Polar vortex

A polar vortex is an upper-level low-pressure area lying near one of the Earth's poles. There are two polar vortices in the Earth's atmosphere, overlying the North and South Poles. Each polar vortex is a persistent, large-scale, low-pressure zone less than 1,000 kilometers (620 miles) in diameter, that rotates counter-clockwise at the North Pole (called a cyclone) and clockwise at the South Pole, i.e., both polar vortices rotate eastward around the poles. As with other cyclones, their rotation is driven by the Coriolis effect. The bases of the two polar vortices are located in the middle and upper troposphere and extend into the stratosphere. Beneath that lies a large mass of cold, dense Arctic air.

The interface between the cold dry air mass of the pole and the warm moist air mass farther south defines the location of the polar front. The polar front is centered, roughly at 60° latitude. A polar vortex strengthens in the winter and weakens in the summer due to its dependence on the temperature difference between the equator and the poles.[1]

The vortices weaken and strengthen from year to year. When the vortex of the Arctic is strong, it is well defined, there is a single vortex, and the Arctic air is well contained; when weaker, which it generally is, it will break into two or more vortices; when very weak, the flow of Arctic air becomes more disorganized, and masses of cold Arctic air can push equatorward, bringing with them a rapid and sharp temperature drop. When the polar vortex is strong, there is a single vortex with a jet stream that is "well constrained" near the polar front. When the northern vortex weakens, it separates into two or more vortices, the strongest of which are near Baffin Island, Canada, and the other over northeast Siberia.[2]

The Antarctic vortex of the Southern Hemisphere is a single low-pressure zone that is found near the edge of the Ross ice shelf, near 160 west longitude. When the polar vortex is strong, the mid-latitude Westerlies (winds at the surface level between 30° and 60° latitude from the west) increase in strength and are persistent. When the polar vortex is weak, high-pressure zones of the mid-latitudes may push poleward, moving the polar vortex, jet stream, and polar front equatorward. The jet stream is seen to "buckle" and deviate south. This rapidly brings cold dry air into contact with the warm, moist air of the mid-latitudes, resulting in a rapid and dramatic change of weather known as a "cold snap".[3]

Ozone depletion occurs within the polar vortices – particularly over the Southern Hemisphere – reaching a maximum depletion in the spring.

November2013 polar vortex geopotentialheight mean Large
A strong polar vortex configuration in November 2013
Jan52014 polar vortex geopotentialheight mean Large
A more typical weak polar vortex on January 5, 2014
Low pressure area over Quebec and Maine, part of the northern polar vortex weakening, on the record-setting cold morning of January 21, 1985


The polar vortex was first described as early as 1853.[4] The phenomenon's sudden stratospheric warming (SSW) develops during the winter in the Northern Hemisphere and was discovered in 1952 with radiosonde observations at altitudes higher than 20 km.[5]

The phenomenon was mentioned frequently in the news and weather media in the cold North American winter of 2013–2014, popularizing the term as an explanation of very cold temperatures.[6]

A deep freeze that gripped much of the United States and Canada in late January 2019 has been blamed on a polar vortex. The US National Weather Service warned that frostbite is possible within just 10 minutes of being outside in such extreme temperatures, and hundreds of schools, colleges and universities in the affected areas were closed. Around 21 people died in US due to severe frostbite.[7][8] States within the midwest region of the United States had windchills just above -50°F (-45°C), which is colder than the frozen tundra and Antarctica. [9]


Polar cyclones are low-pressure zones embedded within the polar air masses, and exist year-round. The stratospheric polar vortex develops at latitudes above the subtropical jet stream.[10] Horizontally, most polar vortices have a radius of less than 1,000 kilometres (620 mi).[11] Since polar vortices exist from the stratosphere downward into the mid-troposphere,[2] a variety of heights/pressure levels are used to mark its position. The 50 mb pressure surface is most often used to identify its stratospheric location.[12] At the level of the tropopause, the extent of closed contours of potential temperature can be used to determine its strength. Others have used levels down to the 500 hPa pressure level (about 5,460 metres (17,910 ft) above sea level during the winter) to identify the polar vortex.[13]

