Not a single official temperature record has been set in Philadelphia since Memorial Day — and that one was for a record-low daily maximum for the date, 54 degrees. It hasn’t come close to hitting 100 in the last three months.
Yet for discomfort, the summer of 2021 evidently ranks among the elite. Based on an analysis of absolute moisture in the air, as measured by the dew points, this could become the muggiest summer since 1995.
“The dew points have definitely been up,” said Jonathan O’Brien, meteorologist at the National Weather Service Office in Mount Holly, and the levels have been particularly notable for their “duration.”
“You usually get some breaks,” said Paul Walker, senior meteorologist with AccuWeather Inc., but “it’s pretty darn consistent.”
Heat and humidity are forecast to persist the rest of the workweek with highs in the 90s and heat indexes flirting with triple digits. A weather service “heat advisory” was in effect for Wednesday. A tepid break is possible this weekend, but with Canadian air masses having trouble getting through customs, it is unlikely that the air will get a decent cleansing before August ends.
The atmospheric moisture — and worldwide warming may well be a conspirator — has been contributing to the frequent rains, which in turn have contributed to the moisture, which in turn has been a challenge to the body’s ingenious cooling system.
When sweat evaporates, it gives off a cooling effect, but if the air is moisture-laden, sweat can become a dripping source of torment. Think of an air-conditioner during a power failure.
It’s not the humidity, it’s the dew point
Dew point is the temperature at which water vapor in the air condenses into liquid. If the overnight temperature reaches 70, and the dew point is 70, dew will form on the grass and your car.
Meteorologists view it as a far better indicator of how much moisture the atmosphere is holding, as opposed to the more popular “relative humidity,” which is relative to the temperature. When it’s 90 degrees, the air can hold a lot more water vapor than when it’s 70.
» READ MORE: Why it’s not the humidity ... it’s the dewpoint
The humidity was forecast to approach 100% early Thursday with readings in the low 70s, but it will feel a whole lot hotter in the afternoon when humidities fall to the mid-50s and temperatures are in the low 90s.
Dew points in the afternoon will rise toward the steamy mid-70s, so chances are if you walk outside with a glass of water, beads of water will form on the outside of the glass.
High dew points have been a staple of recent summers, based on a study of hourly data dating to the summer of 1948 using the Pennsylvania State Climatologist database. In the last four summers, the dew point temperature has averaged just over 64; the weather service considers 65 “oppressive.”
The values have fluctuated from year to year, but the overall average the previous 70 years was 62.5.
Through Monday, the average dew point this summer was 64.8, on pace to be the highest since the 66.4 of 1995, one of the deadliest summers on record in Philadelphia.
Along with the discomfort, the low-grade steam-bath conditions have contributed to showers, thunderstorms, and assorted downpours, with rainfall well above normal.
“That’s been the story the past couple of summers,” said O’Brien, and rain has begotten rain.
“The soil moisture is very high,” he said. “You get a big transpiration component.” That returns some of the moisture to the atmosphere. That also would explain the lack of record highs: Some of the solar energy that would be used to heat the ground would be diverted to evaporation.
“That does keep the temperature down,” said O’Brien. “That’s a trade-off.”
Said AccuWeather’s Walker: “You don’t get the 100s like we’ve seen in past summers.”
» READ MORE: Nights in Philly getting more sultry, data show
But it also keeps the temperature up at night, which can be especially dangerous to people who live alone without air-conditioning and have respiratory or heart conditions or other ailments.
Water vapor inhibits daytime heating from escaping into space after sunset.
Philadelphia hasn’t set any high-minimum temperature records this summer, but so far the temperature failed to get below 75 on 11 dates.
That’s in step with the trend. In the last decade, on average the temperature has stayed above 75 an average of 15 dates, outdoing even the deadly 1990s, when the average was about 12, and the highest of any of the last seven decades.
That trend likely has something to do with urban heating and the site of the thermometer at Philadelphia International Airport, O’Brien said, but increased water vapor also has been a factor.
Various studies have shown nighttime temperatures rising nationwide, likely the result of rising temperatures: Warmer air can hold more moisture.
Relief, not coming soon
The moisture isn’t going anywhere in the near future, forecasters say. Shower chances pop up Thursday through Saturday as a somewhat misnamed “cold front” approaches.
Temperatures on the weekend will back off into the 80s, but dew points will remain high.
In short, said Walker, wait until next month.
“Look at it this way,” he said, “we don’t have much more of this month to go.”