From 20d5291ebb20073e8df062508635ff57dbb6d6d3 Mon Sep 17 00:00:00 2001
From: cherith <315844584+Cherith1222@users.noreply.github.com>
Date: Mon, 19 Oct 2026 02:18:47 +0800
Subject: [PATCH 1/2] test(units): expect the symbol C to stay coulomb

Importing the CF unit registry currently makes C mean degree Celsius, so a thermal velocity built from electron-volts no longer converts to km/s.
---
 cf_xarray/tests/test_units.py | 10 +++++++++-
 1 file changed, 9 insertions(+), 1 deletion(-)

diff --git a/cf_xarray/tests/test_units.py b/cf_xarray/tests/test_units.py
index 36c677cd..0a94ba7a 100644
--- a/cf_xarray/tests/test_units.py
+++ b/cf_xarray/tests/test_units.py
@@ -4,6 +4,7 @@
 Copyright (c) 2017 MetPy Developers.
 """
 
+import numpy as np
 import pytest
 
 pytest.importorskip("pint")
@@ -90,7 +91,14 @@ def test_udunits_format(units, expected):
 
 @pytest.mark.parametrize(
     "alias",
-    [ureg("Celsius"), ureg("degC"), ureg("C"), ureg("deg_C"), ureg("degrees_Celsius")],
+    [ureg("Celsius"), ureg("degC"), ureg("deg_C"), ureg("degrees_Celsius")],
 )
 def test_temperature_aliases(alias):
     assert alias == ureg("celsius")
+
+
+def test_coulomb_symbol_survives_cf_units():
+    assert ureg("C") == ureg("coulomb")
+    speed = np.sqrt(100 * ureg.eV / ureg.m_e).to("km/s")
+    assert speed.units == ureg("km/s")
+    assert speed.magnitude == pytest.approx(4193.828809546234)

From 5aa83c062074d9bdf9fcff000725dbc9461528dc Mon Sep 17 00:00:00 2001
From: cherith <315844584+Cherith1222@users.noreply.github.com>
Date: Mon, 19 Oct 2026 02:20:13 +0800
Subject: [PATCH 2/2] fix(units): stop aliasing C to degree Celsius

UDUNITS uses C for coulomb. degC, Celsius, deg_C, and degrees_Celsius still name degree Celsius.
---
 cf_xarray/units.py | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/cf_xarray/units.py b/cf_xarray/units.py
index 7f0fb281..63de43c2 100644
--- a/cf_xarray/units.py
+++ b/cf_xarray/units.py
@@ -92,7 +92,7 @@ def short_formatter(unit, registry, **options):
 units.define("year = 365.25 * day = yr")
 
 # Define commonly encountered units not defined by pint
-units.define("@alias degC = C = deg_C = Celsius = degrees_Celsius")
+units.define("@alias degC = deg_C = Celsius = degrees_Celsius")
 units.define("@alias degK = deg_K")
 units.define("@alias day = d")
 units.define("@alias hour = h")  # Not the Planck constant...