By Kate Raddatz, Good Question Intern

With the heat of summer upon us, we hear a lot about the heat index in our weather forecasts these days. But what exactly is the heat index and how is it calculated?

“It’s kind of the summer version of the wind chill,” said WCCO meteorologist Mike Augustyniak. “Any kind of excessive heat, advisory watch or warning, is based off the heat index.”

The heat index, or apparent temperature, tells us what the temperature feels like after accounting for the amount of moisture in the air. But it’s not humidity that determines the heat index.

“The only direct measure of how much moisture is in the air is the dew point — that’s what is taken into account when you’re calculating the heat index,” said Augustyniak.

The dew point measures the weight of the water molecules (humidity) in the air. Anyone can calculate the heat index if they know the temperature of the air and the dew point.

The dew point temperature is the temperature at which the air will condense the water vapor into a visible form. The higher the temperature of the air and dew point, the higher the heat index.

“You really start to notice it when we’re at 70 [degrees] or above,” he said.

While we can feel the difference of the heat index, nonliving things are only affected by the temperature of the air.

“The only thing that fuels the heat index is something that’s alive. If the head index goes up, that wouldn’t cause the roads to buckle,” he said.

Just like the wind chill can’t be higher than the actual temperature of the air, the heat index can’t be lower than air temperature. The higher the heat index, the hotter we feel outside.

“We feel uncomfortable when it’s more humid and there’s more moisture in the air because our perspiration doesn’t happen as effectively. The cooling system can’t work as well,” he said.

If you want to calculate the heat index on your own, you can enter the temperature and dew point values from the WCCO website into this calculator from the National Weather Center!

1. Jen says:

Good article, but I have to point out an error. You state:

“Just like the wind chill can’t be higher than the actual temperature of the air, the heat index can’t be lower than air temperature.”

While this is true for wind chill, it is not true that the heat index can’t be lower than the air temperature. If the air is dry enough (low dew points, such as in the Desert Southwest/Arizona-New Mexico), the heat index will be a few degrees below the air temperature. It’s most typical when the dew point is in the lower 50s or lower. In those cases, the air is dry enough that a person’s sweat will evaporate more readily, thus having a minimal cooling effect. Try some numbers in the heat index calculator you linked to see for yourself.