Not your grandparents’ weather: These charts show why Philly summers are hotter than in the 1970s

Philadelphia might not be hitting the record temperatures seen in Europe this week, but it’s definitely in the midst of summer swelter that even a cherry wooder ice might not soothe. The heat wave will culminate this weekend with the thermometer hovering at about100 degrees.

These strings of hotter days, marked by warming summer nights, have helped push long-term temperature averages up over the years under a changing climate.

And Philly is feeling it.

Average summers in the city are now about 3.1 degrees warmer than in 1970, according to data analyzed by The Inquirer and Climate Central, a nonprofit comprising scientists and journalists who research climate science, sea-level rise, and extreme weather. But, maybe worse, nights have warmed 3.9 degrees, The Inquirer found.

July has been brutal — up 3.7 degrees over the same time period (1970 to 2021), according to the analysis, which used a standard statistical technique known as linear regression to demonstrate change over time.

The data downloaded from the National Oceanic and Atmospheric Administration’s Northeast Regional Climate Centers indicate a clear trend when the rate of temperature change is measured over the last 52 years. Climate Central chose a base year of 1970 because it allows for more uniform measurements with other weather stations across the United States.

Climate Central looked at weather data for 235 locations nationwide and found that 96% saw increases in average summer temperatures. More than half warmed by 2 degrees or more for meteorological summers, defined as June, July, and August.

The West, now in a severe drought, has been hit the hardest. Reno, Nev., saw in increase of 10.9 degrees. Las Vegas is up 5.8 degrees, followed by Boise, Idaho, at 5.6.

Jennifer Brady, analysis manager at Climate Central, said the Central Plains region had not been warming until recently, helping keep the overall U.S average down.

Brady, who hunts for patterns, said simply eyeing data, or comparing end points, doesn’t help because temperatures bounce up and down year to year, with some lower than others, so the progression isn’t a straight line.

Data show minimum temperatures in the 1970s for Philadelphia tended to hover around the low to mid-60s, with some poking above. But since the 2000s, they’ve consistently run in the high 60s. For 2020 and 2022, the average nightly temperature for Philly was 69 degrees. The low forecast for Thursday night into Friday is 74, according to the National Weather Service.

Rising temperatures, coupled with warmer nights, can be much harder on people living in cities such as Philadelphia, which has large pockets of heat islands — neighborhoods hotter than others because pavement, asphalt, parking lots, and residential and commercial buildings absorb and retain heat then radiate it back out.

Some neighborhoods in Philly can run 20 degrees warmer than others because of a lack of shade from tree canopy — the layer of leaves, branches, and stems. Those neighborhoods tend to be marked by block after block of rowhouses with flat, black, heat-absorbing roofs.

» READ MORE: Why Philly trees cast more shade on the wealthier

The Hunting Park Neighborhood Advisory Committee surveyed residents last year about how they cope with heat and asked how many had air-conditioning. Out of 563 who answered that question, only 100 had air-conditioning.

There is little other relief. Not all of the city’s 65 public swimming pools opened amid a national lifeguard shortage.

Philadelphia is currently assembling a Philly Tree Plan, a 10-year strategy to grow its tree canopy as a way of combating climate change. In 2008, the city had 18,450 acres of trees but had lost 1,095 acres by 2018. The loss was greatest in residential areas. The densest canopy is in the northwestern and northern parts of the city dominated by green spaces such as Wissahickon and Pennypack Parks.

» READ MORE: Philly asks residents what they think of trees for city’s 10-year ‘urban forest’ plan

City Councilmember Katherine Gilmore Richardson introduced a companion bill in May to the tree plan that, among other things, would require developers to plant or preserve trees for new projects on lots 5,000 square feet or bigger. The bill still needs final adoption.

More trees could help those in the city coping with more hot days. Philadelphia summers are averaging 19 more days with temperatures above normal compared with five decades ago. Excessive heat makes it difficult for people to cool down. Too much heat can cause cramps, heat exhaustion, and heatstroke.

» READ MORE: Here’s what to know about symptoms and treatment for dehydration and heat exhaustion

Researchers from Drexel University have teamed up with the William Penn Foundation and the Hunting Park community to find ways to beat the heat: Professor Franco Montalto, for example, led a team that helped cool the 4400 block of North Marshall Street and recruited residents to help install sprinklers, planters, and umbrellas.

Heat worsens air quality. This week, the Pennsylvania Department of Environmental Protection alerted residents the air quality had reached an unhealthy “orange,” part of the federal Environmental Protection Agency’s color-coded air-quality index, which marks average concentrations of ozone levels. Ozone forms more readily on hot days when nitrogen oxides and volatile organic compounds react with each other.

The code orange triggered an air quality action day, which is a warning that children, the elderly, and those with respiratory problems such as asthma, emphysema, and bronchitis are especially vulnerable.

Saleem Chapman, Philadelphia’s chief resilience officer, said the rising summer temperature data jibe with what city officials see.

“We’re definitely seeing the trajectory of our temperature warming over time,” Chapman said. “And that is concerning.”

He said the impact falls hardest on low-income residents, who either don’t have air-conditioning or are afraid to use it because of the cost.

“Philadelphia is one of the most energy-burdened cities,” Chapman said, referring to people who have to spend a large part of their income on gas and electric.

The city has cooling centers for people to go to, he said, but some residents might have difficulty getting there. So the city has teamed up with SEPTA to provide cooling buses.

Nights are particularly difficult, Chapman said, as the cooling centers are closed and people are largely trapped in their homes.

“The difficult thing for Philly is that those communities who are most exposed to heat risk are most sensitive to it.”