The mugginess in the Philly region has been unrelenting, and near historic

When the sun-cooked air feels like it’s too full of moisture to bother moving, Jeff Kochanowicz has experienced an altogether unpleasant sensation when he brings his bike to a stop.

“You feel like a French fry under a heat lamp,” said Kochanowicz, president of the Bicycle Club of Philadelphia.

It’s perhaps less-than-small compensation, but bicyclists, runners, people who work outside, and just plain pedestrians have been experiencing something special.

Based on an analysis of the moisture content of the Philadelphia air as measured by the hourly dew points, this was the third-sultriest June 1-to-July 15 period in records dating to 1948. And it looks like the mugginess is going to persist, along with the episodic rains.

Thursday is going to be another steamer, with temperatures heading into the 90s and a heat advisory in effect for triple-digit heat indexes.

With the warming of the planet, the atmosphere is capable of holding more moisture. But does it have to hold it all here?

The mugginess, along with the attendant downpours, has been related to a traffic jam in the atmosphere and above-normal sea-surface temperatures in the Gulf and the Atlantic, said Art DeGaetano, director of the Northeast Regional Climate Center, based at Cornell University.

Whatever the cause, “It’s been brutal,” said cyclist Howie Wiener, the bicycle club’s secretary, for whom this has been particularly challenging. He was on vacation above the Arctic Circle in Norway and had the pleasure of returning to Philly just in time for the worst of it.

It was right around the solstice that the atmosphere turned up the steam valves along the I-95 corridor.

That is clearly evident in the dew point data from the official (albeit imperfect) National Weather Service station at Philadelphia International Airport.

The axiom “it’s not the heat, it’s the humidity” is not quite accurate. Humidity is relative to what the atmosphere can hold at a given temperature. The warmer the air, the greater the moisture capacity.

Typically on a summer morning with a temperature around 70 degrees, the humidity is near 100% because that’s as much as the air can hold. When it’s 100 degrees, rarely would the humidity be above 50%.

The dew point is an absolute measure of moisture. It is the temperature at which water vapor condenses and comes out of hiding in liquid form. Water streams down the surface of a glass of ice water because the air around it is cooled to the dew point.

High moisture levels inhibit cooling. The body gets relief when sweat evaporates and gives off a cooling effect. When the air is moisture-clogged, sweat can’t evaporate and becomes an unpleasantly warm coating.

“If it can’t evaporate, then you get in big trouble,” Wiener said.

The weather service has no official standards on dew point comfort levels, said Paul Fitzsimmons, a lead meteorologist in the Mount Holly office. The body can acclimate to higher dew points over time, he said, but 70 is a solid standard for mugginess.

In the June 1-July 15 period, dew points were 70 or higher 45% of the time, a percentage ranking behind only 1973 and 1994 in the 77 years of available data.

Waters in the Gulf and off the Southeast coast have been warmer than normal, DeGaetano said, and they have worked in tandem with the so-called Bermuda high over the North Atlantic.

Winds circulate clockwise in high pressure, and areas to the west of the center experience warm, moist winds from the south and southwest. The heavier air of high pressure also can block systems from moving.

Fronts approaching the region have had a tough time getting here, but some have come close enough to trigger thunderstorms and heavy showers.

The worst of it appears to have kicked in around here with the arrival of a hot spell that began June 19.

Wiener returned from Norway on June 23, just in time to experience Philadelphia’s first 100-degree reading in 13 years. In the June 19-to-July 15 period, dew points remained in the 70-plus level two-thirds of the time, the Inquirer analysis showed.

The summer of 2025 is on track to rank among the top five muggiest, and while the airport is not an ideal measuring station and a variety of factors can contribute to steaminess in a given season, including the “urban heat island” effect, oppressive conditions in urbanized areas are becoming more common as the planet warms.

Not Thursday. A heat advisory is in effect for heat indexes as high as 102 in Philly. Highs are forecast to be in the low 90s, with more showers possible.

However, the dew points are due to fall below 65 Thursday night and might not climb back to 70 until Saturday night.

Few things are more changeable than a summer forecast, but both Friday and Saturday may be dry.

In the meantime, Kochanowicz advises cyclists not to overdo it and stay hydrated.

Said Kochanowicz: “This kind of stickiness does indeed kind of take some of the fun out of riding, although it doesn’t stop us.”