Blocks in
meteorology
Meteorology is the scientific study of the Earth's atmosphere and short-term atmospheric phenomena (i.e. weather), with a focus on weather forecasting. It has applications in the military, aviation, energy production, transport, agricultur ...
are large-scale patterns in the
atmospheric pressure
Atmospheric pressure, also known as air pressure or barometric pressure (after the barometer), is the pressure within the atmosphere of Earth. The standard atmosphere (symbol: atm) is a unit of pressure defined as , which is equivalent to 1,013. ...
field that are nearly stationary, effectively "blocking" or redirecting migratory
cyclone
In meteorology, a cyclone () is a large air mass that rotates around a strong center of low atmospheric pressure, counterclockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere as viewed from above (opposite to an ant ...
s. They are also known as blocking highs or blocking anticyclones.
[Glossary of Meteorology, Second Edition; American Meteorological Society, 2000; .] 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
The Northern Hemisphere is the half of Earth that is north of the equator. For other planets in the Solar System, north is defined by humans as being in the same celestial sphere, celestial hemisphere relative to the invariable plane of the Solar ...
, extended blocking occurs most frequently in the spring over the eastern Pacific and
Atlantic
The Atlantic Ocean is the second largest of the world's five oceanic divisions, with an area of about . It covers approximately 17% of Earth's surface and about 24% of its water surface area. During the Age of Discovery, it was known for se ...
Oceans.
While these events are linked to the occurrence of extreme weather events such as heat waves, particularly the onset and decay of these events is still not well captured in numerical weather forecasts and remains an open area of research.
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
The westerlies, anti-trades, or prevailing westerlies, are prevailing winds from the west toward the east in the middle latitudes between 30 and 60 degrees latitude. They originate from the high-pressure areas in the horse latitudes (about ...
increase in strength. When the polar cyclone is weak, the general flow pattern across mid-latitudes buckles and significant cold outbreaks occur.
Extratropical cyclone
Extratropical cyclones, sometimes called mid-latitude cyclones or wave cyclones, are low-pressure areas which, along with the anticyclones of high-pressure areas, drive the weather over much of the Earth. Extratropical cyclones are capable of p ...
s which occlude and migrate into higher latitudes create
cold-core low
A cold-core low, also known as an upper level low or cold-core cyclone, is a cyclone aloft which has an associated cold pool of air residing at high altitude within the Earth's troposphere, without a frontal structure. It is a low pressure syste ...
s within the polar vortex.
Volcanic
A volcano is commonly defined as a vent or fissure in the crust of a planetary-mass object, such as Earth, that allows hot lava, volcanic ash, and gases to escape from a magma chamber below the surface.
On Earth, volcanoes are most often fo ...
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
The Northern Hemisphere is the half of Earth that is north of the equator. For other planets in the Solar System, north is defined by humans as being in the same celestial sphere, celestial hemisphere relative to the invariable plane of the Solar ...
to gauge its magnitude is the
Arctic oscillation
The Arctic oscillation (AO) or Northern Annular Mode/Northern Hemisphere Annular Mode (NAM) is a weather phenomenon at the Arctic pole north of 55 degrees latitude. It is an important mode of climate variability for the Northern Hemisphere. The s ...
.
Omega blocks
Omega blocks are so-named because the
isobars or
geopotential height
Geopotential height, also known as geopotential altitude or geopotential elevation, is a vertical coordinate (with dimension of length) representing the work involved in lifting one unit of mass over one unit of length through a hypothetical spac ...
contours with which they are associated in the Northern Hemisphere resemble an Ω, the uppercase Greek letter
omega
Omega (, ; uppercase Ω, lowercase ω; Ancient Greek ὦ, later ὦ μέγα, Modern Greek ωμέγα) is the twenty-fourth and last letter in the Greek alphabet. In the Greek numerals, Greek numeric system/isopsephy (gematria), it has a value ...
. They typically have a low-high-low pattern, arranged in the west–east direction.