Duration and power

Polar vortex and weather impacts due to stratospheric warming

Polar vortices are weakest during summer and strongest during winter. Extratropical cyclones that migrate into higher latitudes when the polar vortex is weak can disrupt the single vortex creating smaller vortices (cold-core lows) within the polar air mass.[14] Those individual vortices can persist for more than a month.[11]

Volcanic eruptions in the tropics can lead to a stronger polar vortex during winter for as long as two years afterwards.[15] The strength and position of the polar vortex shapes the flow pattern in a broad area about it. An index which is used in the northern hemisphere to gauge its magnitude is the Arctic oscillation.[16]

When the Arctic vortex is at its strongest, there is a single vortex, but normally, the Arctic vortex is elongated in shape, with two cyclone centers, one over Baffin Island in Canada and the other over northeast Siberia. When the Arctic pattern is at its weakest, subtropic air masses can intrude poleward causing the Arctic air masses to move equatorward, as during the Winter 1985 Arctic outbreak.[17] The Antarctic polar vortex is more pronounced and persistent than the Arctic one. In the Arctic the distribution of land masses at high latitudes in the Northern Hemisphere gives rise to Rossby waves which contribute to the breakdown of the polar vortex, whereas in the Southern Hemisphere the vortex is less disturbed. The breakdown of the polar vortex is an extreme event known as a sudden stratospheric warming, here the vortex completely breaks down and an associated warming of 30–50 °C (54–90 °F) over a few days can occur.

The waxing and waning of the polar vortex is driven by the movement of mass and the transfer of heat in the polar region. In the autumn, the circumpolar winds increase in speed and the polar vortex rises into the stratosphere. The result is that the polar air forms a coherent rotating air mass: the polar vortex. As winter approaches, the vortex core cools, the winds decrease, and the vortex energy declines. Once late winter and early spring approach the vortex is at its weakest. As a result, during late winter, large fragments of the vortex air can be diverted into lower latitudes by stronger weather systems intruding from those latitudes. In the lowest level of the stratosphere, strong potential vorticity gradients remain, and the majority of that air remains confined within the polar air mass into December in the Southern Hemisphere and April in the Northern Hemisphere, well after the breakup of the vortex in the mid-stratosphere.[18]

The breakup of the northern polar vortex occurs between mid March to mid May. This event signifies the transition from winter to spring, and has impacts on the hydrological cycle, growing seasons of vegetation, and overall ecosystem productivity. The timing of the transition also influences changes in sea ice, ozone, air temperature, and cloudiness. Early and late polar breakup episodes have occurred, due to variations in the stratospheric flow structure and upward spreading of planetary waves from the troposphere. As a result of increased waves into the vortex, the vortex experiences more rapid warming than normal, resulting in an earlier breakup and spring. When the breakup comes early, it is characterized by with persistent of remnants of the vortex. When the breakup is late, the remnants dissipate rapidly. When the breakup is early, there is one warming period from late February to middle March. When the breakup is late, there are two warming periods, one January, and one in March. Zonal mean temperature, wind, and geopotential height exert varying deviations from their normal values before and after early breakups, while the deviations remain constant before and after late breakups. Scientists are connecting a delay in the Arctic vortex breakup with a reduction of planetary wave activities, few stratospheric sudden warming events, and depletion of ozone.[19][20]

