It’s been way muggier than this in Philly, but the trends are disturbing

If you feel as though the atmosphere caressing Philly these last few days has become a water-vapor brewery, be assured that the heat isn’t getting to you.

Yes, it has been mightily muggy. Reader Corey Riddle posed a question through Curious Philly — The Inquirer’s forum where readers ask us questions, and our reporters hunt down the answers — that we suspect is on the minds of many people in the great Philadelphia city-state:

“Seriously, has the humidity in the region ever been this bad for this long? To my memory the humidity this summer has been the most intense and sustained that I’ve ever known.”

» ASK US: Have something you’re wondering about the Philly region? Submit your Curious Philly question here.

Seriously, it actually has been steamier around here in summers past, but it may have felt worse this week: Thermal comfort is relative.

“We had such a long, cool spring that went into the early part of June, a lot of people may not be quite as acclimated as they would be by late July,” said Sarah Johnson, the warning coordination meteorologist at the National Weather Service Office in Mount Holly.

That said, an Inquirer analysis showed a rather robust trend in increasing sultriness in Philadelphia this century, and that trend may have consequences for how it actually feels out there.

Like thermal comfort, humidity is “relative” and thus can be misleading. It’s a measure of how much water vapor is in the air relative to how much it could hold. The warmer the air, the more moisture it can hold.

When the heat index reached 101 on Thursday, the temperature at Philadelphia International Airport was 93, and the humidity was an unimpressive sounding 50%. At 6 a.m., it was 94%, but it was 74 degrees, and at that temperature it was near saturation.

» READ MORE: These neighborhoods get the worst of Philly heat

The “dewpoint” is a measure of absolute moisture in the air. It’s the temperature at which the air would be saturated and water vapor would condense into droplets, or more or less comes out of hiding.

On Thursday afternoon, the dewpoints were in the low 70s and again around 70 on Friday. If you had taken a glass of iced tea outside, the glass would have been bleeding water because the temperature of the glass would be well below the dewpoint.

Afternoon dewpoints of 65 and over are considered “uncomfortable,” by weather service standards, with 70 and over approaching oppressive.

Water-vapor content is a major driver of the heat index. For example, around 10 a.m. local time in Phoenix on Monday it was 103 degrees. In Miami, around 10 a.m. local time it was 90. The heat index in Miami was 103. The heat index in Phoenix was … 103. In bone-dry Phoenix the dewpoint temperature was 53. In Miami, where the air is almost always juicy, it was 77.

Thus far the highest dewpoint reading at the airport this week has been 74. That will soak the average shirt, but Ray Martin, a lead meteorologist at the Mount Holly weather service office, can tell you firsthand about July 15, 1995.

At 5 p.m., officially it was 100.9 Fahrenheit, and the dewpoint was 82.9, yielding a heat index of 128.

» READ MORE: Strangely, summers have been hotter but heat-wave deaths are down

“Boy, do I remember that day,” said Martin, a Trenton-area native. “I was a paperboy out collecting. It was awful.”

Deadlier heat waves in Philadelphia occurred in the summers of 1993, 1995, and 1999 than in any of the 21st-century summers. No heat-related deaths have been reported in Philly this year.

Heat waves haven’t been lasting as long in recent years, even as summers are getting warmer, and that paradox may speak to the effects of climate-change worldwide warming.

An Inquirer analysis of hourly dewpoint readings, based on available data from the Pennsylvania State Climatologist office, suggests a rather significant uptick in the mugginess that is such an important component of the heat indexes and discomfort levels.

The annual average of the number of hours that the dewpoints were 70 or higher at the official Philadelphia International Airport observing station have been about 9% higher this century, compared with the 1951-2000 period.

“That makes sense,” said the weather service’s Martin, given that a warmer atmosphere can accommodate more moisture.

Global temperatures in 2022 were about 1.6 degrees higher than the 20th-century average, according to the National Centers for Environmental Information. July is on pace to become the warmest month on record, the World Meteorological Organization said Thursday.

The added moisture may be capping local temperatures in an area that is in close proximity to the Atlantic Ocean and the Delaware Bay.

On Friday, readings were a few degrees below the forecasted highs, and that may have been related to all the additional foliage-retained moisture from those downpours of the last few weeks. The sun can’t heat as efficiently when its energy is diverted to evaporation.

After one more shirt-soaking Saturday with temperatures again the 90s, the forecast calls for a round of showers to herald the passage of a cool front.

Sunday should be gorgeous, with highs in the 80s, and a sequence of clones with comfortable dewpoints, not to mention humidity.