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Diagnosing atmospheric blocking objectively from weather maps is not unambiguous. In the literature, a number of metrics is conventionally used to identify atmospheric blocking.
In the figure on the right, we use a two-dimensional generalization of one of the most commonly used blocking metrics, devised by Tibaldi and Molteni (1990). This index identifies large-scale reversal of the geopotential height field at 500hPa (basically requiring upper-level Easterlies). A five-day running average value is taken (0 means not blocked, 1 means blocked).
By clicking on the image, an animated gif should start in a new window. The animation is of January and February 2012, which produced a very cold period. Relevant to the cold spell in Europe appears to be an atmospheric block that travelled from East Siberia towards Europe. It gradually expanded in size. The flow structure was similar to those that resulted in the coldest February months of the past century. Data source: ECMWF
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