WBGT is a military invention. It was developed by the U.S. Marine Corps and Navy at Parris Island in the 1950s after a run of heat casualties in recruit training, and the services needed something a duty officer could read off a device and act on without interpretation. Everything athletics and industry use today descends from that work.
The military system is also the most prescriptive. Where an athletic chart offers guidance, the military table gives a duty officer a work-rest cycle in minutes and a water intake in quarts for each of three workloads, at each of five categories. The flag goes up, and the answer is already written down.
Heat categories, work-rest cycles and water
The military heat category table ties each category to a work-rest cycle and an hourly water intake for easy, moderate and hard work. These are the figures the flag colors exist to communicate.
| Category | WBGT | Easy work | Moderate work | Hard work |
|---|---|---|---|---|
| 1 (White) | 78 to 81.9°F | No limit, ½ qt | No limit, ¾ qt | 40/20, ¾ qt |
| 2 (Green) | 82 to 84.9°F | No limit, ½ qt | 50/10, ¾ qt | 30/30, 1 qt |
| 3 (Yellow) | 85 to 87.9°F | No limit, ¾ qt | 40/20, ¾ qt | 30/30, 1 qt |
| 4 (Red) | 88 to 89.9°F | No limit, ¾ qt | 30/30, ¾ qt | 20/40, 1 qt |
| 5 (Black) | 90°F and above | 50/10, 1 qt | 20/40, 1 qt | 10/50, 1 qt |
Work-rest figures are minutes of work and rest within each hour. Rest means minimal physical activity, in shade where possible.
Hourly fluid intake should not exceed 1½ quarts, and daily intake should not exceed 12 quarts. Overdrinking causes hyponatremia, which presents much like heat illness and is made considerably worse by the instinctive response to it.
What counts as easy, moderate and hard work
| Category | Examples |
|---|---|
| Easy | Weapon maintenance, walking on hard surface at 2.5 mph with under 30 lb load, marksmanship training, drill and ceremony |
| Moderate | Walking in sand at 2.5 mph with no load, patrolling, individual movement techniques, defensive position construction, calisthenics |
| Hard | Walking in sand at 2.5 mph with a load, field assaults, walking on hard surface at 3.5 mph with over 40 lb load |
Clothing and equipment adjustments
Add the adjustment to the measured WBGT before reading the category. Protective equipment changes the problem dramatically.
| Worn | Add to WBGT |
|---|---|
| Standard combat uniform | 0°F |
| Uniform with body armor | about 5°F |
| MOPP 4 protective posture | about 10°F |
| Uniform, armor and full assault load | about 10°F or more |
In MOPP 4, an 80°F WBGT reading behaves like 90°F, which is category 5. That is the practical reason protective posture training is scheduled for early morning rather than fitted around the rest of the day.
Acclimatization
Heat acclimatization develops over roughly two weeks of progressive exposure and is lost faster than it is gained.
- Plan for 10 to 14 days of progressive exposure for newly arrived personnel.
- Build intensity and duration gradually rather than jumping to a full training load.
- Expect most of the physiological adaptation in the first 5 to 7 days, with the remainder consolidating over the second week.
- Treat personnel returning from leave, illness, injury or a cooler climate as needing re-acclimatization, not as fully adapted.
- Acclimatization reduces risk substantially but does not eliminate it. Fully acclimatized soldiers still become heat casualties in category 5.
Individual risk factors
Prior heat injury
A previous episode substantially raises the risk of another and warrants individual monitoring.
Recent illness
Fever, gastrointestinal illness or dehydration from the preceding days narrows the margin considerably.
Dehydration and sleep
Arriving short on fluid or sleep is a far larger factor than most people assume.
Sickle cell trait
A recognised risk factor for exertional collapse, requiring specific attention during intense exertion in heat.
Warning signs of heat illness
Heat stress shows up before it becomes an emergency. Anyone on site should speak up immediately on any of these:
When in doubt, cool first and call for help. Collapse or confusion in the heat should be treated as possible exertional heat stroke: begin cold-water immersion immediately and call 911. Cooling comes before transport.
Military mode on your phone
The app includes a Military mode with heat categories 1 to 5 and the work-rest and water guidance that goes with each, against a live WBGT reading for your location.
- Military mode with heat categories 1 to 5
- Live WBGT for your location or any city
- Hourly and 14-day forecasts for training plans
- Category and flag color at a glance
WBGT is estimated from weather data with the peer-reviewed Liljegren model. Where a policy requires on-site measurement, use the app for planning and as a cross-check alongside your instrument.
Common questions
What are the military heat categories?
Five categories tied to WBGT ranges: category 1 (white) 78 to 81.9°F, category 2 (green) 82 to 84.9°F, category 3 (yellow) 85 to 87.9°F, category 4 (red) 88 to 89.9°F and category 5 (black) 90°F and above. Each specifies work-rest cycles and water intake by workload.
What is the work rest cycle for category 5?
In category 5, easy work runs 50 minutes work to 10 rest, moderate work 20 to 40, and hard work 10 minutes work to 50 minutes rest, each within the hour, with 1 quart of water per hour.
How much water should soldiers drink in the heat?
Between half a quart and one quart per hour depending on category and workload. Hourly intake should not exceed 1½ quarts and daily intake should not exceed 12 quarts, because overdrinking causes hyponatremia, which looks much like heat illness.
How much does body armor add to WBGT?
Body armor is commonly treated as adding about 5°F to the effective WBGT, and MOPP 4 protective posture about 10°F. Apply the adjustment to the measured reading before determining the category.
How long does heat acclimatization take?
Roughly 10 to 14 days of progressive exposure, with most of the adaptation occurring in the first 5 to 7 days. It is lost faster than it is gained, so personnel returning from leave or a cooler climate need re-acclimatization.
Related WBGT guides
Know the real heat number
Real-time WBGT, flag levels and forecasts. Free on iPhone.
