Spell of hot weather (4,4)
(Other definitions for heat wave that I've seen before include "Prolonged period of very warm weather" , "Period of unusually hot weather" , "Long spell of hot weather" , "Continuous spell of hot weather" , "Long period of hot weather" .)
(Other definitions for heatwave that I've seen before include "Torrid spell" , "unusually warm spell" , "Torrid time" .)