Sudden stratospheric warming events are associated with weaker polar vortices. This warming of stratospheric air can reverse the circulation in the Arctic Polar Vortex from counter-clockwise to clockwise.[21] These changes aloft force changes in the troposphere below.[22] An example of an effect on the troposphere is the change in speed of the Atlantic Ocean circulation pattern. A soft spot just south of Greenland is where the initial step of downwelling occurs, nicknamed the "Achilles Heel of the North Atlantic". Small amounts of heating or cooling traveling from the polar vortex can trigger or delay downwelling, altering the Gulf Stream Current of the Atlantic, and the speed of other ocean currents. Since all other oceans depend on the Atlantic Ocean's movement of heat energy, climates across the planet can be dramatically affected. The weakening or strengthening of the polar vortex can alter the sea circulation more than a mile beneath the waves.[23] Strengthening storm systems within the troposphere that cool the poles, intensify the polar vortex. La Niña–related climate anomalies significantly strengthen the polar vortex.[24] Intensification of the polar vortex produces changes in relative humidity as downward intrusions of dry, stratospheric air enter the vortex core. With a strengthening of the vortex comes a longwave cooling due to a decrease in water vapor concentration near the vortex. The decreased water content is a result of a lower tropopause within the vortex, which places dry stratospheric air above moist tropospheric air.[25] Instability is caused when the vortex tube, the line of concentrated vorticity, is displaced. When this occurs, the vortex rings become more unstable and prone to shifting by planetary waves.The planetary wave activity in both hemispheres varies year-to-year, producing a corresponding response in the strength and temperature of the polar vortex.[26] The number of waves around the perimeter of the vortex are related to the core size; as the vortex core decreases, the number of waves increase.[27]

The degree of the mixing of polar and mid-latitude air depends on the evolution and position of the polar night jet. In general, the mixing is less inside the vortex than outside. Mixing occurs with unstable planetary waves that are characteristic of the middle and upper stratosphere in winter. Prior to vortex breakdown, there is little transport of air out of the Arctic Polar Vortex due to strong barriers above 420 km (261 miles). The polar night jet which exists below this, is weak in the early winter. As a result, it does not deviate any descending polar air, which then mixes with air in the mid-latitudes. In the late winter, air parcels do not descend as much, reducing mixing.[28] After the vortex is broken up, the ex-vortex air is dispersed into the middle latitudes within a month.[29]

Sometimes, a mass of the polar vortex breaks off before the end of the final warming period. If large enough, the piece can move into Canada and the Midwestern, Central, Southern, and Northeastern United States. This diversion of the polar vortex can occur due to the displacement of the polar jet stream; for example, the significant northwestward direction of the polar jet stream in the western part of the United States during the winters of 2013–2014, and 2014–2015. This caused warm, dry conditions in the west, and cold, snowy conditions in the north-central and northeast.[30] Occasionally, the high-pressure air mass, called the Greenland Block, can cause the polar vortex to divert to the south, rather than follow its normal path over the North Atlantic.[31]

Climate change

Jetstream - Rossby Waves - N hemisphere
Meanders of the northern hemisphere's jet stream developing (a, b) and finally detaching a "drop" of cold air (c); orange: warmer masses of air; pink: jet stream
Southern Hemisphere Ozone Concentration, February 22, 2012

A study in 2001 found that stratospheric circulation can have anomalous effects on weather regimes.[32] In the same year, researchers found a statistical correlation between weak polar vortex and outbreaks of severe cold in the Northern Hemisphere.[33][34] In later years, scientists identified interactions with Arctic sea ice decline, reduced snow cover, evapotranspiration patterns, NAO anomalies or weather anomalies which are linked to the polar vortex and jet stream configuration.[32][34][35][36][37][38][39][40] However, because the specific observations are considered short-term observations (starting c. 13 years ago) there is considerable uncertainty in the conclusions. Climatology observations require several decades to definitively distinguish natural variability from climate trends.[41]

The general assumption is that reduced snow cover and sea ice reflect less sunlight and therefore evaporation and transpiration increases, which in turn alters the pressure and temperature gradient of the polar vortex, causing it to weaken or collapse. This becomes apparent when the jet stream amplitude increases (meanders) over the northern hemisphere, causing Rossby waves to propagate farther to the south or north, which in turn transports warmer air to the north pole and polar air into lower latitudes. The jet stream amplitude increases with a weaker polar vortex, hence increases the chance for weather systems to become blocked. A blocking event in 2012 emerged when a high-pressure over Greenland steered Hurricane Sandy into the northern Mid-Atlantic states.[42]