Rex blocks
Rex blocks (or dipole blocks) consist of a high situated poleward (north in the Northern Hemisphere; south in the Southern Hemisphere) of a low. Very often both the high and the low are closed, meaning that the
isobars (or constant
geopotential height
Geopotential height, also known as geopotential altitude or geopotential elevation, is a vertical coordinate (with dimension of length) representing the work involved in lifting one unit of mass over one unit of length through a hypothetical spac ...
lines) defining the high–low close to form a circle.
Rex blocks are named after meteorologist Daniel F. Rex, who first identified them in 1950.
Cut-off highs and lows
When an upper-level
high
High may refer to:
Science and technology
* Height
* High (atmospheric), a high-pressure area
* High (computability), a quality of a Turing degree, in computability theory
* High (tectonics), in geology an area where relative tectonic uplift t ...
- or
low
Low or LOW or lows, may refer to:
People
* Low (surname), listing people surnamed Low
Places
* Low, Quebec, Canada
* Low, Utah, United States
* Lo Wu station (MTR code LOW), Hong Kong; a rail station
* Salzburg Airport (ICAO airport code: LO ...
-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
Jet streams are fast flowing, narrow thermal wind, air currents in the Earth's Atmosphere of Earth, atmosphere.
The main jet streams are located near the altitude of the tropopause and are westerly winds, flowing west to east around the gl ...
retreating poleward, 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 southern United States during 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
Texas ( , ; or ) is the most populous U.S. state, state in the South Central United States, South Central region of the United States. It borders Louisiana to the east, Arkansas to the northeast, Oklahoma to the north, New Mexico to the we ...
and
Oklahoma
Oklahoma ( ; Choctaw language, Choctaw: , ) is a landlocked U.S. state, state in the South Central United States, South Central region of the United States. It borders Texas to the south and west, Kansas to the north, Missouri to the northea ...
(see
June 2007 Texas flooding
The June 2007 Texas flooding occurred after heavy rains hit the Southern Plains of the United States. Moisture from the Gulf of Mexico flowed north creating a slow-moving frontal system. Approximately 200 millimeters (8 inches) of rain poured in ...
), and a cut-off-high near the coast of Georgia that caused a drought in the Southeast that same year. Rainy, cooler weather results if the block is a low in the US.
Hurricane Ian
Hurricane Ian was a devastating tropical cyclone which was the third costliest natural disaster, weather disaster on record worldwide. It was also the deadliest hurricane to strike the state of Florida since the 1935 Labor Day hurricane, and ...
in the last week of September 2022 drifted northward and its remnants became detached from the jet stream, resulting in a stationary low pressure system spinning off the Northeastern US and bringing several days of precipitation until a front finally moved through on 6 October.
Blocking high
If the block is a high, it will usually lead to dry, warm weather as the air beneath it is compressed and warmed, as happened in southeastern Australia in 2006 and 1967 with resultant extreme
droughts
A drought is a period of drier-than-normal conditions.Douville, H., K. Raghavan, J. Renwick, R.P. Allan, P.A. Arias, M. Barlow, R. Cerezo-Mota, A. Cherchi, T.Y. Gan, J. Gergis, D. Jiang, A. Khan, W. Pokam Mba, D. Rosenfeld, J. Tierney, ...
. However, when a blocking high is situated in the
Tasman Sea
The Tasman Sea is a marginal sea of the South Pacific Ocean, situated between Australia and New Zealand. It measures about across and about from north to south. The sea was named after the Dutch explorer Abel Janszoon Tasman, who in 1642 wa ...
it can cause torrential rains in eastern Australia, as in the cases of the
2021
Like the year 2020, 2021 was also heavily defined by the COVID-19 pandemic, due to the emergence of multiple Variants of SARS-CoV-2, COVID-19 variants. The major global rollout of COVID-19 vaccines, which began at the end of 2020, continued ...
and
2022 flood events. A blocking high in the southern Tasman Sea directs low pressure systems and troughs towards eastern Australia, whereby providing rainfall on the east coast of Australia.
In Australia, blocking highs generally occur in the
Great Australian Bight
The Great Australian Bight is a large oceanic bight (geography), bight, or open bay, off the central and western portions of the southern Coast, coastline of mainland Australia.
