Positions of Monthly Mean Troughs and Ridges in the Northern Hemisphere, 1949–1963

Positions of trough lines and ridge lines for each month are shown as they were transcribed from monthly 700-mb. charts for 1949–63. This is the first time that 15 years of reliable upper-air analyses were available over all continents in the Northern Hemisphere. The preference of these circulation features for particular geographical areas in certain months or seasons is clearly depicted. This is a rather small sample, but if one excludes possible secular changes, these charts should be fairly representative of longer periods, especially in those areas where frequency of occurrence was maximum or minimum.