Marine Glossary

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Low

Short for an area of low pressure. Since a low is, on a synoptic map, always associated with cyclonic circulation, the term is used interchangeably with cyclone (tropical cyclone in the Tropics or extratropical cyclone outside of the Tropics).

In this course, we capitalize Low to emphasize this use of the term as an entity rather than just a description of relative pressures and to emphasize the key role in plays in the subject matter. They are characterized by strong winds, rising air, clouds, plus rain or snow. In the NH the circulation is counterclockwise around the Low with the wind at the surface pointing some 15 to 30° across the isobars toward lower pressure.

Lows are discussed in ART-2 on winds. See tropical cyclones and High. Properties of highs and lows are compared in G153. See G180 for circulation of wind around highs and lows. G139 illustrates the wind and pressure changes as a low passes over a location.

Tropical and extratropical cyclones are compared in G189.

Weak Low is one that does not have a very low central pressure. It could be called a shallow Low, as opposed to a deep Low meaning one with very low central pressure. See deepening and filling.

Figure G255 shows wind flowing out of Highs and into Lows.