A fictitious synoptic chart of an extratropical cyclone affecting Great Britain & Ireland. The blue and red arrows between isobars indicate the direction of the wind and its relative temperature, while the "L" symbol denotes the center of the "low". Note the occluded cold and warm frontal boundaries.

This animation shows an extratropical cyclone developing over the United States, starting late on October 25 and running through October 27, 2010.

Extratropical cyclones, sometimes called mid-latitude cyclones or wave cyclones, are a group of cyclones defined as synoptic scale low pressure weather systems that occur in the middle latitudes of the Earth (outside the tropics) having neither tropical nor polar characteristics, and are connected with fronts and horizontal gradients in temperature and dew point otherwise known as "baroclinic zones".^[1] Extratropical cyclones are the everyday phenomena which, along with anticyclones, drive the weather over much of the Earth, producing anything from cloudiness and mild showers to heavy gales and thunderstorms.

Terminology[link]

Extratropical cyclones encompass a class of storms with many names. Although they are sometimes referred to as "cyclones", this is imprecise; cyclone applies to numerous types of low pressure areas. The descriptor extratropical signifies that this type of cyclone generally occurs outside the tropics in the middle latitudes of Earth. The term mid-latitude cyclones may be used because of where they form; "post-tropical cyclones" if extratropical transition has occurred.^[1][2] Weather forecasters and the general public often describe them as "depressions" or "lows". Terms like frontal cyclone, frontal depression, frontal low, extratropical low, non-tropical low and hybrid low are often used as well.

Extratropical cyclones are classified mainly as baroclinic, because they form along zones of temperature and dewpoint gradient known as frontal zones. They can become barotropic late in their life cycle, when the distribution of heat around the cyclone becomes fairly uniform with its radius.^[3]

Formation[link]

Approximate areas of extratropical cyclone formation worldwide

An upper level jet streak. DIV areas are regions of divergence aloft, which will lead to surface convergence and aid cyclogenesis.

Extratropical cyclones form anywhere within the extratropical regions of the Earth (usually between 30° and 60° latitude from the equator), either through cyclogenesis or extratropical transition. A study of extratropical cyclones in the Southern Hemisphere shows that between the 30th and 70th parallels, there are an average of 37 cyclones in existence during any 6-hour period.^[4] A separate study in the Northern Hemisphere suggests that approximately 234 significant extratropical cyclones form each winter.^[5]

Cyclogenesis[link]

Extratropical cyclones form along linear bands of temperature/dewpoint gradient with significant vertical wind shear, and are thus classified as baroclinic cyclones. Initially, cyclogenesis, or low pressure formation, occurs along frontal zones near a favorable quadrant of a maximum in the upper level jetstream known as a jet streak, usually being the right rear and left front quadrants, where divergence ensues.^[6] This causes air to rush out from the top of the air column which in turn forces convergence in the low-level wind field and increased upward motion within the column. The increased upward motion causes surface pressures to lower as the upward air motion counteracts gravity, lessening the weight of the atmosphere (surface pressure) in that location, and thus strengthening the cyclone. As the cyclone strengthens, the cold front sweeps towards the equator and moves around the back of the cyclone. Meanwhile, its associated warm front progresses more slowly, as the cooler air ahead of the system is denser, and therefore more difficult to dislodge. Later, the cyclones occlude as the poleward portion of the cold front overtakes a section of the warm front, forcing a tongue, or trowal, of warm air aloft. Eventually, the cyclone will become barotropically cold and begin to weaken.

A rapidly falling atmospheric pressure is possible due to strong upper level forces on the system, and when pressures fall more than 1 millibar (0.030 inHg) per hour, such a cyclone is sometimes referred to as a bomb.^[7][8][9] These bombs rapidly drop in pressure to below 980 millibars (28.94 inHg) under favorable conditions such as near a natural temperature gradient like the Gulf Stream, or at a preferred quadrant of an upper level jet streak, where upper level divergence is best. The stronger the upper level divergence over the cyclone, the deeper the cyclone can become. Hurricane-force extratropical cyclones are most likely to form in the northern Atlantic and northern Pacific oceans in the months of December and January.^[10] On 14 and 15 December 1986, an extratropical cyclone near Iceland deepened to below 920 hectopascals (27 inHg),^[11] which is a pressure equivalent to a category 5 hurricane. In the Arctic, the average pressure for cyclones is 980 millibars (28.94 inHg) during the winter, and 1,000 millibars (29.53 inHg) during the summer.^[12]

