(NEXSTAR) - Across the Eastern Seaboard, Americans are dimming the lights and drawing the blinds and blasting the fan, but for millions, it's not enough to offer relief from consecutive days of truly oppressive heat. A projected 245 million people were expected to suffer under 90-degree-plus temperatures this week, nearly three-quarters of the country's population. Some projected Tuesday as the peak of the suffering in the North East, but the timing of the relief may depend somewhat on where you live. "The ridge of high pressure that's been the source of the heat for the last week will finally start to split and break down as we head into the weekend," said Brian James, Chief Meteorologist at the Nexstar Weather Center. "One piece will move off the East Coast while the other piece will retreat to the Southern Plains." In practical terms, that means some will see relief in days, but others may have to wait through the work week. "New England will start feeling relief by Thursday. Farther south, it'll take until the end of the weekend and early next week to get temperatures to come down," said James. On Monday, New York City’s John F. Kennedy Airport hit 100 degrees Fahrenheit a little after noon, the first time since 2013. Then Baltimore, Philadelphia and Boston joined the 100 club. More than 150 million people woke up to heat warnings and forecasters at the National Weather Service expected dozens of places to tie or set new daily high temperature records Tuesday. The dangerous heat sent people to the hospital, delayed Amtrak trains and caused utilities to urge customers to conserve power. Tuesday’s heat came on top of 39 new or tied heat records Monday. But just as dangerous as triple-digit heat is the lack of cooling at night, driven by the humidity. A heat dome occurs when a large area of high pressure in the upper atmosphere acts as a reservoir, trapping heat and humidity. A heat wave is the persistence of heat, usually three days or more, with unusually hot temperatures.“You get the combination of the extreme heat and humidity but no relief,” said Jacob Asherman, a meteorologist at NOAA’s Weather Prediction Center. “It’s kind of been just everything stacked on top of itself.... It just speaks to how strong this heat wave is. This is a pretty, pretty extreme event.” The heat wave is especially threatening because it's hitting cities like Boston, New York and Philadelphia early in the summer when people haven't gotten their bodies adapted to the broiling conditions, several meteorologists said. The dome of high pressure that's parking over the eastern United States is trapping hot air from the Southwest that already made an uncomfortable stop in the Midwest. A key measurement of the strength of the high pressure broke a record Monday and was the third-highest reading for any date, making for a “near historic” heat wave, said private meteorologist Ryan Maue, a former National Oceanic and Atmospheric Administration chief scientist. “Like an air fryer, it’s going to be hot," Maue said. ”This is a three-day stretch of dangerous heat that will test the mettle of city dwellers who are most vulnerable to oppressive heat waves.” The Associated Press contributed to this report.