WBGT Monitor & Heat Stress Sensor

01  What Is a Wet Bulb Thermometer? — The Instrument Behind the Index

A wet bulb thermometer is a temperature sensor wrapped in a water-saturated cotton wick and exposed to ambient air. As water evaporates from the wick, it carries heat away from the sensor — cooling the thermometer below the actual air temperature. The degree of cooling depends entirely on the humidity of the surrounding air: in dry conditions, rapid evaporation produces a large temperature depression; in humid conditions, slow evaporation produces little cooling and the wet bulb temperature approaches the air temperature.

This evaporation-driven cooling is the same mechanism the human body uses to regulate temperature through sweating. A wet bulb thermometer is therefore a direct physical analog for the body’s cooling capacity — which is why it carries the dominant 70% weighting in the WBGT heat stress formula.

Wet Bulb Temperature in Meteorology

Meteorologists use the difference between dry-bulb (air) temperature and wet-bulb temperature — the wet-bulb depression — to calculate relative humidity and dew point. A large depression between the two readings indicates dry air with significant evaporative potential; a small depression indicates humid air near saturation. The sling psychrometer — a classic instrument that spins two thermometers (one dry, one wet-wick) through the air — remains a reference standard for humidity measurement in the field.

Wet Bulb Temperature in Agriculture and Industry

In agriculture, wet-bulb temperature determines whether running irrigation can protect crops from frost — a technique that works when the wet-bulb temperature is at or above 32°F (0°C), because the release of latent heat as water freezes provides protection even when air temperature drops below freezing. Ski resorts use wet-bulb readings to determine whether snow-making is possible, since effective snowmaking requires wet-bulb temperatures below approximately 28°F regardless of air temperature.

In industrial applications, the wet-bulb temperature defines the physical efficiency limit of evaporative cooling systems — cooling towers, swamp coolers, and similar equipment cannot cool water below the wet-bulb temperature of the intake air. This makes wet-bulb monitoring essential for industrial cooling system operation in hot weather.

02  What WBGT Measures and Why It Is the Standard

Wet Bulb Globe Temperature (WBGT) is the globally recognized metric for measuring environmental heat stress on the human body. Unlike air temperature or heat index — which measure only one or two environmental variables — WBGT simultaneously accounts for all four factors that determine how effectively the human body can cool itself: air temperature, humidity, radiant heat from the sun, and wind speed.

The result is a single composite index, expressed in degrees Fahrenheit or Celsius, that tells athletes, athletic trainers, coaches, outdoor workers, and safety officers what the heat actually feels like to a body in motion and in direct sunlight — not what it feels like to a stationary person sitting in the shade, as the standard heat index does.

cyclonePort weather surveillance stations include a WBGT monitoring system as part of their sensor suite, delivering continuous real-time WBGT readings through the RadarOmega platform. Where cyclonePort’s competitors — particularly Perry Weather, WeatherSTEM, Baron Weather, and Vaisala — often present WBGT as a calculated approximation derived from temperature and humidity alone, cyclonePort’s system uses a dedicated black globe sensor to directly measure radiant heat: the physically measured component that makes WBGT genuinely different from heat index, and that no algorithmic approximation can fully replicate.

03  How WBGT Is Measured — The Three-Component System

A true WBGT measurement requires three distinct sensor inputs, each measuring a different aspect of the thermal environment. This is where WBGT monitoring systems separate into two categories: instruments that measure all three components directly, and cheaper systems that substitute calculated approximations for one or more components.

Component 1: Dry-Bulb Temperature (Tdb)

The dry-bulb temperature is standard ambient air temperature — the reading you would see on any outdoor thermometer. It is measured with a standard thermistor or RTD sensor housed inside a radiation shield that prevents direct solar heating of the sensor element. This is the baseline air temperature component of the WBGT formula.

Component 2: Natural Wet-Bulb Temperature (Tnwb)

The natural wet-bulb temperature is measured by a thermometer wrapped in a saturated cotton wick, exposed to ambient air without forced airflow. As water evaporates from the wick, evaporative cooling lowers the thermometer reading. The wet-bulb temperature tells you how effectively the atmosphere allows evaporative cooling — which directly mirrors how effectively the human body can cool itself through sweating. In dry air with low humidity, significant evaporation occurs and the wet-bulb temperature drops far below air temperature. In humid air near saturation, very little evaporation occurs and the wet-bulb temperature approaches the dry-bulb temperature.

