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Rossby waves are a belt of upper air westerlies which follow meanders around both hemispheres. The speed of these waves is not uniform. Within them are the very fast jet streams.
Where, in the Northern hemisphere, the jet stream zig-zags south it brings cold air which descends in a clockwise direction to bring the dry, stable conditions which are associated with areas of high pressure i.e. anticyclones.
If the jet stream is meandering only slightly (a strong west circulation) there are wet and cool summers and wet and mild winters in Europe. However, the meanders can also be large. In this case high pressure areas build up in regions where you would normally find middle latitude storms. The dominant western winds are replaced by easterlies. This weather pattern is called a blockade. A blocking anticyclone stops in the way of middle latitude storms.
This means that the low pressure is forced to go around the stationary anticyclone.
If such a blockade stays in place for many weeks it generates extreme weather. This can bring droughts and floods depending on the time of year.
The summer of 1976 in Europe
In June and July Western Europe was hot and dry, with many days with maximum temperatures well above 30 degrees. Moscow in Russia, however, had one of the coldest (3 - 4 °C below normal) and wettest summers in the century. Mid latitude storms which normally move over the UK and Scandinavia were forced on a track round Scandinavia into Russia by a blockade over Western Europe.
Another, more recent, example is the long dry cold spell of December 2001 to January 2002 which was due to a blocking anticyclone over Scandinavia or the north east Atlantic. This caused depressions to travel on a south easterly course over Spain and into the Mediterranean. This gave most parts of the UK a dry and cold spell with hard frosts and an unusually sunny December, whilst the Spanish Mediterranean resorts, Greece and Turkey suffered memorable snowstorms as areas of low pressure made their way through the Mediterranean.
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