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What the flag actually encodes
A flag is a shorthand for a work/rest ratio and a water intake, indexed to WBGT rather than to air temperature, because the thing that limits a body under load is whether it can still shed heat, and that depends on humidity, wind and radiant load as much as on the thermometer.
That is also why a flag condition is not transferable between contexts without thought. The cycles assume a particular workload; heavier work at the same flag is a different exposure. Our guidance shows the standard band, and the app can escalate it for an individual’s own situation: it never relaxes below the published band.
Where a modeled reading helps, and where it doesn’t
It helps for planning: seeing the day’s arc before you build the schedule, checking a training area you’re traveling to, or knowing whether the evening window is genuinely cooler. It gives you a consistent number across sites, which a single kit at a single point cannot.
It does not replace the instrument where the instrument is required. A modeled WBGT is derived from gridded weather; a kit sitting on the actual training area captures that ground, that surface and that shade. When a standing order says measure, measure.
Acclimatization is part of the plan, not a footnote
Heat adaptation takes one to two weeks and decays during time away. New arrivals, personnel returning from leave, and anyone transitioning from a cool climate are all operating without it, and opening days are usually where intensity ramps hardest. That pairing is the risk. See why the first hot week is the most dangerous.
Screening guidance only. This is not an official flag determination and does not supersede your command’s or department’s standing orders, local policy, or a required on-site measurement. Heat stroke is a medical emergency: cool first, transport second.
What to do at each heat category
The flag comes from the WBGT reading itself, on TB MED 507’s own lines. Our six heat-stress levels draw their lines in different places, so one level can hold more than one flag: High runs from green through yellow to red. The lookup above names the flag for the actual reading.
| Heat category | WBGT | Easy work | Moderate work | Heavy work | Very heavy work |
|---|---|---|---|---|---|
| Below category 1 | Below 78 °F | No work/rest cycle set | |||
| 1 (white) | 78 to under 82 °F | No limit½ qt/hr | No limit¾ qt/hr | 40/20¾ qt/hr | 20/401 qt/hr |
| 2 (green) | 82 to under 85 °F | No limit½ qt/hr | No limit¾ qt/hr | 30/301 qt/hr | 15/451 qt/hr |
| 3 (yellow) | 85 to under 88 °F | No limit¾ qt/hr | No limit¾ qt/hr | 30/301 qt/hr | 10/501 qt/hr |
| 4 (red) | 88 to under 90 °F | No limit¾ qt/hr | 50/10¾ qt/hr | 20/401 qt/hr | 10/501 qt/hr |
| 5 (black) | 90 °F and up | No limit1 qt/hr | 20/401 qt/hr | 15/451 qt/hr | 10/501 qt/hr |
From TB MED 507 (April 2022), Table 3-2, for an average-sized, heat-acclimatized Service member in the OCP uniform, for up to 4 hours of work. Each cell is minutes of work / minutes of rest in every hour, then quarts of water per hour; “No limit” is no limit to work per hour. Water should not exceed 1½ qt an hour or 12 qt a day. Heavy protective clothing (CBRN, JSLIST) adds 10 °F to the WBGT for easy work and 20 °F for moderate and heavy work. Your command’s flag call, and any on-site reading it requires, govern.
Common questions
What are the WBGT flag conditions?
The flag system maps WBGT onto work/rest cycles and water intake in five heat categories: white, green, yellow, red and black, each a fixed band of WBGT. The US Army's TB MED 507 is the reference most commonly used, and the guidance shown on this page follows that framework. One 2026 command policy that lists it among its references is US Army Cadet Command's Cadet Summer Training (CST) Policy Memorandum 8, Guidelines for Prevention, Evaluation and Treatment of Casualties with Potential Heat Illnesses (signed 26 June 2026). Individual commands and departments may run stricter local rules.
Is this the same as an on-site WBGT kit reading?
No. Our value is modeled from weather data for your coordinates. Where a unit or department requires a measured on-site WBGT to set the flag, that instrument reading is the one that governs. Use this to plan the day, pick the window, and know what to expect before the kit comes out.
Why do the limits assume trained, monitored people?
The flag system was built for populations that are screened, acclimatized, supervised and have medical support close by. Those assumptions do a lot of work. Applying the same cycles to unacclimatized personnel, or to anyone without monitoring, removes the margin the system depends on.
How does gear change the calculation?
Body armor, turnout gear, CBRN suits and similar impermeable equipment block evaporation, which is the body's principal cooling mechanism. Standard practice is to apply a WBGT adjustment for encumbering clothing: the effective heat stress is higher than the ambient reading. Treat the number here as the ambient case and adjust upward for gear per your own policy.
Does the guidance change at night?
The wording does: after dark it stops referring to shade and shifts to what actually applies. The physiology doesn't get a discount: humid nights with little wind clear heat poorly, and hard surfaces hold the day's load for hours, so night training on a paved area is not the same as night training on grass.
Other audiences
Asphalt runs far hotter than the air. Check the ground temperature where you are before you walk.
Work/rest guidance from the live heat-stress reading, for crews, supervisors and safety officers.
Activity modification for training and competition, plus your state's own high-school WBGT policy.