It’s not just your imagination: July has been Baltimore’s hottest month in our lifetimes.
The 25 days of 90-degree temperatures in one month recorded at BWI Marshall Airport as of Thursday set a 148-year record dating to Civil War hero Ulysses S. Grant’s first term as president, long before the airport was built and became used as the point of record for the region.
Baltimore’s sweltering July was a result of a long-lasting, subtropical area of high pressure in the western Atlantic Ocean known as a “Bermuda High,” according to Dan Hofmann, a National Weather Service meteorologist.
“That kind of pattern puts us in a hot, humid air mass, and there really hasn’t been much of anything to break it,” Hofmann said Thursday.
The 90-degree trend, which surpassed the previous, 24-day record set in July 2011, is expected to end Friday, with a high of 82 degrees and rain in the forecast, according to the National Weather Service.
But Marylanders should expect more long, humid stretches in years to come because of climate change, according to an expert at the University of Maryland Center for Environmental Science in Cambridge.
“It isn’t going to get better,” said Victoria Coles, associate professor of physical oceanography.
Coles said her latest research focuses on the climate extremes rather than averages, in Baltimore and the other areas surrounding the Chesapeake Bay since 1900.
“Is that changing?” she said. “That’s really what affects human health, not the mean temperature change, but the actual temperature on a given day. ”
The researchers noticed a troubling, if not surprising, pattern over the past century: While the hottest temperature measured each year didn’t trend upward noticeably, there was a trend of more warm days and more warm nights.
“It’s the difference between intensity and duration,” she said.
Coles’ research also revealed strong patterns showing increases in the number of “tropical nights,” in which temperatures stay hot after the sun sets, she said.
“In particular, nighttime temperatures can be problematic for vulnerable populations,” she said. “The people sweating outside, working, are pretty habituated to hot summers in Maryland. I really worry about these nighttime temperatures and individuals who can’t afford air conditioning and, when they come into the house at night, really suffer unrelenting heat.”
The University of Maryland Center for Environment Change researchers compared the predictions generated by 10 models from the Intergovernmental Panel on Climate Change to the actual temperatures in the area over the past century, Coles said.
They used their findings to predict the potential extreme temperature increases over the next 100 years, both with and without reductions to global carbon emissions.
“It’s exactly what you would expect, to some extent,” Cole said. “As mean temperatures increase, we’re also seeing increases in these extreme event indices.”
This recent stint of hot weather coincided with surging coronavirus numbers across the U.S., which has killed more than 150,000 Americans, decimated the national economy and forced Marylanders to wear masks inside businesses and outdoor areas.
In Baltimore, COVID-19 also has forced officials to cap the capacity at 25% in the city’s 10 cooling centers, which offer refuge on extremely hot days, said Heang Tan, deputy health commissioner for aging and care services.
Despite the limited space, attendance has been low at the cooling centers during the seven days declared a “Code Red” for extreme heat this summer, Tan said.
Among the COVID-19 precautions, all the cooling centers have received deep cleanings, and staff members and visitors are required to undergo temperatures checks and health screenings before entering, Tan said. While no one has been turned away from a cooling center this summer, free transportation to another location is available to any visitors who arrive to find one full, she said.
“We wanted to make sure safety of our staff as well as the residents that attend cooling centers is first and foremost,” said Tan, who oversees the cooling centers for the Baltimore City Health Department. “Our cooling centers are available, and we are providing a safe space for residents during this very hot month.”
Hofmann, the National Weather Service meteorologist, said nearly whole months of 90-degree days are “definitely unusual” for Baltimore, “given the fact it’s only happened a dozen or so times at the site in a century and a half.”
But those hot stretches have happened more frequently over the past 40 years — and even more in the past decade, he noted.
Ten of the 12 months in Baltimore’s history with 20 or more days of 90-degree temperatures have occurred since 1980, Hofmann said. Six of those months have been since 2010, three of them since 2015, he said.
Baltimore/Washington International Thurgood Marshall Airport has been the point of reference for temperature readings since 1950; temperatures previously were taken in downtown Baltimore, and the National Weather Service’s records date to 1872.
All National Weather Service temperature readings are considered preliminary until they are officially confirmed by the National Centers for Environmental Information in Asheville, N.C.
The weather service is also “keeping an eye on” Tropical Storm Isaias, which is approaching the Dominican Republic and could continue up the East Coast of the United States, Hofmann said.
The mountainous Caribbean island could disrupt the storm and cause it to lose momentum, he said.
“We’re going to have to see what’s left of it in the next 24 hours,” Hofmann said. “It does look like we’re going to have waves of thunderstorms and showers this weekend and into early next week.”