Here on planet Earth?

Colder than a witches tit.
When the temperature falls low enough to freeze mercury in the bulb or t i t of a weather witch or what is commonly known as a mercury thermomoter it is said
to be as cold as a witches tit.

Exactly how cold would that be?

The melting point of mercury is -38.83 °C (-37.89 °F). While that's not an unheard of temperature, I'm sure most of us would agree that it was indeed colder than a witch's t i t long before that. RRC

Especially so in a brass brassiere