Wet bulb, heat index or dew point: which number matters

At 100 degrees Fahrenheit with a dew point of 70, the National Weather Service's heat index reads 108. Its wet bulb globe temperature, for the same air on the same afternoon, reads 90. Eighteen degrees apart, and only one of those numbers is telling a roofer whether to come down.

Three humidity-adjusted heat measurements crowd the American summer forecast: dew point, heat index, and wet bulb globe temperature. They are not competing estimates of the same thing. Each answers a different question, and the one most people quote is the one built on the most assumptions.

"It can be confusing, having multiple parameters or indicators of heat and heat stress."

Kimberly G. McMahon, public weather services program manager at the National Weather Service, speaking to Scientific American

What is wet bulb temperature?

Strip away the jargon and it is a thermometer with a wet sock on it. The Met Office describes wet bulb temperature as the lowest temperature that can be achieved through evaporative cooling, measured with a thermometer whose bulb is wrapped in a moist muslin wick. Air flows over the wet surface, water evaporates, evaporation steals heat, the reading drops.

How far it drops is the whole point. Dry air pulls a lot of moisture off the wick and the wet bulb reading falls well below the air temperature. Saturated air pulls almost none. On a foggy morning at 100 percent relative humidity, evaporation stops entirely and the wet bulb temperature equals the ordinary air temperature.

Your skin runs the same experiment every time you sweat. A wide gap between air temperature and wet bulb temperature means sweat is working. A narrow gap means it is not.

Wet bulb temperature vs dew point: what is the difference?

Dew point is the temperature the air would have to be cooled to before water vapor starts condensing out. It measures moisture content, not heat stress, and it has one useful property: it does not move up and down with the temperature the way relative humidity does.

That makes it the quickest gut check available. Scientific American's rough guide, sourced from the weather service, runs like this: below 55 degrees is dry and pleasant, 55 to 65 is starting to get sticky, above 65 and it can feel like you are swimming through the air. Once dew points hit the 70s, oppressive is the word.

Wet bulb temperature always sits between the dew point and the air temperature. Dew point tells you how much water is up there. Wet bulb tells you how much cooling you can still buy with it.

Why does the heat index assume you are standing in the shade?

Because it was designed that way in 1979, and almost nobody reads the fine print. The heat index comes from Robert G. Steadman's paper "An Assessment of Sultriness," and the weather service's own comparison page lists the 21 parameters and assumptions baked into it: body mass of 147.7 pounds, height of 5 feet 7 inches, actively walking at 3.1 mph, wearing pants and a short sleeve shirt, standing in the shade.

Step into the sun and the model breaks. Direct sunlight, the weather service notes, can add as much as 15 degrees to the heat index. It takes only two inputs, temperature and relative humidity, so it cannot see the sun, the wind, or the cloud deck.

Wet bulb globe temperature can. It is a weighted blend of three thermometers, formalized as WBGT = 0.7Tw + 0.2Tg + 0.1Td: a natural wet bulb, a black globe measuring solar radiation, and a dry bulb in the shade. Temperature, humidity, wind speed, sun angle, and cloud cover all get a vote.

That extra information is why the two numbers diverge so violently on a bright, breezy, low-humidity afternoon.

Same conditions, two very different warnings

What wet bulb globe temperature is dangerous?

The measure was born out of casualties. It dates to the 1950s at the Marine Corps Recruit Depot on Parris Island, South Carolina, where recruits doing high-intensity training in high heat and humidity were falling ill in numbers, and Navy and Army doctors went looking for a better yardstick. The military still uses it. So do OSHA, college and high school athletic departments, and marathon organizers.

Jared Rennie, a NOAA research meteorologist, gave Scientific American the practical threshold: A dangerous value is 90 degrees. At that point, he said, you cannot be outside.

Florida wrote the number into law. The Zachary Martin Act, passed in 2020 and named for a high school athlete who died of heat illness, requires high school sports teams in the state to use wet bulb globe temperature when deciding whether practice can go ahead.

For everyone else, the weather service's guidance on when to use WBGT is refreshingly blunt. For day-to-day activities, heat index will serve you well. If you work outside or plan on any sort of vigorous outdoor activity in the full sun, use WBGT.

Which number should you actually check?

Start with dew point, because it is one number and it is honest. If it is in the 70s, the air has stopped helping you and everything downstream gets harder, which is a large part of why air conditioners struggle during a humid heat wave and why a heat dome parked over a region is more than a temperature story.

Then match the metric to what you are about to do. Sitting on a porch: heat index. Pouring concrete, running two-a-days, cutting grass at noon: WBGT, every time. And if you are about to spend a week doing any of that for the first time this summer, the number that matters most is not on the forecast at all, because your body has not adapted to the heat yet.

McMahon's framing is the one worth keeping. Heat, she told Scientific American, is a really personalized hazard. The forecast gives you three numbers because your body is not the 147.7-pound person walking in the shade that the heat index was built around.