Extratropical transition[link]

Hurricane Florence in the north Atlantic after completing its transition to an extratropical cyclone from a hurricane

Tropical cyclones often transform into extratropical cyclones at the end of their tropical existence, usually between 30° and 40° latitude, where there is sufficient forcing from upper-level troughs or shortwaves riding the Westerlies for the process of extratropical transition to begin. During extratropical transition, the cyclone begins to tilt back into the colder airmass with height, and the cyclone's primary energy source converts from the release of latent heat from condensation (from thunderstorms near the center) to baroclinic processes. The low pressure system eventually loses its warm core and becomes a cold-core system. During this process, a cyclone in extratropical transition (known across the eastern North Pacific and North Atlantic oceans as the post-tropical stage)^[13][14] will invariably form or connect with nearby fronts and/or troughs consistent with a baroclinic system. Due to this, the size of the system will usually appear to increase, while the core weakens. However, after transition is complete, the storm may re-strengthen due to baroclinic energy, depending on the environmental conditions surrounding the system. The cyclone will also distort in shape, becoming less symmetric with time.

On rare occasions, an extratropical cyclone can transit into a tropical cyclone if it reaches an area of ocean with warmer waters and an environment with less vertical wind shear. The peak time of subtropical cyclogenesis (the midpoint of this transition) is in the months of September and October, when the difference between the temperature of the air aloft and the sea surface temperature is the greatest, leading to the greatest potential for instability. An example of this happening is in the 1991 Halloween Nor'easter. ^[15] The process known as "tropical transition" involves the usually slow development of an extratropically cold core vortex into a tropical cyclone.^[16][17]

Structure[link]

Surface pressure and wind distribution[link]

QuikSCAT image of typical extratropical cyclones over the ocean. Note the maximum winds are on the outside of the occlusion.

The windfield of an extratropical cyclone constricts with distance in relation to surface level pressure, with the lowest pressure being found near the center, and the highest winds typically just on the cold/poleward side of warm fronts, occlusions, and cold fronts, where the pressure gradient force is highest.^[18] The area north and west of the cold and warm fronts connected to extratropical cyclones is known as the cold sector, while the area south and east of its associated cold and warm fronts is known as the warm sector.

A clockwise spinning Extratropical cyclone off southern Australia.

The wind flow around a Extratropical cyclone is counterclockwise in the northern hemisphere, and clockwise in the southern hemisphere, due to the Coriolis effect (this manner of rotation is generally referred to as cyclonic). Near this center, the pressure gradient force (from the pressure at the center of the cyclone compared to the pressure outside the cyclone) and the Coriolis force must be in an approximate balance for the cyclone to avoid collapsing in on itself as a result of the difference in pressure.^[19] The central pressure of the cyclone will lower with increasing maturity, while outside of the cyclone, the sea-level pressure is not very low; its typical value is around 1,013 millibars (29.91 inHg), which is the average sea level pressure for Earth. In most extratropical cyclones, the part of the cold front ahead of the cyclone will develop into a warm front, giving the frontal zone (as drawn on surface weather maps) a wave-like shape. Due to their appearance on satellite images, extratropical cyclones can also be referred to as frontal waves early in their life cycle. In the United States, an old name for such a system is "warm wave".^[20]

Once a cyclone occludes, a trough of warm air aloft, or "trowal" for short, will be caused by strong southerly winds on its eastern periphery rotating aloft around its northeast, and ultimately northwestern, periphery (also known as the warm conveyor belt), forcing a surface trough to continue into the cold sector on a similar curve to the occluded front. The trowal creates the portion of an occluded cyclone known as its comma head, due to the comma-like shape of the mid-tropospheric cloudiness that accompanies the feature. It can also be the focus of locally heavy precipitation, with thunderstorms possible if the atmosphere along the trowal is unstable enough for convection.^[21]

Vertical structure[link]

Extratropical cyclones slant back into colder air masses and strengthen with height, sometimes exceeding 30,000 feet (approximately 9 km) in depth.^[22] Above the surface of the earth, the air temperature near the center of the cyclone is increasingly colder than the surrounding environment. These characteristics are the direct opposite of those found in their tropical cyclones; thus, they are sometimes called "cold-core lows".^[23] Various charts can be examined to check the characteristics of a cold-core system with height, such as the 700 millibars (20.67 inHg) chart, which is at about 10,000 feet (3,048 meters) in height. Cyclone phase diagrams are used to tell whether a cyclone is tropical, subtropical, or extratropical.^[24]

