The two-year-old Watha T. Daniel/Shaw Library is one of nine that went dark Wednesday due to heat. (Astrid Riecken for The Washington Post)

On the hottest April 10 in the recorded history of the District of Columbia, nine D.C. Public Library branches closed because conditions became too sultry inside.

DCPL spokesman George Williams said seven of the nine branches were older buildings with climate control systems that cannot quickly be converted from heating to cooling. The remaining two branches — Deanwood and Watha T. Daniel/Shaw — had “mechanical issues which we are addressing.”

The magic number is 82 degrees: If the indoor temperature goes above that, the building closes. Williams said, despite lower forecast temperatures, that there is a chance branches could also close Thursday: “We’ll monitor the temperatures inside the buildings,” he said. “If we go above 82 degrees, those locations will close.”

D.C. Council member Muriel Bowser (D-Ward 4), who represents a ward where three libraries closed Wednesday, sent a message to listservs this afternoon pledging to “monitor these temporary closures carefully.”

The following branches closed Wednesday: Watha T. Daniel/Shaw, 1630 7th St. NW; West End, 1101 24th St. NW; Woodridge, 1801 Hamlin St. NE; Southwest, 900 Wesley Place SW; Lamond-Riggs, 5401 South Dakota Ave. NE; Deanwood, 1350 49th St. NE; Southeast, 403 7th St. SE; Juanita E. Thorton/Shepherd Park, 7420 Georgia Ave. NW; and Takoma Park, 416 Cedar St. NW.