[Cherith1222]

Why

Importing the CF unit registry redefines the symbol C as degree Celsius. UDUNITS uses C for coulomb, and that redefinition makes a thermal velocity built from electron-volts fail to convert to km/s.

What changed

• The degree Celsius alias no longer includes C.
• degC, Celsius, deg_C, and degrees_Celsius still mean degree Celsius.
• A test checks that C stays coulomb and that the thermal velocity converts.

Scope

This covers the coulomb symbol in the example from #543. It leaves the regex preprocessor and the pint minimum alone.

Tradeoffs

Keeping C as Celsius would preserve the old alias and keep breaking coulomb. Removing it matches UDUNITS. A bare unit string C now means coulomb. The other Celsius names are unchanged.

Blast Radius

Code that quantified a value with the unit string C and meant Celsius will now get coulomb. degC and Celsius are unchanged. The in-repo what's new file points at GitHub releases for changes after February 2023, so this diff does not add a section there.

Verification

• Before the alias change, ureg("C") was degree Celsius and np.sqrt(100 * ureg.eV / ureg.m_e).to("km/s") raised DimensionalityError.
• After the change, C stays coulomb, the same conversion is 4193.828809546234 km/s, and degC is still degree Celsius.
• python -m pytest cf_xarray/tests/test_units.py is 17 passed.

Drafted with Cursor.