Blocks in meteorology are large-scale patterns in the atmospheric pressure field that are nearly stationary, effectively "blocking" or redirecting migratory cyclones. They are also known as blocking highs or blocking anticyclones. These blocks can remain in place for several days or even weeks, causing the areas affected by them to have the same kind of weather for an extended period of time (e.g. precipitation for some areas, clear skies for others). In the Northern Hemisphere, extended blocking occurs most frequently in the spring over the eastern Pacific and Atlantic Oceans.
Similarly, in northern Europe anticyclonic blocks over western Russia and Scandinavia during the winter months can bring sub-zero easterly winds on their southern flanks, sometimes extending into the Atlantic Ocean and forcing the prevailing jet stream as far south as Portugal and Spain. Northern and Western European severe winters such as those of 1683–4, 1739–40, 1795, 1895, 1940, 1947, 1962–63, 1978–79, 1986, 2009–10 and December 2010 are caused by such blocks. Blocking highs were a key feature of the extreme winter droughts in southeastern Australia in 2006.
Impact of the polar vortex
Polar cyclones are climatological features which hover near the poles year-round. They are weaker during summer and strongest during winter. When the polar vortex is strong, the Westerlies increase in strength. When the polar cyclone is weak, the general flow pattern across mid-latitudes buckles and significant cold outbreaks occur. Extratropical cyclones which occlude and migrate into higher latitudes create cold-core lows within the polar vortex. Volcanic eruptions in the tropics lead to a stronger polar vortex during the winter for as long as two years afterwards. The strength and position of the cyclone shapes the flow pattern across the hemisphere of its influence. An index which is used in the northern hemisphere to gage its magnitude is the Arctic oscillation.
Omega blocks are so-named because the height fields with which they are associated in the Northern Hemisphere resemble an Ω, the uppercase Greek letter omega. They typically have a low-high-low pattern, arranged in the west–east direction.
Rex blocks consist of a high situated to the north (more generally, poleward) of a low. Very often both the high and the low are closed, meaning that the isobars (or constant geopotential height lines) defining the high–low close to form a circle. Rex blocks are named after the meteorologist who first identified them.
Cut-off highs and lows
When an upper-level high- or low-pressure system becomes stuck in place due to a lack of steering currents, it is known as being "cut off". The usual pattern which leads to this is the jet stream retreating to the north, leaving the then cut-off system behind. Whether or not the system is of high- or low-pressure variety dictates the weather that the block causes. Precisely this situation occurred over the southwestern United States in late spring and early summer of 2007, when a cut-off-low system hovering over the region brought unusually cool temperatures and an extraordinary amount of rain to Texas and Oklahoma (see June 2007 Texas flooding), and a cut-off-high near the coast of Georgia that caused a drought in the Southeast that same year. If the block is a high, it will usually lead to dry, warm weather as the air beneath it is compressed and warmed; and rainy, cooler weather if the block is a low.
Cold air damming
Blocking of atmospheric systems near the surface of the Earth occurs when a well-established poleward high pressure system lies near or within the path of the advancing storm system. The thicker the cold air mass is, the more effectively it can block an invading milder air mass. The depth of the cold air mass is normally shallower than the mountain barrier which created the cold air damming, or CAD. Some events across the Intermountain West can last for ten days. Pollutants and smoke can remain suspended within the stable air mass of a cold air dam.
- Glossary of Meteorology, Second Edition; American Meteorlogical Society, 2000; ISBN 1-878220-34-9.
- Brief page about Omega blocks
-  see p116
- Erik A. Rasmussen and John Turner (2003). Polar lows: mesoscale weather systems in the polar regions. Cambridge University Press. p. 174. ISBN 978-0-521-62430-5. Retrieved 2012-02-24.
- Alan Robock (May 2000). "Volcanic Eruptions and Climate" (PDF). Review of Geophysics (Rutgers University) 38 (2): 171. doi:10.1029/1998rg000054. Retrieved 2012-02-24.
- Todd Mitchell (2004). Arctic Oscillation (AO) time series, 1899 - June 2002. University of Washington. Retrieved on 2009-03-02.
- Brief page about Rex blocks
- Rex, D. F. (1950). "Blocking Action in the Middle Troposphere and its Effect upon Regional Climate". Tellus 2 (4): 275. doi:10.1111/j.2153-3490.1950.tb00339.x.
- Atmospheric Blocking
- C. David Whiteman (2000-05-05). Mountain Meteorology : Fundamentals and Applications. Oxford University Press. p. 166. ISBN 9780198030447. Retrieved 2013-05-17.