Cyclone evolution[link]

Norwegian model and Shapiro-Keyser model differences in frontal structure

There are two models of cyclone development and lifecycles in common use—the Norwegian model and the Shapiro-Keyser Model.^[25]

Norwegian cyclone model[link]

Of the two theories on extratropical cyclone structure and life cycle, the older is the Norwegian Cyclone Model, developed during World War I. In this theory, cyclones develop as they move up and along a frontal boundary, eventually occluding and reaching a barotropically cold environment.^[26] It was developed completely from surface-based weather observations, including descriptions of clouds found near frontal boundaries. This theory still retains merit, as it is a good description for extratropical cyclones over continental landmasses.

Shapiro-Keyser model[link]

A second competing theory for extratropical cyclone development over the oceans is the Shapiro-Keyser model, developed in 1990.^[27] Its main differences with the Norwegian Cyclone Model are the fracture of the cold front, treating warm-type occlusions and warm fronts as the same, and allowing the cold front to progress through the warm sector perpendicular to the warm front. This model was based on oceanic cyclones and their frontal structure, as seen in surface observations and in previous projects which used aircraft to determine the vertical structure of fronts across the northwest Atlantic.

Warm seclusion[link]

A warm seclusion is the mature phase of the extratropical cyclone lifecycle. This was conceptualized after the ERICA field experiment of the late 1980s, which produced observations of intense marine cyclones that indicated an anomalously warm low-level thermal structure, secluded (or surrounded) by a bent-back warm front and a coincident chevron-shaped band of intense surface winds.^[28] The Norwegian Cyclone Model, as developed by the Bergen School of Meteorology, largely observed cyclones at the tail end of their lifecycle and used the term occlusion to identify the decaying stages.^[29]

Warm seclusions may have cloud-free, eye-like features at their center (reminiscent of tropical cyclones), significant pressure falls, hurricane force winds, and moderate to strong convection. The most intense warm seclusions often attain pressures less than 950 millibars (28.05 inHg) with a definitive lower to mid-level warm core structure.^[28] A warm seclusion, the result of a baroclinic lifecycle, occurs at latitudes well poleward of the tropics.

As latent heat flux releases are important for their development and intensification, most warm seclusion events occur over the oceans; they may impact coastal nations with hurricane force winds and torrential rain.^[27][30] Climatologically, the Northern Hemisphere sees warm seclusions during the cold season months, while the Southern Hemisphere may see a strong cyclone event such as this during all times of the year.

In all tropical basins, except the Northern Indian Ocean, the extratropical transition of a tropical cyclone may result in reintensification into a warm seclusion. For example, Hurricane Maria of 2005 reintensified into a strong baroclinic system and achieved warm seclusion status at maturity (or lowest pressure).^[31]

Motion[link]

A zonal flow regime. Note the dominant west-to-east flow as shown in the 500 hPa height pattern.

A February 24, 2007 radar image of a large Extratropical cyclonic storm system at its peak over the central United States.

Extratropical cyclones are generally driven, or "steered", by deep westerly winds in a general west to east motion across both the Northern and Southern hemispheres of the Earth. This general motion of atmospheric flow is known as "zonal".^[32] Where this general trend is the main steering influence of an extratropical cyclone, it is known as a "zonal flow regime".

When the general flow pattern buckles from a zonal pattern to the meridional pattern,^[33] a slower movement in a north or southward direction is more likely. Meridional flow patterns feature strong, amplified troughs and ridges, generally with more northerly and southerly flow.

Changes in direction of this nature are most commonly observed as a result of a cyclone's interaction with other low pressure systems, troughs, ridges, or with anticyclones. A strong and stationary anticyclone can effectively block the path of an extratropical cyclone. Such blocking patterns are quite normal, and will generally result in a weakening of the cyclone, the weakening of the anticyclone, a diversion of the cyclone towards the anticyclones periphery, or a combination of all three to some extent depending on the precise conditions. It is also common for an extratropical cyclone to strengthen as the blocking anticyclone or ridge weakens in these circumstances.^[34]

Where an extratropical cyclone encounters another extratropical cyclone (or almost any other kind of cyclonic vortex in the atmosphere), the two may combine to become a binary cyclone, where the vortices of the two cyclones rotate around each other (known as the "Fujiwhara effect"). This most often results in a merging of the two low pressure systems into a single extratropical cyclone, or can less commonly result in a mere change of direction of either one or both of the cyclones.^[35] The precise results of such interactions depend on factors such as the size of the two cyclones, their strength, their distance from each other, and the prevailing atmospheric conditions around them.

