The “Hoffmeyer Charts”, a joint publication of the Deutsche Seewarte and the Netherlands Meteorological Institute, covering the years 1881–1911, offer an extraordinary opportunity to study the types and behavior of pressure disturbances which form in the trade-wind belt of the eastern North Atlantic. Such disturbances occur relatively infrequently, so that they can only be studied when data from such a long period of years is available.

A systematic review of the “Hoffmeyer Charts” revealed that there are four principal types of disturbances, none of which is as intense in character as middle latitude cyclones, but which nevertheless have definite types of weather phenomena associated with them which may be of considerable significance to ocean and air transportation. These disturbances were classified as follows: Type 1 is a low pressure area centered in middle or higher latitudes of the North Atlantic Ocean, whose influence, however, extends as far south as 25° N. Lat. or more: Type 2 is a disturbance centered at about 30°N, 30°W. Both type 1 and 2 derive from middle latitude disturbances, and usually begin as a secondary. Type 3 is a low-pressure center near the Canary Islands, which may originate on the West African coast south of 20° N and move westward toward the Canaries, or may originate near the Canaries and travel eastward along the same route, or, as is most frequently the case, may originate near or just north of the Canaries, expand or fill up on the spot, or perhaps attach itself to a low moving eastward in higher latitudes. Type 4 is a low on the West African coast between 5° and 20° N which moves westward. These usually move along the southern edge of the trade-wind belt.

“The tracks and monthly frequency of disturbances of these various types is discussed in considerable detail, as well as the duration and tendency to repetition. A detailed synoptic analysis of a type 4 disturbance is given.”

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