units_metadata not allowed if no temperature unit

ch03.adoc

@@ -71,6 +71,7 @@ In order to change the **`units`** correctly, it is essential to know whether a
 Therefore this standard strongly recommends that any variable whose **`units`** involve a temperature unit should also have a **`units_metadata`** attribute to make the distinction.
 This attribute must have one of the following three values: `temperature: on_scale`, `temperature: difference`, `temperature: unknown`.
 The **`units_metadata`** attribute, **`standard_name`** modifier (<<standard-name-modifiers>>) and **`cell_methods`** attribute (<<appendix-cell-methods>>) must be consistent if present.
+A variable must not have a **`units_metadata`** attribute if it has no **`units`** attribute or if its **`units`** do not involve a temperature unit.
 
 [[use-of-units-metadata-ex]]
 [caption="Example 3.1. "]

conformance.adoc

@@ -151,6 +151,7 @@ Exceptions are the units **`level`**, **`layer`**, and **`sigma_level`**.
 * The **`units`** of a variable that specifies a **`standard_name`** must be physically equivalent to the canonical units given in the standard name table, as modified by the **`standard_name`** modifier, if there is one, according to Appendix C, and then modified by all the methods listed in order by the **`cell_methods`** attribute, if one is present, according to Appendix E.
 * If the **`standard_name`** attribute includes the `standard_error` modifier, the **`units_metadata`** attribute, if present, must have the value `temperature: difference`.
 * If the **`cell_methods`** attribute includes any entry with any of the methods `range`, `standard_deviation` or `variance`, the **`units_metadata`** attribute, if present, must have the value `temperature: difference`.
+* A variable must not have a **`units_metadata`** attribute if it has no **`units`** attribute or if its **`units`** do not involve a temperature unit.
 
 *Recommendations:*