It’s official: It’s a heat wave. Boston reached 92 degrees at 10:30 a.m. Sunday, the third straight day of 90 degrees or more, the definition of a heat wave. Above (from left), friends Emma Gresh, Marh Penn, and Katie Bova, along with Dave and Chantal Reed sought relief at the Esplanade. The heat is predicted to last through the week. B1, B13.