Temperatures reached 90 degrees in Philadelphia Monday as 130,000 students returned to start a new school year, with the heat index forecast to reach above 100 degrees. School officials declared that the school days will end at 1 p.m. on Tuesday and Wednesday, the Philadelphia Enquirer said.

Citing high temperatures, schools are also shutting down in other parts of the East Coast, including a number of cities in Massachusetts, New Jersey, Connecticut and Ohio. The forecast for many cities in the region calls for temperatures in the mid-90s, with some experiencing highs in the triple digits.

School children used to celebrate snow days when winter storms made it hazardous to get to and from school, but these days it’s also increasingly likely that school is being canceled because of relentless heat waves. Research shows that school days lost to snowstorms don’t hurt student progress, but they can eat up the school year, which is why some schools—like those in Philadelphia—are starting before Labor Day this year.

The summer of 2018 has seen record temperatures not just in the U.S., but in Japan, South Korea, Sweden, Algeria, and Scotland. A paper published this month on Nature’s web site forecast that the “anomalously warm” temperatures the world is experiencing this year will be with us for the next five years.