The heavens opened British phrase
If the heavens open, it starts to rain heavily.
The heavens opened when we had just gone out.
The concert had to be cancelled because the heavens opened.
Rainy, wet weather
Used to mean that it is the high humidity that makes a person uncomfortable rather than the degree of heat
Intensely hot and irritated
A heavy rain
Wonderful weather, particularly on a day when an outdoor event is scheduled
The word "heavens" is always in plural form which means the sky.
In Old English, the word “heofon” means “home of God” which is the sky that we can see from the earth. In British English, the term “the heavens/sky opened/closed” related to the weather. Then, the heavens opened means there was heavy rain and the heavens closed means the air was hot and humid, it felt like living in a boiler.
Used to emphasize that someone or something is a lot more or a lot better
Mia can paint a damn sight better than I can.