2.1.1 Blocking highs
Areas of high pressure can sometimes be very slow moving, almost stationary. Such a region of slow moving air can prevent other, faster moving pressure systems from moving into a region. For this reason they are known as blocking highs or blocking anticyclones.
The part of the earth's surface beneath a blocking high often experiences the same kind of weather for a long period (perhaps as long as several weeks). In Europe, for example, blocking highs over Western, Russia, Eastern Europe and Scandinavia have caused long, severe winters (see chart). Blocking highs have also led to prolonged droughts.
February 2012: Winter blocking high in Europe
This UK Met Office pressure chart shows a blocking high over Europe, supplying cold, polar air across much of the continent and blocking out milder air from over the Atlantic. This blocking high brought temperatures of -20°C (-4°F), killed hundreds of people in Eastern Europe and even brought snow to the Sahara, as this BBC video explains.