Il fait un soleil de plomb is a useful expression to describe a very hot day. It has been used since 1835. Un soleil de plomb, literally a sun “made of lead” is a blazing sun. It is very intense and the heat that is released weighs down on people very heavily, like lead!
Have you read The Stranger from Albert Camus? In this famous novel, the sun has been referred as very hot and oppressive! It was un soleil de plomb!