Characteristics of northern hemisphere blocking as determined from a long time series of observational data

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.