Component 3: Globe Temperature (Tg) — The Black Globe Sensor

The globe temperature is measured inside a hollow copper sphere approximately 6 inches (150 mm) in diameter, painted matte black. The black surface absorbs all incoming solar radiation — direct sunlight, reflected light from surfaces, and thermal radiation from surrounding objects — and converts it to heat. A temperature sensor inside the globe measures the equilibrium temperature of this radiation-absorbing surface.

The globe temperature is what makes WBGT fundamentally different from heat index. It is the only measurement that captures how hot it actually feels to a person standing in direct sunlight — accounting for solar angle, cloud cover, and radiant heat from artificial sources like track surfaces, turf fields, and industrial equipment. You cannot calculate an accurate globe temperature from humidity and air temperature alone. You must measure it.

The WBGT Formula

04  WBGT vs. Heat Index — Why the Difference Matters

Heat index and WBGT are often conflated — but they measure fundamentally different things, and confusing them carries real safety consequences. OSHA, NIOSH, the U.S. military, and every major athletic governing body that has carefully evaluated both metrics has moved to WBGT as the operative standard. The reasons are specific and compelling.

05  WBGT Activity Guidelines & Regulatory Thresholds

WBGT thresholds translate a single number into specific, actionable decisions about whether to proceed, modify, or cancel outdoor activities. Guidelines are region-specific — athletes and workers in the Southeast who are heat-acclimatized face different thresholds than those in cooler climates encountering peak conditions for the first time. The following represent widely-adopted standards; always consult your governing body’s current policy.

International WBGT Risk Categories — ISO / ACGIH / NWS Reference

The following categories reflect widely adopted international standards expressed in Celsius, used as the basis for governing body guidelines worldwide. Flag colors correspond to the U.S. military’s color-coded installation display system, now broadly adopted across athletic and occupational settings.

GHSA / NFHS Athletics — Georgia High School Association Guidelines

The GHSA mandates WBGT monitoring for all outdoor athletic practices and competitions prior to October 1. Readings must be taken at minimum every 30 minutes, beginning 30 minutes before practice. A scientifically approved instrument with a black globe is required — estimated or calculated WBGT values from apps or non-globe instruments do not meet the standard.

U.S. Military Heat Categories

The U.S. military has used WBGT since the 1950s — developed following nearly 200 heat-related training deaths between 1942 and 1944. Military heat categories govern water intake requirements, allowable activity levels, and work/rest ratios for personnel in various uniform configurations.

OSHA / NIOSH Occupational Heat Exposure

OSHA recommends WBGT as the primary metric for workplace heat hazard assessment, calling heat index a ‘less desirable substitute.’ The NIOSH Recommended Exposure Limit (REL) for heat stress is expressed in WBGT and varies by workload level and acclimatization status. OSHA’s proposed federal heat safety standard (in rulemaking as of this publication) uses WBGT as the operative measurement for high-heat trigger determinations.

• Initial heat trigger: Heat index ≥ 80°F or WBGT equivalent — employers must provide water, rest, and shade access
• High heat trigger: Heat index ≥ 90°F or WBGT at NIOSH REL — mandatory rest breaks every 2 hours, heat illness monitoring, buddy system
• NIOSH REL for light work (acclimatized): WBGT ≈ 86°F; heavy work: WBGT ≈ 77–79°F
• ISO 7243 specifies maximum WBGT limits ranging from 33°C (91°F) for acclimatized sedentary workers to 20°C (68°F) for unacclimatized heavy laborers

06  Operational Applications — Athletics, Workers, and Public Safety

High School and Collegiate Athletics

Heat stroke is one of the leading causes of preventable death in U.S. high school athletics. The Korey Stringer Institute at the University of Connecticut, named for the Minnesota Vikings offensive lineman who died of heat stroke during training camp in 2001, has documented that exertional heat stroke (EHS) is 100% preventable when appropriate environmental monitoring and response protocols are in place.