Effects[link]

General[link]

Preferred region of snowfall in an extratropical cyclone

Extratropical cyclones can bring mild weather with a little rain and surface winds of 15–30 km/h (9.3–19 mph), or they can be cold and dangerous with torrential rain and winds exceeding 119 km/h (74 mph),^[36] (sometimes referred to as windstorms in Europe). The band of precipitation that is associated with the warm front is often extensive. In mature extratropical cyclones, an area known as the comma head on the northwest periphery of the surface low can be a region of heavy precipitation, frequent thunderstorms, and thundersnows. Cyclones tend to move along a predictable path at a moderate rate of progress. During fall, winter, and spring, the atmosphere over continents can be cold enough through the depth of the troposphere to cause snowfall.

Severe weather[link]

Squall lines, or solid bands of strong thunderstorms, can form ahead of cold fronts and lee troughs due to the presence of significant atmospheric moisture and strong upper level divergence, leading to hail and high winds.^[37] When significant directional wind shear exists in the atmosphere ahead of a cold front in the presence of a strong upper level jet stream, tornado formation is possible.^[38] Although tornadoes can form anywhere on Earth, the greatest number occur in the Great Plains in the United States, because downsloped winds off the north-south oriented Rocky Mountains, which can form a dryline, aid their development at any strength.

Explosive development of extratropical cyclones can be sudden. The storm known in the UK as the "Great Storm of 1987" deepened to 953 millibars (28.14 inHg) with a highest recorded wind of 220 km/h (140 mph), resulting in the loss of 19 lives, 15 million trees, widespread damage to homes and an estimated economic cost of £1.2 billion (US$2.3 billion).^[39]

Although most tropical cyclones that become extratropical quickly dissipate or are absorbed by another weather system, they can still retain winds of hurricane or gale force. In 1954, Hurricane Hazel became extratropical over North Carolina as a strong Category 3 storm. The Columbus Day Storm of 1962, which evolved from the remains of Typhoon Freda, caused heavy damage in Oregon and Washington, with widespread damage equivalent to at least a Category 3. In 2005, Hurricane Wilma began to lose tropical characteristics while still sporting Category 3-force winds (and became fully extratropical as a Category 1 storm). ^[40]

Climate and general circulation[link]

In the classic analysis by Edward Lorenz (the Lorenz Energy Cycle),^[41] extratropical cyclones (so called atmospheric transients) acts as a mechanism in converting potential energy that is created by pole to equator temperature gradients to eddy kinetic energy. In the process, the pole-equator temperature gradient is reduced (i.e. energy is transported poleward to warm up the higher latitudes).

The existence of such transients are also closely related to the formation of the Icelandic and Aleutian Low — the two most prominent general circulation features in the mid- to sub-polar northern latitudes.^[42] The two lows are formed by both the transport of kinetic energy and the latent heating (the energy released when water phase changed from vapor to liquid during precipitation) from the extratropical cyclones.

Historic storms[link]

A violent storm during the Crimean War on November 14, 1854 wrecked 30 vessels, and sparked initial investigations into meteorology and forecasting in Europe. In the United States, the Columbus Day Storm of 1962 led to Oregon's lowest measured pressure of 965.5 hPa (28.51 inHg), violent winds, and US$170 million in damage (1964 dollars).^[43] A rapidly strengthening storm struck Vancouver Island on October 11, 1984, and inspired the development of moored buoys off the western coast of Canada.^[44] The Wahine disaster, New Zealand's most infamous maritime disaster, that caused the death of 53 people occurred during a major extratropical storm. The Braer Storm of January 1993 was the strongest extratropical cyclone known to occur across the northern Atlantic ocean, with a central pressure of 914 millibars (27.0 inHg).^[45]

In the Southern Hemisphere, a violent extratropical storm hit Uruguay on August 23–24, 2005, killing 10 people.^[46] The system's winds exceeded 100 mph (160 km/h) while Montevideo, the country's capital with 1.5 million inhabitants, was affected by tropical storm-force winds for over 12 hours and by hurricane-force winds for nearly four hours.^[47] Peak gusts were registered at Carrasco International Airport as 107 mph (172 km/h) and at the Harbour of Montevideo as 116 mph (187 km/h). The lowest reported pressure was 991.7 hPa (29.28 inHg). Extratropical cyclones are common in this part of the globe during fall, winter and spring months. The winds usually peak to 80–110 km/h (50–68 mph), but winds of 116 mph (187 km/h) are very uncommon.^[47]