There are two definitions for its extent—one by the Internation ...
and the
Tasman Sea
The Tasman Sea is a marginal sea of the South Pacific Ocean, situated between Australia and New Zealand. It measures about across and about from north to south. The sea was named after the Dutch explorer Abel Janszoon Tasman, who in 1642 wa ...
, which are powerful
high-pressure systems that usually develop further south than normal. They stay virtually unmoving for a lengthy period (i.e. several days to weeks) and thus block the regular easterly motion of weather systems across southern Australia.
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
The Intermountain West, or Intermountain Region, is a geographic and geological region of the Western United States. It is located between the Rocky Mountain Front on the east and the Cascade Range and Sierra Nevada on the west.
Topography
...
can last for ten days. Pollutants and smoke can remain suspended within the stable air mass of a cold air dam.
Midlatitude cold waves
In the middle latitudes of the Northern Hemisphere, areas on the eastern side of blocking anticyclones or under the influence of anomalous flows from colder continental interiors related to blocks experience severe winters, a phenomenon which has been known since the discovery of the
North Atlantic Oscillation (NAO) in the 1840s.
[van Loon, Harry and Rogers, Jeffrey C.; ‘The Seesaw in Winter Temperatures Between Greenland and Northern Europe: Part I: General Description’; '']Monthly Weather Review
The ''Monthly Weather Review'' is a peer-reviewed scientific journal published by the American Meteorological Society. It covers research related to analysis and prediction of observed and modeled circulations of the atmosphere, including technique ...
'', 106 (1978), pp. 296–310 These blocking patterns also have a tendency to produce anomalously mild conditions at very high latitudes, at least in those regions exposed to anomalous flow from the ocean as in
Greenland
Greenland is an autonomous territory in the Danish Realm, Kingdom of Denmark. It is by far the largest geographically of three constituent parts of the kingdom; the other two are metropolitan Denmark and the Faroe Islands. Citizens of Greenlan ...
and
Beringia
Beringia is defined today as the land and maritime area bounded on the west by the Lena River in Russia; on the east by the Mackenzie River in Canada; on the north by 70th parallel north, 72° north latitude in the Chukchi Sea; and on the south ...
, or from
chinook wind
Chinook winds, or simply Chinooks, are two types of prevailing warm, generally westerly winds in western North America: Coastal Chinooks and interior Chinooks. The coastal Chinooks are persistent seasonal, wet, southwesterly winds blowing in from ...
s as in
Interior Alaska
Interior Alaska is the central region of Alaska's territory, roughly bounded by the Alaska Range to the south and the Brooks Range to the north. It is largely wilderness. Mountains include Denali in the Alaska Range, the Wrangell Mountains, and ...
.
Such cold winters over the contiguous United States and southern Canada as
1911/12,
1935/36, 1949/50,
1977/78 and 1978/79, 1993/94, and 2017/18 resulted from blocks in the
Gulf of Alaska
The Gulf of Alaska ( Tlingit: ''Yéil T'ooch’'') is an arm of the Pacific Ocean defined by the curve of the southern coast of Alaska, stretching from the Alaska Peninsula and Kodiak Island in the west to the Alexander Archipelago in the ...
or to the east of the
Mackenzie Mountains
The Mackenzie Mountains are a Canadian mountain range forming part of the Yukon–Northwest Territories boundary between the Liard and Peel rivers. The range is named in honour of Canada's second prime minister, Alexander Mackenzie. Nahanni ...
directing very cold Arctic air with a long trajectory as far as the
American South
The Southern United States (sometimes Dixie, also referred to as the Southern States, the American South, the Southland, Dixieland, or simply the South) is census regions United States Census Bureau. It is between the Atlantic Ocean and the ...
