The flag system exists because numbers do not travel well across a field. A coach, a drill sergeant or a foreman can shout a color and everyone knows what changes. Behind each color is a wet bulb globe temperature range and a set of required actions.
It began as literal flags. The U.S. military ran colored flags up a pole at training installations so anyone outdoors could look up and see the current heat category without asking. Athletics and industry borrowed both the colors and the logic.
WBGT flag conditions
A number only helps if it tells you what to do. These are the five flag categories the U.S. military established and athletics and industry adopted. Thresholds shift by guideline and by how acclimatized and how heavily dressed people are, so treat these as the common baseline rather than a universal law.
- Normal activity. Heat stress is low for most people.
- Water available and encouraged throughout.
- Normal activity for acclimatized people.
- Watch anyone unacclimatized, unfit, or in heavy gear.
- Reduce intense exertion and total time in the heat.
- Add rest breaks in shade; increase fluid intake.
- Cut hard training sharply. Unacclimatized people should stop strenuous work.
- Frequent breaks, minimal equipment, close supervision.
- Cancel or postpone outdoor exertion.
- Essential work only, with short cycles and active cooling on hand.
A cold-water immersion tub or equivalent rapid-cooling method should be ready and staffed from roughly 80°F WBGT upward, not improvised after someone goes down.
What changes at each flag
The same color, read across three settings. The pattern is consistent even where the exact numbers differ.
| Flag | Athletics | Outdoor work | Military |
|---|---|---|---|
| Green | Normal practice, monitor new athletes | Normal work, water available and encouraged | Normal training, heavy exercise capped for the unacclimatized |
| Yellow | Shorten intense work, add breaks | Add rest cycles, rotate crews through shade | Reduce heavy exertion, extend rest periods |
| Red | Helmets and pads only, cap at two hours | Short work cycles, mandatory shaded rest | Hard training suspended for the unacclimatized |
| Black | Cancel or move indoors | Essential work only, active cooling on site | Nonessential physical training cancelled |
Why the same flag is not the same risk
Two people standing in an identical black flag condition can be in very different trouble. Four things move the line.
Acclimatization
Most exertional heat illness happens in the first three to five days in the heat. A fit but unacclimatized body is the high-risk case, not the unfit one.
Clothing and equipment
Anything that blocks evaporation raises effective WBGT. Full pads, turnout gear and body armor all shift the threshold down.
Workload
Standing in red flag heat and sprinting in it produce different core temperatures. Intensity and duration matter as much as the reading.
Individual risk
Prior heat illness, illness or fever, certain medications, sickle cell trait, dehydration and poor sleep all narrow the margin.
Running a flag system that works
- Decide in advance who takes the reading and who has authority to call the flag. That person needs to be able to stop practice without negotiating.
- Take the first reading about 15 minutes before activity and repeat on a fixed interval, commonly every 30 minutes.
- Post the current flag somewhere everyone can see it, and say it out loud at the start of the session.
- Write every reading down with the time. The log is what protects you and your athletes later.
- Have the cooling response staffed before you need it: shade, cold water, ice towels, and a cold-water immersion tub from roughly 80°F WBGT upward.
- Brief everyone that reporting symptoms is expected, not weakness. A flag system fails quietly when nobody speaks up.
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.
Flag conditions on your phone
The app shows the current flag level for your location with the guidance that goes with it, and forecasts the flag forward hour by hour so you know whether a four o'clock practice is going to turn red before it ends.
- Current flag level, color-coded
- Hourly flag forecast for the rest of the day
- Guidance that changes with your selected mode
- Your location or any city you search
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 WBGT flag colors in order?
White, green, yellow, red and black, from least to most severe. White is below 80°F WBGT, green 80 to 84.9°F, yellow 85 to 87.9°F, red 88 to 89.9°F and black 90°F and above in the common baseline chart.
What does a black flag mean?
Black flag is the stop condition: 90°F WBGT and above. Outdoor exertion should be cancelled or postponed, with essential work only, short cycles and active cooling available on site.
What does red flag WBGT mean for football practice?
In most athletic charts, red means helmets and shoulder pads only with no other equipment, practice capped at around two hours, and four breaks per hour. Unacclimatized athletes should not do strenuous work at all.
Are flag conditions the same as military heat categories?
They map onto each other closely. The military numbers its categories 1 through 5 and attaches work-rest cycles and water intake amounts to each; the flag colors are the visual shorthand for the same ranges. See the heat category chart.
Who decides the flag at a school or worksite?
Whoever your policy names, and the policy should name someone. In athletics it is usually the athletic trainer; on a jobsite it is usually the competent person or site safety lead. The key is that the role has authority to stop activity.
Related WBGT guides
Know the real heat number
Real-time WBGT, flag levels and forecasts. Free on iPhone.
