30 years of observational 500 mb geopotential height data have been used to assess the characteristics of northern hemisphere blocking situations. A zonal index suitable for identification of blockings is defined and translated into a computer program. Characteristics of blocking situations have been computed and are presented as statistics.As expected, there are 2 preferred regions for blocking, the Atlantic region and the Pacific region. The results show that the number of days with blocked flow has a maximum over the eastern part of the Atlantic region, while the maximum is found over the western part of the Pacific region. The annual variation shows that there is an extended maximum from February through April in the Atlantic region, while there is a pronounced maximum in January in the Pacific region.The occurrence of simultaneous blocking in the 2 regions has also been investigated. The results show that there is no preference for a connection.Investigation of individual blocking situations reveals that the shortest ones are also formed in preferred geographical locations, and are not just random configurations in the changing pattern of waves in the Westerlies. There is a tendency for blocking episodes to seem to be concentrated in certain geographical locations, the longer the duration of the episode. The results also show that long-lasting episodes are notably more frequent in the Atlantic region than in the Pacific.

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A time series of 500 mb geopotential heights has been used to assess characteristics of southern hemisphere blocking. A zonal index similar to the index used in an analogous study of northern hemisphere blocking has been used. Characteristics of blocking situations have been computed with a simple algorithm, which has been programmed, and are presented as statistics. We found one preferred region for blocking action: the Australian‐New Zealand region. Two other regions where blocks occur are the Atlantic east of South America, and the Indian Ocean south‐east of South Africa. The number of days with blocked flow is considerably less over the Atlantic and the Indian Ocean than in the Australian‐New Zealand region. The annual variation shows two maxima: one peak value of 19% in April and one extended maximum of c. 20% during June–August in the Australian‐New Zealand region. There is no clear indication of a seasonal variation in the Atlantic and the Indian Ocean region. Investigation of individual blocking episodes reveals that the majority of the blocks have a mean eastward velocity, although the longer they last the more of them are westward‐moving. Longlasting episodes are almost exclusively found in the Australian region.

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