The NFHS placed a formal emphasis on WBGT adoption across all state associations during the 2021–22 school year, funding a grant program to distribute WBGT devices to schools nationwide. Georgia’s GHSA has since made WBGT monitoring mandatory for all outdoor practices and competitions prior to October 1 — covering not only in-season practices but all summer activities, including 7-on-7, strength and conditioning, and camps affiliated with the school.

cyclonePort weather surveillance stations at athletic facilities deliver continuous WBGT readings to coaches, athletic trainers, and administrators through RadarOmega — eliminating the need for manual handheld readings every 30 minutes and providing timestamped records that document compliance with state association requirements.

Beyond threshold compliance, continuous WBGT data enables proactive scheduling decisions that reduce heat exposure without cancelling activity entirely. Practice times shifted to early morning typically see WBGT readings 5–8°C (9–14°F) lower than peak afternoon conditions — a difference that can move a session from the red flag zone to the green flag zone. cyclonePort’s historical WBGT data archive allows athletic programs to analyze seasonal patterns, optimize practice scheduling, and document the environmental conditions behind every scheduling decision.

Outdoor Workers — OSHA and Occupational Heat Safety

The U.S. Department of Labor’s proposed federal heat safety standard would establish the first enforceable occupational heat exposure regulations in U.S. history, with WBGT as the operative measurement metric. Industries with the highest occupational heat illness risk — construction, agriculture, landscaping, roofing, utilities, and outdoor industrial operations — would face mandatory monitoring, work/rest protocols, and documentation requirements triggered by WBGT thresholds.

Organizations that deploy cyclonePort WBGT monitoring at work sites gain two simultaneous advantages: the ability to implement evidence-based work/rest protocols that protect workers, and the documented, timestamped WBGT records that demonstrate due diligence for OSHA compliance and reduce liability exposure in heat-related incident investigations.

Military and First Responder Training

The U.S. Army, Navy, Air Force, and Marine Corps have used WBGT to govern physical training since the 1950s, when WBGT protocols were developed specifically in response to heat training casualties. Military installations in hot climates display color-coded flag systems (White through Black) corresponding to WBGT heat categories, governing allowable training intensity for all personnel on base.

First responder agencies — fire departments, law enforcement, search and rescue — face similar heat stress risks during training exercises, wildfire operations, and extended outdoor deployments. cyclonePort WBGT monitoring at training facilities and forward operating locations provides the real-time data needed to implement heat safety protocols without compromising operational readiness.

Large Events and Public Gatherings

Stadium operators, race directors, festival organizers, and municipality emergency managers use WBGT to make real-time decisions about outdoor events: whether to modify schedules, activate additional cooling resources, issue public heat advisories, or suspend outdoor activities entirely. Unlike heat index — which does not account for direct solar radiation and is calculated for shade conditions — WBGT reflects what spectators and participants standing on sun-exposed surfaces actually experience.

cyclonePort’s RadarOmega platform delivers WBGT alongside wind speed, humidity, temperature, and camera feeds in a single dashboard — giving event operations teams the complete picture, not just a heat number divorced from context.

Emergency Management

During heat waves — increasingly frequent and severe events — public health and emergency management agencies issue heat advisories, activate cooling centers, and coordinate wellness checks for vulnerable populations based on environmental heat data. WBGT provides the most complete picture of heat stress conditions for these decisions, particularly when populations at risk include older adults, people engaged in outdoor labor, or those without air conditioning who are exposed to both ambient heat and solar radiation.

07  Instrument Selection Guide — What Separates Professional WBGT Systems

WBGT instruments span a wide range — from inexpensive handheld devices that approximate WBGT through calculation to ISO-compliant research-grade systems that directly measure all three components. The differences matter operationally, legally, and in terms of athlete and worker safety.

08  Installation & Maintenance

WBGT is one of the most siting-sensitive measurements in meteorological instrumentation. Because the black globe must be exposed to direct solar radiation to accurately measure radiant heat — and the wet-bulb sensor must be in natural ambient airflow — placement and exposure choices have outsized effects on reading accuracy.

