The county's 122 public elementary, middle and high schools will close three hours early today.
Morning and afternoon kindergarten classes have been canceled.
The Board of Education announced the closings yesterday afternoonafter weather forecasters projected continued high temperatures and humidity.
The National Weather Bureau at Baltimore-Washington International Airport reported that afternoon temperatures reached 97 degrees yesterday.
Today's forecast calls for hot and humid conditions, with a chance of afternoon thundershowers.
"It's not so much the heat as the humidity" that led to the school closings, board spokeswoman Jane Doyle said.
Lunch schedules will be adjusted so that students will eat before they leave school.