posted 05-21-2002 11:49 PM            

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A blocking pattern is an interruption of the normal west-to-east flow at middle and high latitudes. It persists from one to several weeks.

Blocks occur in both hemispheres but much more often in the Northern Hemisphere, suggesting that the distribution of continents and oceans may play a role in their formation and maintenance.

In Volume II of Synoptic-Dynamic Meteorology in Midlatitudes, Howard Bluestein describes three kinds of blocks. His figures, illustrating Northern Hemisphere blocks, are reproduced here. Each figure traces the air flow at 500 millibars. The contours are labeled in tens of meters. A contour label of 552 indicates that the 500-millibar pressure surface along that contour lies 5,520 meters above sea level.

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