As outlined in Section 14.1, operators are used to construct various terms in the tracer and momentum equations. They are implemented as statement functions requiring no storage and their details are expanded out in Sections 22.9.5 and 22.8.7. It is important to note that for operators to work correctly, all items buried within the details of the operator must be defined correctly at the time when the operator is used. As with variables, operators also have a placement on the grid as explained below: