Bergen School of Meteorology The early realisation that the large-scale behaviour of the atmosphere is due to physical processes that govern the behaviour of fluids – mainly hydrodynamics, thermodynamics and mechanics – and that it can be described in mathematical terms defines an approach to meteorology that became known as the Bergen School of Meteorology. It was pioneered by a group of scientists that built up around and was influenced by Norwegian physicist Vilhelm Bjerknes, who was based at the University of Bergen in Norway. This scientific approach started to take hold in the early twentieth century immediately after the end of the First World War and led to many significant developments in the science of meteorology.
Continental climate A climate with a large variation in temperature between the summer and winter months. These conditions occur in the interior of continents because such areas are away from the moderating influence of the sea on air temperature. This means that continental interiors tend to be hotter than coastal areas during summer and cooler than the coast in winter.
Equinox/equinoctial seasons The equinoctial seasons contain the equinoxes – when day and night are equally long – which occurs twice yearly on or around 21 March and 21 September. They are the seasons between winter and summer; so spring and autumn are equinoctial seasons. In meteorological terms equinoctial seasons are usually considered to be the three-month periods of March, April and May and September, October and November. The equinoctial seasons are transition seasons between the more extreme summer and winter.
Maritime climate Coastal regions that benefit from a prevailing wind that comes from the sea have a maritime climate where the ocean acts to cool the air in the summer and warm it in the winter. Annual temperature variations in a maritime climate are moderate and much less than those found in continental interiors. Many western coasts of continents in mid-latitudes experience a maritime climate thanks to prevailing westerly winds.
Polar night/day At both the poles, the period when the darkness of the night lasts for 24 hours is known as the polar night and when the light of the day lasts for 24 hours it is known as the polar day. Polar days and polar nights are due to the tilt of the Earth’s axis of rotation to the plane of its orbit around the Sun. It means that for a period around the summer solstice the Sun does not set and the surface of the Earth continues to benefit from warming sunlight. Similarly, for a period around the winter solstice the Sun does not rise in polar regions and this means that the surface is not warmed by sunlight and so cannot warm the air above it. The result is that temperatures drop significantly and it becomes extremely cold. The coldest ever temperature recorded on Earth was -89°C in Antarctica during the polar night of July 1983.
Stratosphere The layer of the Earth’s atmosphere between an altitude of around 12 and 50 kilometres above sea level. The stratosphere begins closer to the surface at the poles (around 8 kilometres) and higher above the surface at the equator (around 18 kilometres). It features extremely cold, thin, dry air and is home to the ozone layer that protects us from much of the Sun’s damaging ultraviolet light. Unlike the lower atmosphere, air temperature in the stratosphere increases with altitude due to the warming effect of this ozone, which is heated by the energy from the ultraviolet light it absorbs.
Tropopause The boundary between the troposphere, the lowest layer of the Earth’s atmosphere, and the stratosphere which begins about 12 kilometres above sea level. The tropopause is the border of a temperature inversion, marking the point at which temperatures stop falling with altitude in the troposphere and start rising with altitude in the stratosphere. It also acts as a boundary between layers of the atmosphere with different chemical compositions dividing the troposphere, which contains a lot of water vapour and very little ozone, from the stratosphere, which is very dry and includes the ozone layer.
Troposphere The lowest layer of the atmosphere where most of the weather that we experience takes place. The troposphere ranges from sea level to the edge of the stratosphere around 12 kilometres in altitude -higher at the equator and lower at the poles. The troposphere contains around 75 per cent of the total mass of the atmosphere and nearly all its water vapour. Both the temperature of the air and the amount of moisture in the air decrease with altitude in the troposphere.
Westerlies and Easterlies The prevailing winds that blow from the west towards the east in the mid-latitudes of both hemispheres between 30 and 60 degrees are known as the Westerlies. Another set of winds, the Easterlies, blow from an easterly direction in a band between 30 degrees and the equatorial region of each hemisphere and are known as the Trade Winds; they blow from the northeast towards the equator in the northern hemisphere and from the southeast towards the equator in the southern hemisphere. In the polar regions above 60 degrees latitude in both hemispheres there are other easterly winds although these tend to be less regular.
On 11 November 1911 in Springfield, Missouri, USA, the temperature plummeted from 27°C in mid-afternoon to -6°C by 7pm as cold air surged in from the northwest. This drastic drop was accompanied by thunderstorms, hail and wind gusts in excess of 110 kph that caused damage to buildings. Similar changes were experienced across much of central USA, spawning large numbers of deadly tornadoes. Although an extreme case, what Springfield experienced dramatically illustrates that atmospheric temperature and humidity in mid-latitudes tend to transition abruptly rather than varying gradually over long distances. The interfaces between air types are called weather fronts, and their movement accounts for much of the change in day-to-day weather. Between fronts, air is more uniform and is characteristic of its origin, leading to the idea of distinct air masses. For example, air that has spent time over the subpolar ocean will have different properties to air that has been over subtropical waters, and air masses of continental origin are different again. Air masses meet at fronts, where the sharp difference in temperature and humidity can produce clouds and precipitation. Most vigorous fronts occur as part of low-pressure cyclones, which act to sharpen frontal contrasts by making the opposing air masses spiral together.