, as did the Western cold waves of 1889/90 and January 1950. In
Northern and Western Europe, cold winters such as 1683/84, 1739/40, 1794/95, 1829/30,
1894/95, 1916/17, 1941/42, February 1947 and
1962/63 are almost always associated with high latitude Atlantic blocking and an equatorward shift of the polar jet stream to
Portugal
Portugal, officially the Portuguese Republic, is a country on the Iberian Peninsula in Southwestern Europe. Featuring Cabo da Roca, the westernmost point in continental Europe, Portugal borders Spain to its north and east, with which it share ...
and even
Morocco
Morocco, officially the Kingdom of Morocco, is a country in the Maghreb region of North Africa. It has coastlines on the Mediterranean Sea to the north and the Atlantic Ocean to the west, and has land borders with Algeria to Algeria–Morocc ...
.
Over Central Asia, unusually cold winters like 1899/1900, 1929/30 and 1930/31, 1944/45, 1954/55 and
1968/69 are associated with blocking near the
Ural Mountains
The Ural Mountains ( ),; , ; , or simply the Urals, are a mountain range in Eurasia that runs north–south mostly through Russia, from the coast of the Arctic Ocean to the river Ural (river), Ural and northwestern Kazakhstan. extending the
Siberian High
The Siberian High (also Siberian Anticyclone; (''Aziatsky antitsiklon''); zh, 西伯利亞高壓; Pinyin ''Xībólìyǎ gāoyā''; Kazakh Азия антициклоны (''Aziya antitsiklonı'')) is a massive collection of cold dry air that a ...
westwards to push the very cold air from the Siberian "
cold pole" outward towards the
Aral and
Caspian Sea
The Caspian Sea is the world's largest inland body of water, described as the List of lakes by area, world's largest lake and usually referred to as a full-fledged sea. An endorheic basin, it lies between Europe and Asia: east of the Caucasus, ...
s. Unlike other midlatitude regions of the Northern Hemisphere, however, cold winters in Europe (e.g. 1916/17, 1962/63) are often very mild over Central Asia, which can gain warm air advection from subtropical cyclones pushed to the south under negative NAO conditions.
Heat waves
Heat waves in summer are the result of similar blocking patterns, typically involving the placement of the semi-permanent
subtropical ridge
The horse latitudes are the latitudes about 30 degrees north and south of the Equator. They are characterized by sunny skies, calm winds, and very little precipitation. They are also known as subtropical ridges or highs. It is a high-pressur ...
. Some unusually intense summers such as 1936 in the United States, 1999, 2002, and 2011, and in Europe summers such as 1976, 2003 European heat wave, and 2019, were the result of entrenched highs that became detached from the jet stream for a prolonged period of time and allowed warm, dry air to build in place. In many cases such as the 1999 US drought, the heat wave was preceded by prior months of below normal precipitation that prevented temperatures from cooling. The 2003 heat wave in Europe occurred, conversely, during a year that North America experienced markedly below normal temperatures and higher than normal precipitation, especially during the spring months. The high amount of rain in North America increased the energy levels in the polar jet, driving it far to the north in Europe and resulting in a prolonged, static high pressure ridge that drove up hot air from the Sahara Desert into Europe.
See also
*
Anticyclone
A high-pressure area, high, or anticyclone, is an area near the surface of a planet where the atmospheric pressure is greater than the pressure in the surrounding regions. Highs are middle-scale meteorological features that result from interpl ...
*
Geopotential height
Geopotential height, also known as geopotential altitude or geopotential elevation, is a vertical coordinate (with dimension of length) representing the work involved in lifting one unit of mass over one unit of length through a hypothetical spac ...
*
Heat dome
*
High-pressure area
A high-pressure area, high, or anticyclone, is an area near the surface of a planet where the atmospheric pressure is greater than the pressure in the surrounding regions. Highs are middle-scale meteorological features that result from interpl ...
*
Ridiculously Resilient Ridge
*
Trough (meteorology)
A trough is an elongated region of relatively low atmospheric pressure without a closed Isobar (meteorology), isobaric contour that would define it as a Low-pressure area, low pressure area. Since low pressure implies a low Geopotential height, he ...
References
External links
What Is a Greenland Block?– Jim Andrews,
Accuweather.com
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Anticyclones