See also[link]

Weather portal

References[link]

Alfred Wegener
Alfred Wegener, around 1925
Born	(1880-11-01)November 1, 1880 Berlin, German Empire
Died	November 1930 (aged 50) Clarinetania, Greenland
Residence	Germany
Citizenship	German
Nationality	German
Fields	Meteorology, Geology, Astronomy
Alma mater	University of Berlin
Doctoral advisor	Julius Bauschinger
Known for	Continental drift theory
Influenced	Johannes Letzmann
Signature

Alfred Lothar Wegener (November 1, 1880 – November 1930) was a German polar researcher, geophysicist and meteorologist.

During his lifetime he was primarily known for his achievements in meteorology and as a pioneer of polar research, but today he is most remembered for first proposing the theory of continental drift (Kontinentalverschiebung) in 1912, which hypothesized that the continents were slowly drifting around the Earth. His hypothesis was controversial and not widely accepted until the 1950s, when numerous discoveries such as palaeomagnetism provided strong support for continental drift, and thereby a substantial basis for today's model of Plate tectonics.^[1][2] Wegener was involved in several expeditions to Greenland to study polar air circulation before the existence of the jet stream was accepted. Expedition participants made many meteorological observations and achieved the first-ever overwintering on the inland Greenland ice sheet as well as the first-ever boring of ice cores on a moving Arctic glacier.

Biography[link]

Early life and education[link]

On November 1, 1880, Alfred Wegener was born in Berlin as the youngest of five children in a clergyman's family. His father, Richard Wegener, was a theologian and teacher of classical languages at the Berlinisches Gymnasium zum Grauen Kloster. In 1886 his family purchased a former manor house near Rheinsberg, which they used as a vacation home. Today there is an Alfred Wegener Memorial site and tourist information office in a nearby building that was once the local schoolhouse^[3].

Commemorative plaque on Wegener's former school in Wallstrasse

Wegener attended school at the Köllnische Gymnasium on Wallstrasse in Berlin (a fact which is memorialized on a plaque on this protected building, now a school of music), graduating as the best in his class. Afterward he studied Physics, meteorology and Astronomy in Berlin, Heidelberg and Innsbruck. From 1902 to 1903 during his studies he was an assistant at the Urania astronomical observatory. He obtained a doctorate in astronomy in 1905 based on a dissertation written under the supervision of Julius Bauschinger at Friedrich Wilhelms University (today Humboldt University), Berlin. Wegener had always maintained a strong interest in the developing fields of meteorology and climatology and his studies afterwards focused on these disciplines.

In 1905 Wegener became an assistant at the Aeronautischen Observatorium Lindenberg near Beeskow. He worked there with his brother Kurt, two years his senior, who was likewise a scientist with an interest in meteorology and polar research. The two pioneered the use of weather balloons to track air masses. On a balloon ascent undertaken to carry out meteorological investigations and to test a celestial navigation method using a particular type of quadrant (“Libellenquadrant”), the Wegener brothers set a new record for a continuous balloon flight, remaining aloft 52.5 hours from April 5-7, 1906.^[4]

First Greenland expedition and years in Marburg[link]

In that same year 1906, Wegener participated in the first of his three Greenland expeditions, later regarding this experience as marking a decisive turning point in his life. The expedition was led by the Dane Ludvig Mylius-Erichsen and charged with studying the last unknown portion of the northeastern coast of Greenland. During the expedition Wegener constructed the first meteorological station in Greenland near Danmarkshavn, where he launched kites and tethered balloons to make meteorological measurements in an Arctic climatic zone. Here Wegener also made his first acquaintance with death in a wilderness of ice when the expedition leader and two of his colleagues died on an exploratory trip undertaken with sled dogs.

After his return in 1908 and until World War I, Wegener was a lecturer in meteorology, applied astronomy and cosmic physics at the University of Marburg. His students and colleagues in Marburg particularly valued his ability to clearly and understandably explain even complex topics and current research findings without sacrificing precision. His lectures formed the basis of what was to become a standard textbook in meteorology, first written In 1909/1910: Thermodynamik der Atmosphäre (Thermodynamics of the Atmosphere), in which he incorporated many of the results of the Greenland expedition.

