GEOGRAPHICAL FREQUENCY OF TROUGHS AND RIDGES ON MEAN 700-MB. CHARTS

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  • 1 Extended Forecast Section, U. S. Weather Bureau, Washington, D. C.
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Abstract

The geographical frequencies of occurrence of troughs and ridges on both 5-day and 30-day mean 700-mb. charts for the Northern Hemisphere during a long period of record are presented for individual months. Many regions of maximum and minimum frequency show a close relationship to trough and ridge positions on long-period mean, or normal 700-mb. charts. However, even a cursory inspection of these frequency charts shows that they yield considerably more information for both the practicing forecaster and the research worker than can be derived solely from normal maps. Preferred trough and ridge locations and their seasonal changes appear to be related largely to land-sea boundaries with their associated thermal influences and also to prominent orography.

Abstract

The geographical frequencies of occurrence of troughs and ridges on both 5-day and 30-day mean 700-mb. charts for the Northern Hemisphere during a long period of record are presented for individual months. Many regions of maximum and minimum frequency show a close relationship to trough and ridge positions on long-period mean, or normal 700-mb. charts. However, even a cursory inspection of these frequency charts shows that they yield considerably more information for both the practicing forecaster and the research worker than can be derived solely from normal maps. Preferred trough and ridge locations and their seasonal changes appear to be related largely to land-sea boundaries with their associated thermal influences and also to prominent orography.

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