Measurements · Meteorology
Heat Index
Also known as feels-like temperature, apparent temperature, humidex
Heat index / apparent temperature combining heat and humidity.
What it is
What is heat index?
The heat index is a derived "feels-like" temperature that combines air temperature and relative humidity to express how hot conditions feel to the human body, returning an equivalent value in degrees Celsius. As humidity rises, sweat evaporates less readily, so the body cools less effectively and a given temperature feels hotter; the heat index captures this. It is calculated from temperature and humidity using a standard formula, the same relationship behind the US National Weather Service "apparent temperature". It applies in warm, humid conditions and is the summer counterpart to wind chill.
Health and operational effects
The heat index warns of heat-related illness in everyday and indoor settings, where high humidity turns a merely warm day into a dangerous one. As it climbs, the risk of heat cramps, exhaustion, and heat stroke rises, particularly for the elderly, children, and anyone exerting themselves. It is widely used in public heat warnings and in general comfort management for offices, schools, and homes.
Two important caveats govern its use. The heat index assumes shade and light wind, so it understates risk in direct sun, and it is a general-comfort index rather than an occupational one. For managing heat stress in physically demanding or industrial work the proper index is WBGT (wet bulb globe temperature), which also accounts for radiant heat and workload (see the WBGT parameter on this site). The heat index is best read as a comfort and public-health guide, not a work-safety limit.
How it is measured
How Heat Index Is Measured
Like other apparent-temperature indices, the heat index is calculated rather than sensed: it is computed from a co-located air-temperature measurement and a relative-humidity measurement. A device that reports heat index therefore contains an accurate temperature element, properly shielded from radiant heat and self-heating, and a capacitive humidity sensor, and applies the standard formula. Its accuracy is entirely inherited from those two inputs, so a biased temperature or humidity reading produces a biased heat index. Because it needs only temperature and humidity, it can be provided anywhere those two are already measured.
Built into Aethair PRO and Aethair IAQ2
Measured by the monitor's own sensors, calibrated before it ships: nothing to add.
An instrument on Thiamis3
A third-party instrument connected to a Thiamis gateway, reporting into the same Environet account.
Placing heat index monitoring
Site the underlying temperature and humidity sensors in the occupied zone at body height, out of direct sun and away from supply air, heat-emitting equipment, and moisture sources that would bias either input. Since the standard heat index assumes shade, an instrument in sunlight will not match the formula's assumptions. For sun-exposed outdoor work, use WBGT to account for the radiant load. For indoor comfort, a well-placed temperature and humidity sensor in the room people occupy gives a representative heat index directly.
Every device and option that measures Heat Index 5
| Device / option | Type | Base platform | Measures |
|---|---|---|---|
Aethair PRO built-in sensors | Built-in sensors | Aethair PRO | |
 Aethair IAQ built-in sensors | Built-in sensors | Aethair IAQ | |
 Airmar · 110WX, 110WXS, 120WX… | Weather Stations | Thiamis | |
 Davis · Davis Wizard III, Davis Wizard II, Davis Monitor… | Weather Stations | Thiamis | |
 TSI · QuestTemp-32, QuestTemp-34, QuestTemp-36… | Heat Stress & Comfort | Thiamis |
Where it is monitored
Solutions That Monitor Heat Index
Each one links to the solution page showing what that industry measures it for.
Heat Stress & Outdoor Work
Often addedWBGT for worksites, athletics, and events
Weather Stations & Meteorology
Often addedProfessional multi-parameter weather monitoring
Article Library
Reading on Heat Index
Aethair IAQ vs Kaiterra Sensedge Mini: IAQ Monitors Compared
Aethair IAQ vs Kaiterra Sensedge Mini compared on connectivity, sensors, accuracy, and platform, and how to choose the right commercial IAQ monitor.
How to Choose an Air Quality Monitor: PRO, IAQ, or Thiamis
Compare Aethair PRO, IAQ, and Thiamis to match the right device to your site, from commercial interiors to industrial and fenceline monitoring.
OSHA Air Quality Standards 2026: PELs & 29 CFR 1910.1000
OSHA air quality standards for indoor and outdoor work: permissible exposure limits, Table Z-1, 29 CFR 1910.1000, and the heat and wildfire smoke triggers.
Related
More in Meteorology
Questions
Heat Index FAQ
What is the difference between heat index and WBGT?
The heat index combines only temperature and humidity and assumes shade, so it is a general public-comfort measure. WBGT (wet bulb globe temperature) also folds in radiant heat and is weighted for workload, which makes it the standard for occupational and sports heat-stress safety. For managing physical work in the heat, use WBGT; for everyday and indoor comfort, the heat index is appropriate.
Does the heat index account for sun and wind?
No. It assumes shade and light wind, so full sun makes real conditions feel considerably hotter than the heat index suggests, and strong wind can make them feel cooler. This is one reason WBGT, which includes a black-globe radiant term, is preferred for sun-exposed work.
Why does humidity make heat feel worse?
The body sheds most of its heat by evaporating sweat, and humid air already holds a lot of moisture, so evaporation slows and cooling becomes less effective. The heat index quantifies this, raising the feels-like temperature as humidity climbs even when the air temperature is unchanged.
How do I monitor heat index?
The Aethair family (Aethair PRO and Aethair IAQ) derives the heat index from its built-in temperature and humidity sensors, so indoor comfort monitoring needs no extra instrument. For occupational heat stress in demanding or sun-exposed work, use the configurator to add a WBGT heat-stress monitor connected through Thiamis instead.
Monitor Heat Index on the Record
Schedule a callConfigure Heat Index Monitoring All parameters
A 30-minute call: you tell us about your application, then we demo the solution for it.