diff --git a/umpost/um2netcdf.py b/umpost/um2netcdf.py
index f1a4e76..9515fc1 100644
--- a/umpost/um2netcdf.py
+++ b/umpost/um2netcdf.py
@@ -738,6 +738,13 @@ def fix_level_coord(cube, z_rho, z_theta, tol=1e-6):
 
 
 def convert_32_bit(cube):
+    """
+    Convert 64 bit int/float data to 32 bit (in place).
+
+    Parameters
+    ----------
+    cube : iris.cube object to modify.
+    """
     if cube.data.dtype == 'float64':
         cube.data = cube.data.astype(np.float32)
     elif cube.data.dtype == 'int64':