Ozone depletion

The chemistry of the Antarctic polar vortex has created severe ozone depletion. The nitric acid in polar stratospheric clouds reacts with chlorofluorocarbons to form chlorine, which catalyzes the photochemical destruction of ozone.[43] Chlorine concentrations build up during the polar winter, and the consequent ozone destruction is greatest when the sunlight returns in spring.[44] These clouds can only form at temperatures below about −80 °C (−112 °F). Since there is greater air exchange between the Arctic and the mid-latitudes, ozone depletion at the north pole is much less severe than at the south.[45] Accordingly, the seasonal reduction of ozone levels over the Arctic is usually characterized as an "ozone dent", whereas the more severe ozone depletion over the Antarctic is considered an "ozone hole". This said, chemical ozone destruction in the 2011 Arctic polar vortex attained, for the first time, a level clearly identifiable as an Arctic "ozone hole".[46]

Outside Earth

Mars cyclone
Hubble view of the colossal polar cloud on Mars

Other astronomical bodies are also known to have polar vortices, including Venus (double vortex – that is, two polar vortices at a pole),[47] Mars, Jupiter, Saturn, and Saturn's moon Titan.

Saturn's south pole is the only known hot polar vortex in the solar system.[48]

See also


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Further reading

External links

2013–14 North American winter

The 2013–14 North American winter refers to winter in North America as it occurred across the continent from late 2013 through early 2014. The winter of 2013–14 was one of the most significant for the United States, due in part to the breakdown of the polar vortex in November 2013, which allowed very cold air to travel down into the United States, leading to an extended period of very cold temperatures. The pattern continued mostly uninterrupted throughout the winter and numerous significant winter storms affected the Eastern United States, with the most notable one being a powerful winter storm that dumped ice and snow in the Southeast and Northeast in mid-February. Most of the cold weather abated by the end of March, though a few winter storms did affect the western portions of the U.S. towards the end of the winter.

While there is no well-agreed-upon date used to indicate the start of winter in the Northern Hemisphere, there are two definitions of winter which may be used. Based on the astronomical definition, winter begins at the winter solstice, which in 2013 occurred on December 21, and ends at the March equinox, which in 2014 occurred on March 20. Based on the meteorological definition, the first day of winter is December 1 and the last day February 28. Both definitions involve a period of approximately three months, with some variability.

2017–18 North American cold wave

The 2017–18 North American cold wave was an extreme weather event in North America in which record low temperatures gripped much of the Central, Eastern United States, and parts of Central and Eastern Canada. Starting in late December as a result of the southward shift of the polar vortex, extremely cold conditions froze the eastern United States in the last few days of 2017 as well as into the new year. Following a brief respite in mid-January, cold temperatures swung back into the eastern U.S. shortly afterwards. The cold wave finally dissolved by around January 19, as near-average temperatures returned.

Several winter weather events accompanied the cold wave, the most significant one was a powerful blizzard that impacted the Northeastern U.S. in the first few days of 2018. Some of these events impacted areas that normally do not receive snow, such as Louisiana and Texas. In an extremely rare event, Tallahassee, Florida in extreme north Florida received trace amounts of frozen precipitation for the first time in more than 30 years. In addition, many places broke records for coldest temperatures in the final week of 2017 and the first part of 2018.

2018 Great Britain and Ireland cold wave

Beginning on 22 February 2018, Great Britain and Ireland were affected by a cold wave, dubbed the Beast from the East by the media and officially named Anticyclone Hartmut, which brought widespread unusually low temperatures and heavy snowfall to large areas. The cold wave combined with Storm Emma, part of the 2017–18 UK and Ireland windstorm season, which made landfall in southwest England and southern Ireland on 2 March.

