ABSTRACT
We have seen in the last chapter examples of the types of variation to which the worlds wind circulation is liable. The circumpolar vortex over either hemisphere is a constant feature, as are the belts of prevailing high and low surface pressure that go with it. An oversimplified view of climatic variation might regard it as all a matter of ‘expansion’ of the circumpolar vortex in cold epochs, when the area of the polar regime expands, carrying the belt of westerlies to lower latitudes than before, and ‘contraction’ of the vortex in warm epochs when the polar cap contracts. There is some truth in this in so far as such expansions and contractions are indeed observed to take place in the course of the yearly round of the seasons and longer-term variations. But we have to consider other elements of the situation that change.
