hot summer; sweltering summer
Refers to a summer characterized by intense heat; often used in literary or emphatic contexts.
This year's hot summer brought record-breaking heat.
midsummer; middle of summer
Specifically indicates the peak or middle period of summer, often with a nuance of intense heat.
In this midsummer season, I hope you are all in good health.
Compound of 炎 (flame, heat) and 夏 (summer), literally 'flaming summer'. The exact historical derivation is uncertain, but the kanji combination suggests a vivid image of a scorching summer.