Marine Glossary

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blocking high

Large scale anticyclone that deflects or slows the usual west-to-east progression of weather systems. From the author's experience, typical characteristics of a blocking High apparent from weather maps include:

(1) that the High be in its proper (climatic) location for that season,
see World Atlas under world pressures,

(2) that it be smooth and round in shape with at least two isobars circling it, and

(3) that it be indeed high, that is above some 1030 mb or so. Chances are, with these conditions, the High will look pretty much the same the following day. Eventually, though, one or more of these characteristics will weaken, and at that point things are likely to start to change even more.

See WXT G159 for surface maps comparing a blocking high with a weak high. WXT Q-1907 discusses the role of such a High in sailing tactics. Blocking Highs occur much more frequently in the Northern Hemisphere than in the Southern.