Cruel hot weather is the kind that's too much for anyone to handle. It's intense heat. Stranded in the desert in summer, heat.

This is a jukugo word that uses the on'yomi readings of the kanji. You didn't learn this reading for 暑 yet, so here's a mnemonic to help you:

Intense heat will show (しょ) on your face. You'll turn red and get all sweaty. You really can't hide it.