Siting Guidelines

• Open sky exposure: The black globe must have unobstructed exposure to the sky above and to the horizon in all directions. Shade from trees, structures, or overhangs blocks solar radiation from reaching the globe, causing the system to systematically underestimate radiant heat and produce artificially low WBGT readings.
• Representative location: Place the system where athletes or workers actually operate. A WBGT reading at a shaded equipment shed tells you nothing about conditions on the sun-exposed practice field 100 yards away. If the activity is on an artificial turf field in direct sun, the instrument must be on or immediately adjacent to that surface.
• GHSA requirement — 15 minutes pre-equilibration: The instrument must be set up and allowed to equilibrate at the activity venue at least 15–20 minutes prior to the start of activity. A permanently installed cyclonePort station eliminates this requirement entirely — it is always in equilibration.
• Away from heat sources: Artificial heat sources — HVAC exhausts, generators, cooking equipment, industrial processes — must not be within the sensing radius of the WBGT system. These inflate globe and dry-bulb readings independently of ambient conditions.
• Standard height: Mount the system at approximately 1.2–1.5 meters (4–5 feet) above the ground surface, consistent with the breathing zone of a standing person. Mounting too high or too low can misrepresent the radiation environment experienced at human height.

Black Globe Sensor Maintenance

• Surface condition: The matte black finish of the globe must be maintained. Oxidation, bird droppings, pollen, or any coating that changes the surface from matte black to reflective or different-colored alters the radiation absorption characteristics and introduces calibration error. Clean the globe surface gently with a soft cloth and mild soap periodically.
• Structural integrity: Inspect the globe for cracks, dents, or physical damage that could allow moisture intrusion or alter the internal thermal environment. A cracked globe must be replaced before the sensor is used for safety decisions.
• Internal sensor check: Verify the globe temperature sensor is reading plausibly relative to ambient temperature — on a clear, sunny day, globe temperature should significantly exceed air temperature. A globe reading close to air temperature in full sun indicates possible sensor failure or surface reflectivity change.

Wet-Bulb Sensor Maintenance

• Wick replacement: The cotton wick on the natural wet-bulb sensor must be kept moist and clean. Replace the wick periodically — frequency depends on water quality and ambient conditions. A contaminated or dried wick produces inaccurate wet-bulb readings.
• Water reservoir: The wet-bulb sensor requires a water reservoir that keeps the wick saturated. Check and refill the reservoir regularly, particularly during prolonged hot, dry periods. Use distilled water where possible to minimize mineral deposits on the wick.

Calibration

cyclonePort WBGT systems ship with factory calibration documentation. GHSA regulations require recalibration at minimum every two years. The individual component sensors (temperature, humidity) can be verified against calibrated reference instruments in controlled conditions. Full system WBGT calibration verification is performed by comparing readings against a reference instrument meeting ISO 7243 specifications under known conditions.

09  cyclonePort WBGT Monitoring System

cyclonePort weather surveillance stations include a WBGT monitoring system with a dedicated 6-inch black globe sensor, delivering continuous real-time WBGT readings through the RadarOmega platform alongside all other station sensor streams.

Technical Specifications

Specifications may vary by model. Contact cyclonePort for current engineering documentation.

What the System Delivers

• Continuous real-time WBGT updated throughout every practice, game, and work shift
• Automated threshold alerts — SMS and email the instant WBGT crosses the 82°F, 87°F, 90°F, and 92°F GHSA action levels, or any custom threshold
• Timestamped compliance log — every WBGT reading archived with timestamp for GHSA documentation, OSHA record-keeping, and post-incident review
• Full weather picture — WBGT alongside wind speed, humidity, temperature, dew point, rain gauge, and barometric pressure from the same station
• PTZ camera integration — correlate live video with WBGT readings for situational awareness during events
• Multi-station comparison — monitor WBGT across multiple venues or field locations simultaneously
• Remote access — coaches, athletic trainers, and administrators access live WBGT from any device via RadarOmega
• Historical archive — full WBGT history accessible for trend analysis, policy review, and after-action documentation

Who Deploys cyclonePort WBGT Monitoring

10  Frequently Asked Questions