See http://en.wikipedia.org/wiki/Metric_system

He means, why not just say "minus 10 celsius". Or in print -10c

The SI unit is "degree Celsius", the original statement looks perfectly good English to me.

Looks fine to me. We use it the same way in India.

And both celsius and Kelvin are SI units.

http://en.wikipedia.org/wiki/Metric_system#Conversion_and_ca...

The SI unit for temperature is kelvin.

Kelvin and Celsius are the same, modulo a shift.

"Ten below", "ten below zero", and "ten degrees below zero" all seem natural to me (BrE speaker), but I'm not sure about "ten degrees celsius below zero". That strikes me as an attempt to shoehorn units into an ambiguous* idiom. It doesn't work, and "-10C" or "minus ten celsius" would be better, in my opinion.

* Both F and C are used in the UK, but I think it's an age divide. I doubt many people under around 40 would use Fahrenheit as their measurement system of choice. I'm 35 and I have no intuitive understanding of Fahrenheit temperatures.

I'm British and read that without thinking anything of it, but I can see where you're coming from - it could mistakenly be read as "plus ten degrees below zero" perhaps.

Must be literal translation from Russian, "N degrees above/below zero" and "N degrees of warmth/cold" are common expressions in it.

Seems fine to me. How would you express -10°C?

Negative (or minus) ten degrees celsius.

Ten degrees below zero celsius.

I would argue that the unit of measure is "degrees Celcius" rather than individually degrees or Celcius. So I would agree with either ten-below-zero degrees Celcius (which is a little awkward), minus ten degrees C, or as the author wrote it.