The character is composed of two "火" (fire). The image of one fire piled on the other apparently indicates the fire is getting bigger. Later, it is used to refer to something broiling
The glowing summer is called "yanxia" (scorching summer). In ancient Chinese poems, there is such a line as "The hot bright sun is just like a fire that burns the paddies semi-dried in the field", which means the summer is extremely hot as if being scorched by the fire.
