Marine WBGT Heat Stress Calculator

Frequently Asked Questions

What is WBGT and how is it computed?

WBGT (outdoor, sunny) = 0.7 × Twb + 0.2 × Tg + 0.1 × Tdb, where Twb is natural wet-bulb, Tg is black-globe (radiant), and Tdb is dry-bulb air temperature. Indoor/shaded: 0.7 × Twb + 0.3 × Tg.

What WBGT triggers heat-stress concern?

NIOSH/ACGIH thresholds for acclimatized workers: 25 °C light work, 27 °C moderate, 28 °C heavy work. WBGT > 32 °C is the "extreme caution" zone; sustained work above 35 °C is dangerous.

How does humidity dominate the calculation?

Wet-bulb gets 70% of the weight because evaporative cooling (sweat) is the human body's main heat shed. 35 °C/30% RH and 30 °C/80% RH feel different but yield similar WBGT - the muggy day is more dangerous than its lower dry-bulb suggests.

How is the rule applied at sea or outdoors?

Pair WBGT readings with work-rest cycles: e.g. 75% work / 25% rest at WBGT 28 °C for moderate effort, dropping to 25/75 at WBGT 32 °C. Educational only - consult OSHA/USCG guidance and on-vessel safety procedures.

Provided by AllCalculators.io
Free online calculators for everyday. No registration required.

Educational only. Not for flight or navigation operations.

Aviation & Marine Disclaimer: Educational only. Not for flight or navigation operations.

This calculator is for educational and informational purposes only and is not a substitute for official flight or navigation planning. Always use current performance charts, an approved POH/AFM, certified navigation tools, and follow all applicable FAA, ICAO, USCG, and other regulatory guidance. Verify all results independently before operating any aircraft or vessel.

JavaScript is required to use the interactive calculator above. The questions and answers below remain readable without JavaScript.

How This Calculator Works

Heat stress is a leading cause of incapacitation on open boats, fishing decks, and ship engine rooms. Air temperature alone is a poor predictor - humidity prevents sweat evaporation, sun loading raises skin temperature directly, and a brisk wind can mask actual core-temperature stress until it is too late. The Wet-Bulb Globe Temperature (WBGT) index combines three measurements into one number: the natural wet-bulb temperature (T_wb), which captures humidity; the globe temperature (T_g), a black-globe thermometer that captures solar and radiant load; and the dry-bulb (ambient) temperature (T_db). It is the worldwide standard for occupational heat-stress management (ISO 7243, US military, ACGIH). This calculator implements the outdoor-with-solar formula and maps the result to flag categories used by navies and the Coast Guard for work-rest cycling. Use for planning study; physiology and conditions vary - supervise crew and stop work if anyone shows heat-stress symptoms.

Formula / Method

Outdoor, with solar load: WBGT = 0.7 · T_wb + 0.2 · T_g + 0.1 · T_db, all in °C. (Indoor or no-solar form drops the dry-bulb term and uses 0.7·T_wb + 0.3·T_g.) The weighting reflects the dominance of humidity in heat stress - sweat evaporation is the body's primary cooling mechanism and humidity blocks it. Categories used here approximate the US Navy / ACGIH flag system: <27°C low, 27–29 moderate (yellow), 29–31 high (red), 31–32 very high (black), ≥32 extreme. Acclimatised personnel tolerate higher WBGT than unacclimatised; clothing (foul-weather gear, immersion suits) adds significantly to effective WBGT.

Worked Example

Working deck on a hot, humid summer day: wet-bulb 26°C, globe 38°C (full sun on a dark surface), dry-bulb 32°C. WBGT = 0.7 × 26 + 0.2 × 38 + 0.1 × 32 = 18.2 + 7.6 + 3.2 = 29.0°C - into the red/high category. Recommended action: reduce continuous work to 30 min on / 30 min off cycles, mandate water every 15 min, monitor crew for headache or nausea. In an enclosed wheelhouse without air-conditioning the temperatures could be higher still and the indoor WBGT formula (no solar term) applies if the space is shaded.

Common Mistakes

Confusing wet-bulb with dewpoint - they are related but not identical; the wet-bulb temperature requires a wetted, ventilated thermometer. Treating heat-index (US NWS) as WBGT - heat-index is for shaded indoor conditions and ignores solar and wind; WBGT is the outdoor standard. Ignoring clothing - foul-weather gear, life vests, and immersion suits add 1–3°C to the effective WBGT; military tables include clothing-adjustment factors. Assuming acclimatised performance for new crew - full acclimatisation takes 7–14 days of progressive exposure. Reading the category boundaries as absolute - individual tolerance varies with hydration, sleep, medication, and fitness.

Tips & FAQ

Symptoms to watch for: heat cramps, dizziness, confusion, hot-dry skin (a late and dangerous sign of impending heat stroke). Hydration rule: 1 cup (250 ml) of water every 15–20 min in red/black conditions; do not wait for thirst. Where do I get T_g? A black-globe thermometer (150 mm copper globe painted matte black, with thermometer at centre) is the standard; portable WBGT meters integrate all three sensors. What about cold-water immersion risk? WBGT addresses heat only; cold-water survival is a separate planning question. Disclaimer: planning study only; supervise crew physiologically, follow your flag-state and operator heat-stress guidance, and stop work at the first sign of heat illness.

Marine WBGT Heat Stress Calculator

Frequently Asked Questions

Related Aviation & Marine Calculators