In contrast to usual winter storms, Emma was not formed as a normal low pressure area along with the jetstream; the initial event was an arctic outbreak due to a disordered polar vortex into Central Europe, transporting not only cold air from Siberia to Europe, but on the way to the British Islands according to the lake effect sent a lot of snow into areas of Great Britain and Ireland.

This weather situation repeated itself on the weekend of 17 and 18 March, but was less severe than on the previous occasion due to the onset of spring. This briefer cold snap was given the name "Mini Beast from the East".

Arctic sea ice decline

Arctic sea ice decline refers to the sea ice loss observed in recent decades in the Arctic Ocean. The Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report states that greenhouse gas forcing is predominantly responsible for the decline in Arctic sea ice extent. A study from 2011 suggested that internal variability enhanced the greenhouse gas forced sea ice decline over the last decades. A study from 2007 found the decline to be "faster than forecasted" by model simulations. The IPCC Fifth Assessment Report concluded, with high confidence, that sea ice will continue to decrease in extent, and that there is robust evidence for the downward trend in Arctic summer sea ice extent since 1979. It has been established that the region is at its warmest for at least 40,000 years and the Arctic-wide melt season has lengthened at a rate of 5 days per decade (from 1979 to 2013), dominated by a later autumn freeze-up. Sea ice changes have been identified as a mechanism for polar amplification.

BOOMERanG experiment

In astronomy and observational cosmology, The BOOMERanG experiment (Balloon Observations Of Millimetric Extragalactic Radiation ANd Geophysics) was an experiment which measured the cosmic microwave background radiation of a part of the sky during three sub-orbital (high-altitude) balloon flights. It was the first experiment to make large, high-fidelity images of the CMB temperature anisotropies, and is best known for the discovery in 2000 that the geometry of the universe is close to flat, with similar results from the competing MAXIMA experiment.

By using a telescope which flew at over 42,000 meters high, it was possible to reduce the atmospheric absorption of microwaves to a minimum. This allowed massive cost reduction compared to a satellite probe, though only a tiny part of the sky could be scanned.

The first was a test flight over North America in 1997. In the two subsequent flights in 1998 and 2003 the balloon was launched from McMurdo Station in the Antarctic. It was carried by the Polar vortex winds in a circle around the South Pole, returning after two weeks. From this phenomenon the telescope took its name.

The BOOMERanG team was led by Andrew E. Lange of Caltech and Paolo de Bernardis of the University of Rome La Sapienza.

Block (meteorology)

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.

Climate of North Carolina

North Carolina's climate varies from the Atlantic coast in the east to the Appalachian Mountain range in the west. The mountains often act as a "shield", blocking low temperatures and storms from the Midwest from entering the Piedmont of North Carolina. Most of the state has a humid subtropical climate (Köppen climate classification Cfa), except in the higher elevations of the Appalachians which have a subtropical highland climate (Köppen Cfb). The USDA hardiness zones for the state range from zone 5a (-20°F to -15°F) in the mountains to zone 8b (15°F to 20°F) along the coast. For most areas in the state, the temperatures in July during the daytime are approximately 90 °F (32 °C). In January the average temperatures range near 50 °F (10 °C). (However, a polar vortex or "cold blast" can significantly bring down average temperatures, seen in the winters of 2014 and 2015.)

Climate of Titan

The climate of Titan, the largest moon of Saturn, is similar in many respects to that of Earth, despite having a far lower surface temperature. Its thick atmosphere, methane rain, and possible cryovolcanism create an analogue, though with different materials, to the climatic changes undergone by Earth during its far shorter year.

Early 2014 North American cold wave

The 2014 North American cold wave was an extreme weather event that extended through the late winter months of the 2013–2014 winter season, and was also part of an unusually cold winter affecting parts of Canada and parts of the north-central and upper eastern United States. The event occurred in early 2014 and was caused by a southward shift of the North Polar Vortex. Record-low temperatures also extended well into March.

