Heat Stress and Weather Terms Glossary
Heat safety involves a specific vocabulary that mixes meteorology and occupational health - terms like WBGT, heat index, dew point, heat advisory, and heat dome all describe different things and shouldn't be used interchangeably. WBGT measures actual heat stress on the body using temperature, humidity, radiant heat, and wind; heat index estimates how hot it "feels" in the shade; dew point measures how much moisture is in the air; and terms like heat advisory or extreme heat warning are official National Weather Service alerts tied to specific heat index thresholds. Knowing the difference helps you interpret weather forecasts, safety alerts, and heat illness prevention plans correctly.
Heat-related terms get used loosely in everyday conversation - "it's so humid" or "the heat index is brutal today" - but in occupational safety, sports medicine, and meteorology, these terms have precise, technical meanings. Using the wrong measurement to make a safety decision (relying on a forecasted heat index instead of an on-site WBGT reading, for example) can lead to inadequate protections. This glossary breaks down the terms you're most likely to encounter, grouped by category.
Measuring Heat: Core Terms
Wet Bulb Globe Temperature (WBGT) - A composite heat stress index that combines air temperature, humidity, radiant heat (from the sun or hot surfaces), and wind speed into a single reading. WBGT is measured directly at a location using a specialized instrument and is the standard used by OSHA, NIOSH, the military, and sports medicine organizations to decide when to modify work or athletic activity.
Heat Index (Apparent Temperature) - A measurement of how hot it "feels" to the human body, combining air temperature and relative humidity. Heat index assumes a person is in the shade with light wind and isn't exerting themselves, which is why it tends to understate real heat stress for anyone working or exercising outdoors in direct sun. Heat index is what's typically reported in a weather forecast.
Dry Bulb Temperature - The standard air temperature you'd read from an ordinary thermometer, unaffected by humidity. It's called "dry bulb" to distinguish it from wet bulb temperature, which factors in moisture.
Wet Bulb Temperature - The temperature read from a thermometer with a wet wick over its bulb, exposed to airflow. As water evaporates from the wick, it cools the thermometer, and the amount of cooling depends on humidity. Low humidity allows more evaporation and more cooling; high humidity allows less. This is one of the core inputs to WBGT.
Globe Temperature - The temperature measured inside a matte black copper sphere (a "globe thermometer"), which absorbs radiant heat from sunlight and hot surfaces the way a person's body would. It's the component of WBGT that accounts for radiant heat load - something air temperature alone completely misses.
Relative Humidity - The amount of moisture in the air compared to the maximum amount the air could hold at that temperature, expressed as a percentage. Higher relative humidity slows sweat evaporation, which is the body's primary cooling mechanism, making hot conditions feel - and physiologically become - more dangerous.
Dew Point - The temperature at which air becomes saturated with moisture and water vapor begins to condense. Unlike relative humidity, dew point doesn't change much throughout the day and gives a more consistent read on how muggy conditions actually are. Meteorologists generally consider a dew point above 65°F humid and above 75°F oppressive.
National Weather Service Alerts and Warnings
Heat Advisory - An alert issued when the heat index is expected to reach a locally defined threshold - commonly in the range of 100–109°F depending on region - for a sustained period, typically within the next 12 to 24 hours. Thresholds vary by NWS regional office based on how acclimated the local population is to heat.
Extreme Heat Warning - The more serious alert level, issued when the heat index is expected to reach or exceed a higher regional threshold - often 105–110°F or above. This alert was renamed from "Excessive Heat Warning" by the National Weather Service in 2025; the criteria stayed the same, only the name changed to better communicate urgency.
Extreme Heat Watch - Issued when conditions capable of triggering an Extreme Heat Warning are possible but not yet certain, giving people and organizations advance notice to prepare. Like the warning, this was renamed from "Excessive Heat Watch" in 2025.
Heat Wave - A period of abnormally and uncomfortably hot weather, typically lasting two or more days, though there's no single universal temperature threshold - what counts as a heat wave in Seattle would be an ordinary summer day in Phoenix.
The Weather Patterns Behind Heat Events
Heat Dome - A large-scale weather pattern in which a strong area of high pressure traps warm air in place over a region, acting like a lid. Heat domes are associated with clear skies and minimal cloud cover, which let more solar radiation reach the ground, intensifying and prolonging extreme heat over the affected area for days at a time.
Urban Heat Island - The effect where urban and metropolitan areas experience significantly higher temperatures than surrounding rural areas, largely due to pavement, buildings, and reduced vegetation absorbing and retaining heat. The difference is usually most pronounced at night, when rural areas cool off faster than cities.
