A study of blocking in the Atlantic‐European of the northern hemisphere

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Abstract
Various interpretations of what is meant by ‘blocking’ in the atmosphere are discussed. The three main aspects referred to by writers on the subject are: (a) a sharp local diminution of the mean zonal (tropospheric) flow, often associated with a bifurcation of the jet stream, which diminution is (b) rather persistent and (c) displaced westwards with time. Not all of these attributes have been insisted upon as defining blocking but usually (a) and (b) are required. The view is taken herein that, for forecasting purposes, only (a) is admissible, and a selection of instantaneous patterns having this characteristic is made, two broad types being distinguished, for the period 1949‐52 and in the Atlantic‐European sector of the northern hemisphere.Some statistics relating to the seasonal and geographic distribution of these patterns are given, which in the main confirm previous results. Blocking patterns were found to occur on more than half the days, individual blocks lasting on an average 16.5 days. They were most frequent in May and least frequent in July, and tended to be displaced from Europe to mid‐Atlantic during the first six months of the year. Within this general westward drift several successive ‘waves’ of progression and retrogression were in evidence. Blocking over N. America was extremely rare.This position and movement of individual blocks is presented in some detail, and notes on their formation, dissipation and general character are also given.