On February 4, 1960, a polar air mass penetrated in depth southward to the latitude of a trough in the subtropical jet stream. The subtropical and polar fronts amalgamated into a remarkably deep inversion, with temperatures as high as − 16° C. at 340 mb., much warmer than the tropical air farther south.
A brief discussion is given of the influence of flow curvature, and of curvature variation with height, on the vertical shear of the wind. The actual shear differed in certain localities, by a factor of two to five, from the geostrophic thermal shear.
Temporary retrogression of the subtropical trough was accompanied by pronounced variations of kinematic and thermal structure, which were in some ways similar to those observed when a minor trough deepens into a major trough of the polar-front wave system. This retrogression was preceded by a westward movement of the wind-speed maximum upstream, and followed by a westward movement of the one downstream. The subtropical front was strong on the western side of the trough and absent on the eastern side early in the development; the reverse was true in later stages.