Ridge (High-Pressure Ridge) - An elongated area of relatively high atmospheric pressure extending from a larger high-pressure system. Ridges are associated with sinking air, clear skies, and warming temperatures, and are frequently the mechanism that sets up a heat dome or prolonged heat wave.
Wind Chill - The counterpart to heat index on the cold end of the spectrum - a measurement of how cold moving air makes exposed skin feel by accelerating heat loss from the body. It's worth knowing alongside heat index because both are "feels like" temperatures, but they measure opposite phenomena and use entirely different formulas.
Heat-Related Illness: Quick Reference
Heat Cramps - Painful, involuntary muscle spasms caused by fluid and electrolyte loss during heavy sweating, usually in the legs or abdomen.
Heat Syncope - Brief fainting or lightheadedness caused by a temporary drop in blood pressure, often after standing for a long time or standing up quickly following exertion in the heat.
Heat Exhaustion - A moderate heat illness marked by heavy sweating, weakness, nausea, and a rapid pulse. The person remains alert, but body temperature is elevated and the condition can progress to heat stroke if not addressed.
Heat Stroke (including Exertional Heat Stroke) - The most severe heat-related illness, defined by a dangerously elevated core body temperature combined with central nervous system symptoms such as confusion or loss of consciousness. It is a medical emergency requiring immediate cooling and emergency care.
Heat Rash - A skin irritation caused by blocked sweat ducts trapping perspiration under the skin, common in hot, humid conditions.
Acclimatization - The physiological process by which the body gradually adapts to heat exposure over repeated days, improving sweat efficiency and heat tolerance. Acclimatization typically takes one to two weeks and is lost after about a week away from heat exposure.
Quick Comparison: Which Measurement Does What?
| Term | What It Measures | Accounts for Sun/Radiant Heat? | Typically Used By |
|---|---|---|---|
| Dry Bulb Temperature | Basic air temperature | No | General weather reporting |
| Relative Humidity | Moisture in the air vs. capacity | No | Weather forecasting |
| Dew Point | Moisture saturation point | No | Meteorologists |
| Heat Index | “Feels like” temperature in shade | No | Weather forecasts, general public |
| WBGT | Real-world heat stress on the body | Yes | OSHA, NIOSH, military, sports medicine |
Why This Matters for Heat Safety Programs
Weather forecasts report heat index because it's a reasonable general-public guideline, but it was never designed for people working or training outdoors in direct sun. That gap is exactly why occupational and athletic heat safety programs rely on WBGT instead - it's the only common measurement that accounts for radiant heat, which can be the difference between a comfortable shaded area and a dangerous stretch of asphalt or turf a few feet away. Understanding these terms isn't just trivia: it's the foundation for reading a forecast correctly, interpreting a weather alert appropriately, and knowing which measurement should actually drive a work-rest or practice decision.
Frequently Asked Questions
What's the difference between heat index and WBGT?
Heat index estimates how hot it feels in the shade using only air temperature and humidity. WBGT measures actual on-site heat stress by also factoring in radiant heat from the sun and wind speed, making it far more accurate for anyone working or exercising outdoors in direct sunlight.
What is the difference between a Heat Advisory and an Extreme Heat Warning?
A Heat Advisory is issued for a lower, still-significant heat index threshold, while an Extreme Heat Warning (renamed from Excessive Heat Warning in 2025) is issued for more dangerous, higher heat index thresholds. Both are based on locally defined criteria set by regional National Weather Service offices.
Is dew point or relative humidity a better measure of how muggy it feels?
Dew point is generally considered more reliable because it doesn't fluctuate with temperature the way relative humidity does. A dew point above 65°F is considered humid, and above 75°F is considered oppressive, regardless of the air temperature.
What causes a heat dome?
A heat dome forms when a strong high-pressure ridge traps warm air over a region, combined with clear skies that allow maximum solar heating. It can keep an area locked in extreme heat for days because the same pressure pattern also suppresses clouds and rain.
Why isn't heat index enough for workplace or sports safety decisions?
Heat index assumes a person is in the shade with light wind and not exerting themselves, which understates real conditions for anyone in direct sun performing physical activity. That's why safety programs use WBGT, which is measured on-site and accounts for radiant heat and exertion-relevant conditions.
What is the difference between heat exhaustion and heat stroke?
Heat exhaustion involves heavy sweating, weakness, and nausea while the person remains alert. Heat stroke is a medical emergency involving a dangerously high core temperature and altered mental status, such as confusion or loss of consciousness, and requires immediate cooling and emergency care.
Sources:
National Weather Service (NWS) Glossary and Heat Alert Criteria; National Oceanic and Atmospheric Administration (NOAA); OSHA Technical Manual, Section III, Chapter 4 - Heat Stress; NIOSH Criteria for a Recommended Standard: Occupational Exposure to Heat and Hot Environments.
