Rainfall from above 6,000 feet, in relation to upper wind and fronts

Rainfall from above 6,000 feet, and the movement of rain areas with the winds at those levels, are of importance in forecasting, and sometimes cause difficulties. A suggested classification of “thermal” upper winds is into ideal frontal, frontal zonal, and non‐frontal types. The upper wind system of an ideal front is briefly discussed. Non‐frontal “thermal)” winds sometimes cause an exceptional forward extension of a rain area (example in Fig. 2). A rain area is also liable t o travel in an abnormal manner along a front, owing to upper winds of frontal zonal type (see Figs. 4 and 5). Quasi‐frontal and non‐frontal rainfall, in connection with which upper winds are often important, are discussed briefly. In an appendix it is shown that the angle of slope of a surface of discontinuity should increase as a depression deepens, and that in consequence large‐scale energy transformations are probably related mainly to converging and diverging movements over large areas, and only indirectly to surfaces of discontinuity.