Potential vorticity 1

LTEX source, defined here to print as italic. Possible cross-references to other articles are flagged \crossref{...}. For visibility, \crossref{...} is temporarily defined to print as sans-serif. Therefore, sans-serif italic signals both a new term and a possible cross-reference. I have used \openingscarequote...\closingscarequote in a few places. This bends the house-style rules, but unfortunately we don’t live in an ideal world with perfectly logical terminology. A standard example, that of the variable solar ‘constant’, is enough to make the point. Another is the slow ‘manifold’. It is not a manifold.