On 6 January 1912 he publicized his first thoughts about continental drift in a lecture at a session of the Geologischen Vereinigung at the Senckenberg-Museum, Frankfurt/M and in three articles in the journal Petermanns Geographischen Mitteilungen^[5]

Second Greenland expedition[link]

After a stopover in Iceland to purchase and test ponys as pack animals, the expedition arrived in Danmarkshavn. Even before the trip to the inland ice began the expedition was almost annihilated by a calving glacier. The Danish expedition leader, Johan Peter Koch, broke his leg when he fell into a glacier crevasse and spent months recovering in a sickbed. Wegener and Koch were the first to winter on the inland ice in northeast Greenland.^[6] Inside their hut they drilled to a depth of 25 m with an auger. In summer 1913 the team crossed the inland ice, the four expedition participants covering a distance twice as long as Fridtjof Nansen's southern Greenland crossing in 1888. Only a few kilometers from the western Greenland setlement of Kangersuatsiaq the small team ran out of food while struggling to find their way through difficult glacial breakup terrain. But at the last moment, after the last pony and dog had been eaten, they were picked up at a fjord by the clergyman of Upernavik, who just happened to be visiting a remote congregation at the time.

Later in 1913 after his return Wegener married Else Köppen, the daughter of his former teacher and mentor, the meteorologist Wladimir Köppen. The young pair lived in Marburg, where Wegner resumed his university lectureship.

World War I[link]

As an infantry reserve officer Wegener was immediately conscripted when war began in 1914. On the war front in Belgium he experienced fierce fighting but his term lasted only a few months: after being wounded twice he was declared unfit for active service and assigned to the army weather service. This activity required him to travel constantly between various weather stations in Germany, on the Balkans, on the Western Front and in the Baltic region.

Nevertheless he was able in 1915 to complete the first version of his major work, Die Entstehung der Kontinente und Ozeane (“The Origin of Continents and Oceans”). His brother Kurt remarked that Alfred Wegener’s motivation was to “reestablish the connection between geophysics on the one hand and geography and geology on the other, which had become completely ruptured because of the specialized development of these branches of science.”

Interest in this small publication was however low, also because of wartime chaos. By the end of the war Wegener had published almost 20 additional meteorological and geophysical papers in which he repeatedly embarked for new scientific frontiers. In 1917 he undertook a scientific investigation of the Treysa meteorite.

Postwar period and third expedition[link]

Wegener obtained a position as a meteorologist at the German Naval Observatory (Deutsche Seewarte) and moved to Hamburg with his wife and their two daughters. In 1921 he was appointed senior lecturer at the new University of Hamburg. From 1919 to 1923 Wegener worked on Die Klimate der geologischen Vorzeit (“The Climates of the Geological Past”), published together with his father-in-law, Wladimar Köppen. In 1922 the third, fully revised edition of “The Origin of Continents and Oceans” appeared, and discussion began on his theory of continental drift, first in the German language area and later internationally. Withering criticism was the response of most experts.

In 1924 Wegener was appointed to a professorship in meteorology and geophysics in Graz, which finally provided him with a secure position for himself and his family. He concentrated on physics and the optics of the atmosphere as well as the study of tornados. Scientific assessment of his second Greenland expedition (ice measurements, atmospheric optics, etc.) continued to the end of the 1920s.

In November 1926 Wegener presented his continental drift theory at a symposium of the American Association of Petroleum Geologists in New York City, again earning rejection from everyone but the chairman. Three years later the fourth and final expanded edition of “The Origin of Continents and Oceans” appeared.

In 1929 Wegener embarked on his third trip to Greenland, which laid the groundwork for a later main expedition and included a test of an innovative, propeller-driven snowmobile.

Fourth and last expedition[link]

Wegener (left) and Villumsen (right) in Greenland; November 1st, 1930.

Wegener's last Greenland expedition was in 1930. The 14 participants under his leadership were to establish three permanent stations from which the thickness of the Greenland ice sheet could be measured and year-round Arctic weather observations made. Wegener felt personally responsible for the expedition's success, as the German government had contributed $120,000 ($1.5 million in 2007 dollars) at a time when Germans were starving to death owing to post-war shortages. Success depended on enough provisions being transferred from West camp to Eismitte ("mid-ice") for two men to winter there, and this was a factor in the decision that led to his death. Owing to a late thaw, the expedition was six weeks behind schedule and, as summer ended, the men at Eismitte sent a message that they had insufficient fuel and so would return on October 20.

