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The large-scale air-flow over a region of the earth, which is governed by the height contour patterns, can be considered as inducing or controlling typical weather conditions in an area.   The patterns are spatially fairly large, they affect the weather over regions as large as continents and have a persistence of typically 10 ten days or more.   These patterns over the Euro-Atlantic region may be described by a few weather regime definitions:  

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• +ve Blocking (BLO+) with ridging over Western Europe and Scandinavia:  Spells of wintertime easterly winds bring increased occurrences of very cold and snowy weather to much of Europe while  Scandinavia Scandinavia sees very cold, dry and calm conditions. 
• -ve Blocking (BLO-) with ridging over the Atlantic: Spells  Spells of northerly winds bring increased occurrences of wintertime snow to western Europe while Central Europe is more likely to see very cold, dry and calm winters 

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• 4-regime scheme.  Shown as cumulative histograms using a four regime scheme (NAO+, NAO-, Blocking, ATR) where NAO denotes North Atlantic Oscillation, and ATR denotes Atlantic ridge.  The signs show positive or negative characteristics.  All regimes are independent of one another.  There is also a fifth category of no (clearcut) regime.  This scheme is used to construct Regime Frequency Histogramsregime frequency histograms.
• Mirrored 2-regime scheme.  Shown as phase-space diagrams using a two regime scheme ( NAO and BL) where NAO denotes North Atlantic Oscillation and BL denotes Blocking (both represented somewhat differently to the 4-regime scheme).   Each has a negative and a positive component that are in both cases exactly opposite to one another; these are NAO+/- and BL+/-. There is also a fifth category of no (clearcut) regime which would be denoted by the area close to the centre of the phase-space diagram.  This scheme is used to construct to construct Phase Space or Regime Projection diagrams.

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