Why 90°F Can Be Deadly: A Mentor's Guide to Wet Bulb Globe Temperature (WBGT)

We’ve all said it: “It’s not the heat, it’s the humidity.”

It’s the perfect example of how the simple number on our thermometer fails to tell the whole story. A 95°F day in the Arizona desert (10% humidity) is tolerable. A 95°F day in Florida (90% humidity) is dangerous.

Why does one feel like a warm oven and the other like a suffocating steam bath?

As a mentor in this field, I can tell you it’s because a simple thermometer is only measuring one of the three factors that determine how your body actually experiences heat. If you’re a coach, a safety manager, or a worksite foreman, relying on that single number isn’t just inaccurate—it’s negligent.

To keep people safe, you have to stop thinking about “air temperature” and start thinking about the “Heat Stress Triangle.” And to measure that, you need a specific tool for the job. Let’s dive in, using a device like the Extech HT30 Heat Stress WBGT Meter as our classroom example.

Lesson 1: The “Liar” — Air Temperature (Dry Bulb)

First, let’s talk about that standard thermometer reading. In the science of heat stress, this is called Dry Bulb Temperature (Ta).

• What it is: The ambient temperature of the air.
• How it’s measured: With a standard thermometer, shielded from all sunlight and moisture.
• The Problem: It completely ignores the two most important factors: humidity and radiant heat. It tells you the temperature of the shade, but your athletes or workers aren’t standing in the shade. They’re on a sunny field or a hot asphalt roof.

Relying only on the dry bulb temperature is the #1 mistake in assessing heat risk.

Lesson 2: The “Killer” — Humidity (Wet Bulb Temperature)

This is the most important factor in how you feel heat. Your body is a brilliant machine. When you get hot, you sweat. That sweat evaporates, creating a personal, evaporative cooling system (like a tiny swamp cooler) all over your skin.

But what if that system breaks?

That’s what humidity does. Wet Bulb Temperature (Tw) is the true measure of how much cooling your sweat can actually provide. * How it works: A wet bulb thermometer has a wet cloth wick over its sensor. As the water evaporates, it cools the sensor. * In low humidity (Arizona): Water evaporates easily. The wet bulb temp will be very low. Your sweat works overtime, and you feel comfortable. * In high humidity (Florida): The air is already full of water. Evaporation is slow or impossible. The wet bulb temp will be very high, almost the same as the air temp. * The “Aha!” Moment: When the wet bulb temp is high, your body’s air conditioner is broken. You keep sweating, but you get no cooling effect. This is when heatstroke happens.

This is why in the official Wet Bulb Globe Temperature (WBGT) formula used by OSHA and other safety bodies, the Wet Bulb (humidity factor) accounts for 70% of the total heat stress calculation.

Lesson 3: The “Invisible Oven” — Radiant Heat (Black Globe Temperature)

The third piece of the triangle is the one you feel but can’t see. Black Globe Temperature (Tg) measures the heat being radiated onto your body from the sun or hot surfaces.

• What it is: Think of the heat you feel beaming off a black asphalt parking lot, a brick wall, or a piece of machinery—even if the air is cool.
• How it’s measured: A tool like the Extech HT30 uses that small black globe on top. This globe is painted matte black to absorb all incoming radiant heat, just like your skin or clothing. A sensor inside measures how hot that globe actually gets.
• Why it matters: On a sunny field, the air temp (Ta) might be 85°F, but the Black Globe temp (Tg) could be 120°F! Your body is forced to absorb all that radiant heat, adding a massive load to its cooling system.

Putting It All Together: The WBGT Index

Now you can see why you need a specialized tool. The WBGT (Wet Bulb Globe Temperature) isn’t a single measurement. It’s a calculated index that combines all three factors to produce a single number that truly represents the heat stress on a human body.

The (simplified) outdoor formula is:
WBGT = (0.7 x Wet Bulb) + (0.2 x Black Globe) + (0.1 x Dry Bulb)

Notice the weights. Humidity is 70% of the equation. Radiant heat is 20%. The simple air temperature you’ve been trusting? It’s only 10% of the real story.

The Tool for the Job: From Clunky Science to Handheld Tool

In the old days (and in official labs), you’d have to measure this with three separate, clunky devices, including a “sling psychrometer” that you physically spin around your head. This is completely impractical for a worksite or athletic field.

A modern Heat Stress WBGT Meter like the Extech HT30 is an all-in-one diagnostic tool. * It has the Black Globe (Tg) sensor on top for radiant heat. * It has internal sensors that measure the Air Temp (Ta) and Humidity, which it uses to instantly calculate the Wet Bulb (Tw). * It then does the math for you and displays the final, actionable WBGT number. * It even includes an “In/Out” function. This is critical. You select “Out” for direct sun (using the Black Globe in the formula) or “In” for indoors/shade (which uses a 2-factor formula: 70% Wet Bulb + 30% Air Temp).

A Mentor’s Pro-Tip: How to Use It Correctly (and Avoid a Common Flaw)

Now, a critical tip. One user, SLS, made an excellent observation about this class of “low-end” (sub-$500) all-in-one meters: they can “read hot in direct sunlight.”

He’s right, and here’s why. The Black Globe is supposed to get hot in the sun. But on a cheap, compact meter, the body of the tool itself gets hot, too. This heat can “soak” into the other sensors, “artificially elevating” the Air Temp (Ta) reading.

To get a more accurate reading:
1. Hold the unit so the Black Globe is fully exposed to the sun.
2. Use your own body or a clipboard to cast a shadow over the body of the meter itself (where the air temp/humidity sensors are).
3. Let the readings stabilize for a minute.

This simple step ensures you are measuring the sun’s effect on the globe and the air’s effect on the other sensors, giving you a much more reliable WBGT reading.

Conclusion: Stop Measuring the Air, Start Measuring the Risk

A simple thermometer tells you what the weather is. A WBGT meter tells you what that weather is doing to your body.

For anyone responsible for the safety of people working or playing in the heat, that single number on the weather app is a liability. It’s giving you incomplete (and therefore false) information.

To truly understand the risk, you must measure the full triangle of heat stress: air temp, humidity, and radiant heat. A dedicated tool that calculates the WBGT index is the only way to do this. It’s not just a “fancy thermometer”; it’s a critical safety device that replaces guesswork with science, allowing you to make the right call before it becomes a medical emergency.