Weathercatch: The heat index is more than a lot of hot air

2020’s first widespread heatwave brought more than intense heat to a large portion of the southern U.S. this week, as high humidity catapulted the heat index into the triple digits.

For more than 20 million Americans, the midsummer heat was miserably muggy. “Yucky” is how one meteorologist in Fort Worth, Texas, described it.

Though a number of communities in the Inland Northwest experienced their first 90-degree day of the year on Saturday of last weekend, conditions didn’t reach the yucky criteria. Thanks to our geographic location, we had to contend with the heated temperature of the air – not stifling humidity on top of it.

There’s a reason for the adage: It’s not the heat, it’s the humidity. And it’s due to the heat index, a measure of how hot it feels to the human body when the air temperature is combined with the amount of moisture in the air.

Using a blend of meteorology and biology, the heat index was developed in the early 1980s as a way to assess the threat of combined heat and humidity to the average person.

So why does humidity during a hot day make people feel hotter? The answer lies in our ability to sweat. When combined with high temperatures, humidity makes it difficult to cool off by sweating. Perspiration doesn’t evaporate as quickly as it does in dry heat.

And dry heat is just what we enjoy in our region. That’s why meteorologists seldom refer to the heat index when giving local weather forecasts. Also, when the National Weather Service issues a heat advisory or warning for our area, the heat index is typically not part of the equation. Instead, the alerts are based on tried-and-true thermometer readings.

When relative humidity gets high enough, the difference between the thermometer reading and the heat index can be impressive. For example, high temperatures ran in the mid- to upper 90s across the state of Louisiana on Monday and Tuesday. But with the humidity level factored in, the heat index reached the 108 to 115 range. Likewise, while the thermometer read 96 degrees in Dallas-Fort Worth, the air’s water vapor pushed the “feels like” temperature to 109.

Why is humidity such a prominent factor in much of the country, but not here? East of the Rockies, warm waters off the Gulf of Mexico generate a major source of humidity to much of the country’s eastern half. Conversely, cool waters off the Pacific Ocean bordering Washington state don’t produce nearly as much water vapor. Also, whenever moist airflow heads our way from the Pacific, it encounters the Cascade Range. There, the air becomes drier and hotter as it sinks down the western slopes. We can thank the Desert Southwest for our dry weather as well, which is a major source of our summertime air masses.

But every now and then, humidity sneaks in and surprises us. In late May, the Inland Northwest experienced warm temperatures combined with a relative humidity at 50%, a moisture level commonly seen in the Midwest. This unusual surge of humid air, combined with instability in the atmosphere and wind sheer, helped fuel what became the storm of the decade. On May 30, a complex thunderstorm system toppled trees and power lines and knocked out electricity to thousands of residents throughout the region.

Fortunately, the surge in humidity was short term, unlike the prolonged heatwave that spanned from Alabama to Texas this week. If you’ve ever spent an extended period wilting in intense heat and muggy air, then you appreciate the temperate climate of the Inland Northwest. Because yes, it is the humidity.