry bulb temperature, dry air temperature, or outside temperature.
See also: Dry bulb temperature · Heat Safety Index
Safety limit
The point beyond which the body can no longer cool itself, regardless of fitness, hydration, or willpower.
When wet bulb temperature reaches this point, sweat stops evaporating effectively, core body temperature begins climbing, and heat stroke becomes a matter of time.
Penn State experiments measured the safety limit at around 30.6°C wet bulb for young, healthy adults at rest in shade with full hydration. Real-world conditions — older age, exertion, illness, sun exposure — push the limit lower. The limits HeatSafe displays are set conservatively because a safety tool should warn early, not late.
In research literature, this is called the uncompensable heat stress threshold. HeatSafe uses safety limit in plain language because threshold is technical jargon. Both refer to the same physical reality.
See also: Threshold · Uncompensable heat · Zones · Science page → "HeatSafe's danger limits"
Skin temperature
The temperature of the body's outer surface, normally around 35°C in heat-shedding mode.
Skin is roughly 2°C cooler than the core because it is constantly losing heat to the surrounding air and to evaporating sweat.
Skin temperature matters for the Heat Safety Index because heat flows from hot to cold. As long as wet bulb is below 35°C, heat can still travel from skin into the air. Above 35°C wet bulb, the gradient inverts — heat starts flowing into the body — and survival becomes thermodynamically impossible. This is the basis of the theoretical 35°C survival limit.
See also: Wet bulb temperature · Safety limit
Stull's formula
The mathematical formula HeatSafe uses to calculate wet bulb temperature from outdoor temperature and humidity.
Published in Stull (2011), it is accurate to within about 0.3°C in normal atmospheric conditions and is the standard approach in applied meteorology when no physical wet bulb thermometer is available.
See also: Wet bulb temperature · About page → Sources
Sweat / sweating
The salty fluid the body produces in heat to trigger evaporative cooling.
Sweating is not the cooling itself. Producing sweat is necessary, but it does nothing on its own — the cooling happens only when sweat evaporates off the skin.
This is the most common misconception about heat physiology. We feel sweat and we feel hot, and our intuition tells us "I am sweating, so I am cooling." If sweat sits on the skin without evaporating — because the air is too humid, or because you wipe it off — the body has spent water and salt for no cooling benefit. Wiping sweat actively interrupts the cooling process.
Anchoring this in something familiar: pouring water on a hot pot does nothing if the water just pools. The pot only cools if the water boils off — that is, evaporates.
See also: Evaporation · Science page → "Sweating is necessary, evaporation is sufficient" · FAQ → "Why doesn't wiping sweat cool me down?"
Threshold
A scientific term for the point at which a system shifts from one state to another — used in heat physiology to mark where the body's cooling fails.
In research literature, the uncompensable heat stress threshold is the wet bulb temperature beyond which the body can no longer dissipate metabolic heat.
HeatSafe uses safety limit in its own writing because threshold is jargon — accurate, but a barrier between the science and the reader. Threshold appears here only when quoting research directly (Vecellio's paper title, technical citations) so anyone fact-checking the source recognises the language.
See also: Safety limit · Uncompensable heat
Uncompensable heat
Heat conditions in which our bodies can no longer cool themselves, no matter how much we sweat.
Wet bulb temperature has crossed the level where sweat evaporation can not keep up with the body's own heat production. Core body temperature begins rising; heat exhaustion progresses to heat stroke.
This is the physiological state HeatSafe is built to warn about. Its Danger and Critical zones correspond to conditions approaching or entering uncompensable heat — for vulnerable groups first, and eventually for everyone.
See also: Compensable heat · Heat stroke · Safety limit
Wet bulb temperature
The lowest temperature skin can reach by sweating in the current air — the actual measure of whether heat is survivable.
The "bulb" here refers to the mercury bulb of a thermometer — the rounded reservoir at the base — not an electric bulb.
It is named after the original measurement method: a thermometer wrapped in a wet wick, cooled by evaporation, exactly as skin is cooled by sweat. The drier the air, the more vigorous the evaporation, and the lower the wet bulb reading. In fully saturated air, wet bulb equals dry bulb because no evaporation occurs.
Wet bulb is the number that matters for survival because the cooling mechanism it measures (water evaporating into air) is physically the same as the body's cooling mechanism (sweat evaporating into air). When wet bulb is high, both fail together.
The Heat Safety Index is the wet bulb temperature, presented in everyday language and zones.
See also: Heat Safety Index · Dry bulb temperature · Safety limit · Science page → "Why wet bulb, not dry bulb"
Zones (Manageable, Caution, Danger, Critical)
The four colour-coded bands HeatSafe uses to communicate risk at a glance.
Each maps to a wet bulb temperature range and a single-line verdict the body can act on.
- Manageable (below 26°C wet bulb, green) — Your body can cool itself with adequate hydration.
- Caution (26–28°C, yellow-orange) — Your body's cooling is becoming less efficient. Take precautions.
- Danger (28–32°C, orange) — Your body is struggling to cool itself. Limit exposure.
- Critical (32°C and above, red) — Your body cannot cool itself in these conditions. Stay indoors.
The zones follow standard hazard-communication convention (green / yellow / orange / red — same logic as traffic, fire warnings, weather alerts). The boundaries are conservative; the experimental safety limit for healthy young adults at rest sits around 30.6°C, and real-world conditions push the effective limit lower.
See also: Safety limit · Wet bulb temperature · Science page → "HeatSafe's danger limits"
Last updated: May 11, 2026