That woul be: we are at 5 celsius degrees below zero, right?AmE - "we are at 5 degrees below zero" or "it is 5 below."
Be sure to include whether this is Fahrenheit or Celsius, since the typical American would understand it to be Fahrenheit.
Celsius, with an "s".I would say "it's -5" (It's minus five).
The longer, more formal version would be "it's minus five degrees Celcius", but people rarely feel the need to add the word Celcius here in the UK, because it is quite obvious which scale is being used.
(That said, I personally am still more likely to say "centigrade" than "Celcius"!)