"It's not the heat. It's the humidity." Sound familiar?

I think we're all guilty of saying that at some point between June and September.

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That's why meteorologists use things like the heat index and the wet bulb globe temperature to convey how things feel.

Heat Index

The heat index isn't just some made-up number to inflate temperature readings. Yes, that's an accusation I (and I'm sure many other meteorologists) have faced in the past.

The heat index is used, because our bodies respond differently when there's more moisture in the atmosphere vs. when there isn't.

Sweat is our body's natural way of cooling off. Sweat droplets evaporate off our skin. Evaporation is a cooling process. That process is more efficient when the air is dry, and it's less efficient when the air is humid.

Why the heat index matters
Why the heat index matters (© WRAL)

Between now (June 18) and next week, there will be several days in which the heat index eclipses 100°.

Peak heat index in the Triangle through Sunday
Peak heat index in the Triangle through Sunday (© WRAL)

The National Weather Service issues Heat Advisories when the heat index is between 104 and 109° for two hours or longer. An Extreme Heat Warning is issued when it reaches 110° or higher.

When heat alerts are issued by the National Weather Service in Raleigh
When heat alerts are issued by the National Weather Service in Raleigh (© WRAL)

There's actually a mathematical equation to quantify heat index.

Heat Index = -42.379 + 2.04901523T + 10.14333127R - 0.22475541TR - 6.83783 x 10-3T2 - 5.481717 x 10-2R2 + 1.22874 x 10-3T2R + 8.5282 x 10-4TR2 - 1.99 x 10-6T2R2

T - air temperature (F)

R - relative humidity (percentage)

Wet Bulb Globe Temperature

There is another number called the Wet Bulb Globe Temperature. It is used to take other factors into account.

These factors include sunlight, wind, cloud cover and sun angle. It's often used in high school athletics and in the military.

UNC has a forecast for the WBGT here.

The North Carolina State Climate Office keeps track of this each day at multiple weather observation stations in the state.

WRAL previously spoke with Dr. Jordan Clark of Duke University about how it's used for high school athletes.

NOAA's Heat Risk

The "Heat Risk" tool is something that's been used in the Western U.S. for years, and it's now getting more widespread use.

Heat Risk takes into account the obvious meteorological factors, but it also takes into account socioeconomic factors, urban heating, among other things.

The Heat Risk in our part of North Carolina reaches an "Extreme" level next Monday and Tuesday.

NOAA's Heat Risk through Tuesday, 6/24/2025
NOAA's Heat Risk through Tuesday, 6/24/2025 (© WRAL)

This means that this level of heat poses a risk to a majority of people, but especially to those without proper cooling and those who work outdoors. It also takes into account the fact that there will be very little cooling at night.

Extreme heat risk expected Monday and Tuesday, according to NOAA
Extreme heat risk expected Monday and Tuesday, according to NOAA (© WRAL)

Have questions about the weather and how it works?

Send me an email with the subject line 'Ask the Meteorologist:' to cmichaels@jshu.net.