Weather fronts are sharp transitions between air masses – air of different characteristics. The sharp temperature and humidity gradients produce clouds, precipitation and sometimes storms.
The term ‘weather front’ was coined shortly after the First World War, because plotted on weather maps these features resembled the front lines of armies on military maps of the time. The concept of fronts was introduced by the Bergen School of Meteorology, which established the principles of how mid- latitude cyclones form at the interface between cold polar and warm subtropical air masses.
JACOB BJERKNES
1897–1975
Norwegian-American meteorologist who, with colleagues at the Bergen School, developed the Norwegian cyclone model
Jeff Knight
The atmosphere is a restless multitude of shifting air masses with differences in humidity and temperature. Weather maps allow air masses to be visualized by marking the weather fronts where air masses meet.
In middle latitudes the winds are generally from west to east and become stronger with height, peaking near 10 kilometres on the tropopause, the boundary between the troposphere and stratosphere. This region of maximum westerly winds near the tropopause is called the jet stream. Speeds are usually 40 ms-1(144 km/h) but can be two to three times as fast. The jet stream forms a broken, wavy ribbon around the Earth, typically 3 kilometres deep and 300 kilometres wide, but extends many thousands of kilometres. It was discovered in the 1920s by a Japanese meteorologist, Ooishi, who observed the behaviour of special balloons he released. It became well known during the Second World War because of its impact on aircraft, an aspect that is still important. The westerly winds increasing with height are related to the Earth’s rotation and the contrast between the cold air at high latitudes and the warm air at low latitudes. The jet stream is located where the temperature contrast is strong and so its speed is greatest in winter when this contrast is strongest. Weather systems grow on the strong temperature contrast in the jet stream region, drawing their energy from it and being steered by it.
The jet stream is a narrow region of highlevel westerly winds that meanders around the Earth and drives weather.
The Earth has one or two westerly jet streams: separate polar and subtropical jets are found during winter in the southern hemisphere and also at some longitudes in the northern hemisphere. At other longitudes they combine together to form one jet. In the summer there are easterly jets south of the Indian monsoon and over West Africa. Saturn and Jupiter have many jet streams at different latitudes because of their size and rapid rotation.
WASABRO OOISHI
1874–1950
Japanese meteorologist and Director of Japan’s first upper-air observatory. He wrote his 1926 report in Esperanto
Brian Hoskins
Such is the strength of the jet stream that it can dramatically affect flight times, depending on whether aircraft are going with or against the flow.
Storms in the middle latitudes typically move eastwards across the major oceans along definite paths called storm tracks. Early sailors soon appreciated the existence of ‘preferred’ regions for stormy weather, and by the middle of the eighteenth century there were detailed maps of the North Atlantic storm track. This begins near the east coast of North America and usually tilts slightly northeast across the Atlantic towards northwest Europe. The North Pacific storm track is oriented more west-east, from Japan to near the west coast of North America. In the southern hemisphere the main winter storm track spirals eastwards and polewards across the South Atlantic and Indian Oceans, finishing close to the coast of Antarctica. In summer it encircles Antarctica. Storm tracks are closely related to the westerly jet streams that cross the oceans in middle latitudes. This is because the jet streams are regions of strong north-south temperature contrast, and this contrast provides much of the energy for the growth of the storms. In turn, the storms drive the near-surface westerly winds that accompany the jet streams. The storm tracks are strongest in winter when they are also generally furthest from the poles.
Low-pressure weather systems, or cyclones, develop and move eastwards across the North Atlantic and Pacific Oceans on preferred paths called storm tracks.
Storm tracks can vary in latitude and length. Sometimes the North Atlantic storm track finishes near Norway, and at other times closer to southern Europe. Sometimes it continues into western Europe, and the low-pressure systems are young and vigorous rather than mature and inactive. If, however, a blocking high pressure dominates northern Europe the storm track and its associated weather systems are unable to get close.
BLOCKING, HEATWAVES & COLD SNAPS
Brian Hoskins
Storm tracks map the typical pathways of cyclonic storms from their birth to their death. Early navigators caught in the path of these storms were at the mercy of the wind and waves but modern weather forecasting skill means that ships are now typically given a week’s warning of where and how strong such storms are likely to be.
Waves affect everyone and everything, from waving goodbye to the Mexican wave at a stadium, from waves on a pond’s surface to devastating tsunamis. Energy comes from the Sun in light waves, microwaves heat our coffee, and sound waves bring us the joys of Mozart or the Rolling Stones. Almost anything that is periodic or regular can be thought of as a wave – light waves are just a form of ‘electromagnetic’ wave that oscillate trillions of times a second. The atmosphere contains waves, and not just thundering sound waves. Rossby waves circumnavigate the globe with a wavelength about the width of the Atlantic, excited by air flowing over the oceans and mountain ranges, modified by the rotation of the Earth, and they organize the weather on timescales of days and weeks. Ever wondered why the weather can get stuck in a particular pattern – rain for two weeks solid, or sunshine for days on end? Most likely it is due to Rossby waves forming a particularly persistent pattern. If we could predict these patterns better, just as surfers do for waves coming onshore, we could predict the weather further ahead in time.
