Hot tropical air continues to blow upward from the Gulf of Mexico, making today the fourth consecutive day of over-90 degree weather—and the fourth day of the season’s second heat wave.

Most of Eastern Massachusetts, including Boston, will see a high in the lower 90s today, said Charles Foley, a meteorologist for the National Weather Service, and a heat advisory for the region remains in effect. The dewpoint, which measures moisture in the air, is roughly 70 degrees.

“That means the heat is oppressive,” Foley said. “Since the air already has a lot of moisture, it won’t evaporate your sweat. Around the area, it’s going to feel like New Orleans or Cuba or Miami.”

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There will be some relief on the south coast and Cape Cod, he said, where temperatures will likely peak in the 80s or, in some cases, the high 70s. Elsewhere, the heat wave is expected to continue into Sunday, he said, though there will be a higher chance of rain and temperatures may not be quite as high.

To deal with an uptick in calls, EMS had two extra trucks on patrol Friday and again today, said Nick Martin, spokesman for the Boston Public Health Commission. On Friday, EMS received about 400 calls, he said, up from the average 340. In the system used to log calls, there were 12 cases Friday where heat was noted specifically, he said, and, as of 3 p.m. today there were nine.

“Things are calm at the moment,” he said. “Today’s volume is definitely lower than the previous two days.”

EMS has seen no extreme cases of heat-related illness, like heat stroke or altered mental status, Martin said. He said most calls are from people feeling dizzy, weak, or dehydrated. Children and the elderly are particularly susceptible, though precautions can be taken, including wearing sunscreen, drinking plenty of fluids, and avoiding strenuous labor.

The mayor’s office issued its own heat advisory Friday, urging residents to stay in shady or air-conditioned spaces. Boston Centers for Youth & Families have become cooling centers, with air-conditioned space and available water, and will be open today until 8 p.m.

As of 5 p.m. Saturday, 2,534 people had visited 25 of the sites, said Emilee Ellison, a spokeswoman for the mayor.