Vehicles used by the 1930 expedition (stored).

On September 24, although the route markers were by now largely buried under snow, Wegener set out with thirteen Greenlanders and his meteorologist Fritz Loewe to supply the camp by dog sled. During the journey the temperature reached −60 °C (−76 °F) and Loewe's toes became so frostbitten they had to be amputated with a penknife without anesthetic. Twelve of the Greenlanders returned to West camp. On October 19, the remaining three members of the expedition reached Eismitte. There being only enough supplies for three at Eismitte, Wegener and Rasmus Villumsen took two dog sleds and made for West camp. They took no food for the dogs and killed them one by one to feed the rest until they could run only one sled. While Villumsen rode the sled, Wegener had to use skis. They never reached the camp. The expedition was completed by his brother, Kurt Wegener.

Death[link]

Six months later, on May 12, 1931, Wegener's body was found halfway between Eismitte and West camp. It had been buried (by Villumsen) with great care and a pair of skis marked the grave site. Wegener had been fifty years of age and a heavy smoker and it was believed that he had died of heart failure brought on by overexertion. His body was reburied in the same spot by the team that found him and the grave was marked with a large cross. After burying Wegener, Villumsen had resumed his journey to West camp but was never seen again. He was twenty three when he died and it is estimated that his body, and Wegener's diary, now lie under more than 100 metres (330 ft) of accumulated ice and snow.

Continental drift theory[link]

Alfred Wegener first thought of this idea by noticing that the different large landmasses of the Earth almost fit together like a jigsaw. The Continental shelf of the Americas fit closely to Africa and Europe, and Antarctica, Australia, India and Madagascar fit next to the tip of Southern Africa. But Wegener only took action after reading a paper in Autumn 1911 and seeing that a flooded land-bridge contradicts isostasy.^[7] Wegener's main interest was meteorology, and he wanted to join the Denmark-Greenland expedition scheduled for mid 1912. So he hurried up to present his Continental Drift hypothesis on January 6, 1912. He analyzed either side of the Atlantic Ocean for rock type, geological structures and fossils. He noticed that there was a significant similarity between matching sides of the continents, especially in fossil plants. His hypothesis was thus strongly supported by the physical evidence, and was a pioneering attempt at a rational explanation.

Fossil patterns across continents (Gondwanaland).

From 1912, Wegener publicly advocated the theory of "continental drift", arguing that all the continents were once joined together in a single landmass and have drifted apart. He supposed the cause might be the centrifugal force of the Earth's rotation ("Polflucht") or the astronomical precession. Wegener also speculated on sea-floor spreading and the role of the mid-ocean ridges, stating: the Mid-Atlantic Ridge ... zone in which the floor of the Atlantic, as it keeps spreading, is continuously tearing open and making space for fresh, relatively fluid and hot sima [rising] from depth.^[8] However, he did not pursue these ideas in his later works.

In 1915, in The Origin of Continents and Oceans (Die Entstehung der Kontinente und Ozeane), Wegener published the theory that there had once been a giant continent, he named "Urkontinent" (German word meaning "origin of the continents",^[9] in a way equivalent to the Greek "Pangaea",^[10] meaning "All-Lands" or "All-Earth") and drew together evidence from various fields. Expanded editions during the 1920s presented the accumulating evidence. The last edition, just before his untimely death, revealed the significant observation that shallower oceans were geologically younger.

Wegener during the J.P. Koch's Expedition 1912 - 1913 in the winter base "Borg".

Reaction[link]

In his work, Wegener presented a large amount of very strong evidence in support of continental drift, but the mechanism remained elusive. While his ideas attracted a few early supporters such as Alexander Du Toit from South Africa and Arthur Holmes in England, the hypothesis was generally met with skepticism from largely conservative scientists, who were resistant to any change in the status quo. The one American edition of Wegener's work, published in 1925, was received so poorly that the American Association of Petroleum Geologists organized a symposium specifically in opposition to the continental drift hypothesis. Its opponents could argue, as did the Leipziger geologist Franz Kossmat, that the oceanic crust was too "firm" for the continents to "simply plough through", a suggestion which ignored the plasticity of all rocks at depth and at high temperatures and pressures. The comment also ignored the vast time-scale over which continental drift has occurred, effectively the total age of the earth of about 4.5 billion years.