On January 2, an Arctic cold front initially associated with a nor'easter tracked across Canada and the United States, resulting in heavy snowfall. Temperatures fell to unprecedented levels, and low temperature records were broken across the United States. Business, school, and road closures were common, as well as mass flight cancellations. Altogether, more than 200 million people were affected, in an area ranging from the Rocky Mountains to the Atlantic Ocean and extending south to include roughly 187 million residents of the Continental United States.

Extraterrestrial vortex

Extraterrestrial cyclones are cyclones found on other planets or natural satellites other than the Earth.

February 2015 North American cold wave

The February 2015 North American cold wave was an extreme weather event that affected most of Canada and the eastern half of the United States. Following an earlier cold wave in the winter, the period of below-average temperatures contributed to an already unusually cold winter for the Eastern U.S. Several places broke their records for their coldest February on record, while some areas came very close. The cause of the cold wave was due to the polar vortex advancing southwards into the eastern parts of the U.S, and even making it as far south as the Southeast, where large snow falls are rare. By the beginning of March, although the pattern did continue for the first week, it abated and retreated near the official end of the winter.

In addition to the extremely cold weather, multiple winter storms affected nearly the entire United States, especially in the snow-weary Northeast, which had already seen nearly 3 feet (0.91 m) of snow in the latter part of January; this was added to by roughly 3–4 feet (0.91–1.22 m) more snow, leading to Boston having its highest seasonal snowfall on record.

January–February 2019 North American cold wave

In late January 2019, a severe cold wave caused by a weakened jet stream around the Arctic polar vortex hit the Midwestern United States and Eastern Canada, killing at least 22 people. It came after a winter storm brought up to 13 inches (33 cm) of snow in some regions from January 27–29. On February 2, the polar vortex moved west, and later affected Western Canada and the Western United States.

November 2014 North American cold wave

The November 2014 North American cold wave was an extreme weather event that occurred across most of Canada and the contiguous United States, including parts of the Western United States up to western California. One of the first events of the winter, the cold wave was caused by the northward movement of an extremely powerful bomb cyclone associated with Typhoon Nuri's remnant, which shifted the jet stream far northward, creating an omega block pattern. This allowed a piece of the polar vortex to advance southward into the Central and Eastern United States, bringing record-cold temperatures to much of the region. In contrast, Alaska experienced above-average temperatures.

This was the worst cold wave that the North American region had experienced since an earlier cold wave in early 2014. The cold wave was expected to last for a few weeks, extending at least until American Thanksgiving. Although the Omega Block broke down on November 20, due to a powerful storm moving into the Gulf of Alaska, frigid conditions continued to persist across much of the United States. There was also concern among some meteorologists that another cold wave or abnormally cold trend might persist throughout the winter of 2014–15, the chances of which were "above average." On November 23, a warming trend primarily in the Eastern United States brought an end to the cold wave; however, below-average temperatures were forecast to return to the Midwest by November 24. Despite the development of a second cold wave, it ended on December 6, when a ridge of high pressure brought above-average temperatures to the region, especially in the Central United States.

Polar High

The polar highs are areas of high atmospheric pressure around the north and south poles; the north polar high being the stronger one because land gains and loses heat more effectively than sea. The cold temperatures in the polar regions cause air to descend to create the high pressure (a process called subsidence), just as the warm temperatures around the equator cause air to rise to create the low pressure intertropical convergence zone. Rising air also occurs along bands of low pressure situated just below the polar highs around the 50th parallels of latitude. These extratropical convergence zones are occupied by the polar fronts where air masses of polar origin meet and clash with those of tropical or subtropical origin. This convergence of rising air completes the vertical cycle around the polar cell in each latitudinal hemisphere. Closely related to this concept is the polar vortex.

Surface temperatures under the polar highs are the coldest on Earth, with no month having an average temperature above freezing. Regions under the polar high also experience very low levels of precipitation, which leads them to be known as "polar deserts".

