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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Mesoscale structures in the wind and pressure fields are demonstrated by case studies. Two main types are identified. The one is characteristic for the coast of Northern Norway and is called coastal trough in this paper. The other, typical for the south‐eastern coast, has similarities to the New England phenomenon named coastal front. Both occur during the winter season. The structures are quasi‐permanent in certain weather conditions, which suggests a description in terms of a stationary model in two space dimensions, assuming that the variables do not change in a direction parallel to the straight coastline. The scaled equations depend on two non‐dimensional numbers, which in general are small, permitting a series expansion of the perturbation variables. Solutions of zero and first order are derived, essentially by analytical methods. The size and structure of the solutions are compared to the case studies. The results seems to verify the initial hypothesis, namely that the trough is caused mainly by differential heating, while the front also depends on the discontinuity in surface friction.