In 1943 George Gaylord Simpson wrote a vehement attack on the theory (as well as the rival theory of sunken land bridges) and put forward his own permanentist views.^[11] Alexander du Toit wrote a rejoinder in the following year,^[12] but G.G.Simpson's influence was so powerful that even in countries previously sympathetic towards continental drift, like Australia, Wegener's hypothesis fell out of favour.

Modern developments[link]

The tectonic plates of the world were mapped in the second half of the 20th century.

In the early 1950s, the new science of paleomagnetism pioneered at Cambridge University by S. K. Runcorn and at Imperial College by P.M.S. Blackett was soon producing data in favour of Wegener's theory. By early 1953 samples taken from India showed that the country had previously been in the Southern hemisphere as predicted by Wegener. By 1959, the theory had enough supporting data that minds were starting to change, particularly in the United Kingdom where, in 1964, the Royal Society held a symposium on the subject.^[13]

Additionally, the 1960s saw several developments in geology, notably the discoveries of seafloor spreading and Wadati-Benioff zones, led to the rapid resurrection of the continental drift hypothesis and its direct descendant, the theory of plate tectonics. Alfred Wegener was quickly recognized as the founding father of one of the major scientific revolutions of the 20th century.

With the advent of the Global Positioning System (GPS), it became possible to measure continental drift directly.^[14]

Awards and honors[link]

The Alfred Wegener Institute for Polar and Marine Research in Bremerhaven, Germany, was established in 1980 on his centenary. It awards the Wegener Medal in his name.^[15] The crater Wegener on the Moon and the crater Wegener on Mars, as well as the asteroid 29227 Wegener and the peninsula where he died in Greenland (Wegener Peninsula near Ummannaq, 71°12′N 51°50′W / 71.2°N 51.833°W / 71.2; -51.833), are named after him.^[16]

The European Geosciences Union sponsors an Alfred Wegener Medal & Honorary Membership "for scientists who have achieved exceptional international standing in atmospheric, hydrological or ocean sciences, defined in their widest senses, for their merit and their scientific achievements."^[17]

See also[link]

• Hair ice

References[link]

This article incorporates information from the equivalent article on the German Wikipedia.

Selected works[link]

• Wegener, Alfred (1911) (in German). Thermodynamik der Atmosphäre [Thermodynamics of the Atmosphere]. Leipzig: Verlag Von Johann Ambrosius Barth. http://books.google.com/?id=slxDAAAAIAAJ&pg=PR1&dq=alfred+wegener. 
• Wegener, Alfred (1912). "Die Herausbildung der Grossformen der Erdrinde (Kontinente und Ozeane), auf geophysikalischer Grundlage" (in German). Petermanns Geographische Mitteilungen 63: 185–195, 253–256, 305–309. Presented at the annual meeting of the German Geological Society, Frankfurt am Main (January 6, 1912). 
• Wegener, Alfred (July 1912). "Die Entstehung der Kontinente" (in German). Geologische Rundschau 3 (4): 276–292. Bibcode 1912GeoRu...3..276W. DOI:10.1007/BF02202896. 
• Wegener, Alfred (1922) (in German). Die Entstehung der Kontinente und Ozeane [The Origin of Continents and Oceans]. ISBN 3-443-01056-3. LCCN unk83068007. 
• Wegener, Alfred (1929) (in German). Die Entstehung der Kontinente und Ozeane [The Origin of Continents and Oceans] (4 ed.). Braunschweig: Friedrich Vieweg & Sohn Akt. Ges. ISBN 3-443-01056-3. 
• Wegener, Elsie; Loewe, Fritz, eds. (1939). Greenland Journey, The Story of Wegener’s German Expedition to Greenland in 1930-31 as told by Members of the Expedition and the Leader’s Diary. London: Blackie & Son Ltd.. Translated from the seventh German edition by Winifred M. Deans. 
• Wegener, Alfred (1966). The Origin of Continents and Oceans. New York: Dover. ISBN 0-486-61708-4. Translated from the fourth revised German edition by John Biram. 
• Wegener, Alfred (1968). The Origin of Continents and Oceans. London: Methuen. ISBN 0-486-61708-4. Translated from the fourth German edition by John Biram with an introduction by B.C. King. 

External links[link]

Wikimedia Commons has media related to: Alfred Wegener

• Wegener Institute website
• USGS biography of Wegener
• Wegener biography at Pangaea.org
• Alfred Wegener (1880-1930) - Biographical material