Air flows outwards from the poles to create the polar easterlies in the arctic and antarctic areas.

Saturn's hexagon

Saturn's hexagon is a persisting hexagonal cloud pattern around the north pole of Saturn, located at about 78°N.

The sides of the hexagon are about 14,500 km (9,000 mi) long, which is more than the diameter of Earth (about 12,700 km (7,900 mi)). The hexagon may be a bit greater than 29,000 km (18,000 mi) wide, may be 300 km (190 mi) high, and may be a jet stream made of atmospheric gases moving at 320 km/h (200 mph). It rotates with a period of 10h 39m 24s, the same period as Saturn's radio emissions from its interior. The hexagon does not shift in longitude like other clouds in the visible atmosphere.Saturn's hexagon was discovered during the Voyager mission in 1981 and was later revisited by Cassini-Huygens in 2006. During the Cassini mission, the hexagon changed from a mostly blue color to more of a golden color. Saturn's south pole does not have a hexagon, according to Hubble observations; however, it does have a vortex, and there is also a vortex inside the northern hexagon. Multiple hypotheses for the hexagonal cloud pattern have been developed.


The stratosphere () is the second major layer of Earth's atmosphere, just above the troposphere, and below the mesosphere. The stratosphere is stratified (layered) in temperature, with warmer layers higher and cooler layers closer to the Earth; this increase of temperature with altitude is a result of the absorption of the Sun's ultraviolet radiation by the ozone layer. This is in contrast to the troposphere, near the Earth's surface, where temperature decreases with altitude. The border between the troposphere and stratosphere, the tropopause, marks where this temperature inversion begins. Near the equator, the stratosphere starts at as high as 20 km (66,000 ft; 12 mi), around 10 km (33,000 ft; 6.2 mi) at midlatitudes, and at about 7 km (23,000 ft; 4.3 mi) at the poles. Temperatures range from an average of −51 °C (−60 °F; 220 K) near the tropopause to an average of −15 °C (5.0 °F; 260 K) near the mesosphere. Stratospheric temperatures also vary within the stratosphere as the seasons change, reaching particularly low temperatures in the polar night (winter). Winds in the stratosphere can far exceed those in the troposphere, reaching near 60 m/s (220 km/h; 130 mph) in the Southern polar vortex.

Sudden stratospheric warming

A sudden stratospheric warming (SSW) is an event in which the observed stratospheric temperature rises by several tens of kelvins (up to increases of about 50 °C (about 120 °F)), over the course of a few days. The change is preceded by a situation in which the Polar jet stream of westerly winds in the winter hemisphere is disturbed by natural weather patterns or disturbances in the lower atmosphere.

The E and B Experiment

The E and B Experiment (EBEX) will measure the cosmic microwave background radiation of a part of the sky during two sub-orbital (high-altitude) balloon flights. It is an experiment to make large, high-fidelity images of the CMB polarization anisotropies. By using a telescope which flies at over 42,000 metres high, it is possible to reduce the atmospheric absorption of microwaves to a minimum. This allows massive cost reduction compared to a satellite probe, though only a small part of the sky can be scanned and for shorter duration than a typical satellite mission such as WMAP.

The first flight was an engineering flight over North America in 2009. For the science flight, EBEX was launched on 29 December 2012, near McMurdo Station in Antarctica. It circled around the South Pole using the polar vortex winds before landing on 24 January 2013 about 400 miles from McMurdo.

Winter 1985 cold wave

The winter 1985 cold wave was a meteorological event, the result of the shifting of the polar vortex farther south than is normally seen. Blocked from its normal movement, polar air from the north pushed into nearly every section of the central and eastern half of the United States and Canada, shattering record lows in a number of areas. The event was preceded by unusually warm weather in the eastern U.S. in December 1984, suggesting that there was a build-up of cold air that was suddenly released from the Arctic, a meteorological event known as a mobile polar high, a weather process identified by Professor Marcel Leroux.


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