Baltimore County's 37 schools without air conditioning will be closed on Friday because the heat index is forecast to approach 100 degrees.
A recently adopted county policy requires those schools to close when the heat index is forecast to reach at least 90 degrees.
The National Weather Service is forecasting temperatures in the lower 90s and high humidity by early Friday afternoon. That translates to a heat index of 98 or 99 degrees, according to the weather service.
Superintendent Dallas Dance said he is also concerned about the forecast of high temperatures next week, which could require him to close schools without air conditioning the entire week. Highs are forecast around 90 degrees forecast through the first half of the week and humidity pushing the heat index well into the 90s.
A county policy passed in early August requires the superintendent to make the decision to close schools by 8 p.m. the day before the forecast is expected to reach at least 90 degrees.
The rest of the county's 173 schools have air conditioning and will remain open despite the forecast.
County schools opened Wednesday for the 2016-2017 school year.
On Thursday, classrooms remained open despite heat indices in the lower 90s around the region. By 2 p.m., the heat index reached 91 degrees at Baltimore/Washington International Thurgood Marshall Airport, the region's point of record.
In Baltimore City, where many schools also lack air conditioning, classes begin Monday. School closures are made at the discretion of the city schools CEO.
In other school districts around the region, all facilities are air-conditioned. Students in Anne Arundel and Harford counties also went back this week. Classrooms in Howard and Carroll counties reopen Monday.