Heat—the state or degree of being hot (weather, temperature, humidity), expressed as the noun form of 暑い.
今年の夏は暑さが厳しく、熱中症の患者が増えた。
This summer's heat was severe, and cases of heatstroke increased.
暑さ対策としてエアコンを低めに設定した。
As a measure against the heat, I set the air conditioner a bit lower.
朝晩は暑さが和らいで過ごしやすくなる。
The heat eases in the mornings and evenings, making it more comfortable.