It’s late summer. The sun has been blazing all day, and you and your kid have been dreading soccer practice.

Then your phone buzzes. It’s a text from the coach.

Practice is still on, even though the heat index is around 100 degrees.

So what gives?

Heat index is combines two things: air temperature and humidity. It can help you understand how hot it feels, but it does not tell the full story when it comes to outdoor sports.

That’s where wet-bulb globe temperature, or WBGT, comes in.

WBGT takes into account air temperature, humidity, wind, cloud cover and sun angle. That makes it a better tool for understanding how your body may respond to heat during outdoor activity.

Put simply: heat index tells you how hot it feels. WBGT gives you a better idea of how hard it is for your body to cool itself down.

WBGT levels and outdoor sports

WBGT guidance can vary by region, school, league and activity level, so always follow local officials, coaches and athletic trainers.

But in general, as WBGT rises, the risk of heat illness increases.

WBGT in the 70s

It’s hot and humid, but most outdoor activity is usually still okay with precautions.

Take water breaks and spend at least 15 minutes each hour resting in the shade, AC or another cooler place.

Heat stress may begin after around 45 minutes of working or exercising in direct sunlight.

WBGT around 80

This is when the risk starts to climb.

Plan for at least 30 minutes of breaks each hour during strenuous outdoor activity. Practices may need to be shortened, moved to shade or adjusted with more frequent water breaks.

Heat stress may begin after around 30 minutes in direct sunlight.

WBGT around 85

This is a higher-risk range.

Strenuous outdoor activity should be limited, especially for athletes wearing heavy equipment or doing repeated high-intensity drills.

Plan for at least 40 minutes of breaks each hour in AC, shade or another cooler area.

Heat stress may begin after around 20 minutes in direct sunlight.

WBGT near 90 or higher

This is the danger zone.

Outdoor activity should be avoided, postponed or moved indoors until conditions improve.

At this level, heat stress can begin in 15 minutes or less, and the body may not be able to cool itself fast enough during intense activity.

Why WBGT matters

WBGT may be new to you, but it has been around for decades.

In the 1950s, the U.S. Army was seeing high numbers of heat illnesses, including heat exhaustion and heat stroke, in training camps.

To help reduce those illnesses, scientists studied the conditions soldiers were training in and how their bodies reacted to heat.

They found that sweating was only part of the equation. If outdoor conditions made it harder for sweat to evaporate, the body could start overheating quickly.

That research helped lead to what we now know as wet-bulb globe temperature, or WBGT.

WBGT vs. heat index

Heat index is still useful, but it has limits.

It uses air temperature and humidity, and it is based on shady conditions.

WBGT goes a step further by factoring in the sun, wind and cloud cover. That is why it can be especially helpful for outdoor sports, marching band, outdoor work and summer camps.

And just because the heat index is high does not always mean the WBGT will be just as high.

For example, while you may see a heat index near 90, your coach may be looking at a WBGT closer to 77 because of lower humidity, light winds or a few clouds.

With extra water breaks, shade breaks and close monitoring, practice may still be safe.

WeatherBug Takeaway

Heat index tells you how hot it feels.

WBGT gives you a better idea of how stressful that heat may be on the body during outdoor activity.

So before the next hot practice, don’t just ask what the temperature is.

Ask: What is the WBGT, and what changes are being made to keep athletes safe?

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