Atmospheric waves try to bring order to the anarchist that is weather, by adding structure to the chaos of the circulation.
Large-scale atmospheric motion is chaotic and unpredictable, but not entirely so – it is organized by planet-sized waves that periodically traverse the globe. Thus, storm tracks arise because the weather is channelled into certain regions by these waves. The weather we experience is a competition between chaotic, unpredictable storms and these more regular waves. When the waves win, we experience predictable, regular weather patterns; when chaos wins forecasts go awry. Better understanding of the atmosphere will depend on our ability to extract all the information possible from these waves.
BLOCKING, HEATWAVES & COLD SNAPS
LEONARDO DA VINCI
1452–1519
Italian artist and inventor, probably the first person to realize that sound travelled in waves, who also made some beautiful sketches of waves, among other things
Geoffrey K. Vallis
Waves refer to many different things but in physics waves are oscillations – ranging from the micro to the macro scale – that transfer energy through space or mass.
A whole new way of thinking about the behaviour of our atmosphere was spearheaded by pioneering Swedish-American meteorologist Carl-Gustaf Rossby. By combining ideas from aeronautical engineering with the emerging field of mathematical meteorology, Rossby developed the concept of large-scale waves in the atmosphere that span a significant fraction of the Earth’s circumference and gradually ripple around the planet. These waves have a major effect on the weather, particularly in mid-latitudes, and because their behaviour is governed by mathematical equations linking changes in wind speed and pressure, Rossby’s work contributed hugely to the development of modern computerized weather forecasting.
Rossby studied mathematics and physics in his native Stockholm before taking a job at the Geophysical Institute in Bergen at a time when atmospheric science was developing apace. He went from Bergen to Leipzig and spent much of 1921 at Lindenberg’s Meteorological Observatory. To advance his understanding of upper-air data, Rossby returned to study mathematical physics in Stockholm, financing his studies by working for the Swedish Meteorological-Hydrological Service, which included serving on research expedition vessels in the North Atlantic.
In 1926 Rossby was granted a fellowship by the American-Scandinavian Foundation which took him to America and to a job with the US Weather Bureau, then to a post at MIT, the Massachusetts Institute of Technology, where he established the first dedicated university meteorology course in the USA and set up the first-ever civil aviation weather forecasting service. His role in the aeronautical engineering department allowed Rossby to see the practical value of key physical concepts in fluid dynamics and thermodynamics.
This engineering influence resulted in two ground-breaking contributions to meteorology. The first, published in 1939, included the discovery of the equation that governs the speed of planetary-scale Rossby waves traversing through the atmosphere and shows how this is related to wind speed, latitude and wavelength. The second, a year later, was on a quantity that is conserved in the atmosphere as air flows around, which he called barotropic vorticity. It describes how a mass of air spins as it flows around the Earth. The mathematician John von Neumann later used Rossby’s equation in early computer programmes for weather forecasting.
During the Second World War Rossby helped to organize courses for the systematic training of meteorologists in the military. After the war he spent time both in the USA and his Swedish homeland, collaborating widely to develop our understanding of the atmosphere including the jet stream and atmospheric Rossby waves.
Rossby spent his final years investigating the chemistry of the atmosphere but his name will forever be connected with meteorology and the global-scale waves that organize our weather.
Leon Clifford
28 December 1898
Born in Stockholm, Sweden
1918
Graduates in mathematics, mechanics and astronomy from the University of Stockholm, aged 19
1919
Joins the Norwegian Geophysical Institute in Bergen, Norway, to pursue an interest in meteorology
1921
Returns to University of Stockholm to study mathematical physics
1923
Publishes his first scientific paper, ‘On the Origin of Traveling Discontinuities in the Atmosphere’
1926
Relocates to America and joins the US Weather Bureau in Washington DC
1928
Joins the newly created aeronautical engineering department at the Massachusetts Institute of Technology (MIT)
1939
Appointed assistant chief of US Weather Bureau and becomes a US citizen
1939-40
Authors key scientific papers that include the fundamental equation for what we now call Rossby waves
1947
Becomes founding director of the Institute of Meteorology in Stockholm
1948
Starts spending more time in Sweden where he helps to establish a national weather service
1955
Publishes a scientific paper reinvigorating the field of atmospheric chemistry
1956
Time magazine celebrates his contributions to meteorology in its December issue
19 August 1957
Dies in Stockholm, Sweden
A blocking high is a large high- pressure system that sits over a region such as northern Europe, often for an extended period. Its name refers to the fact that it appears to block the prevailing westerly winds and storms from the Atlantic. Typically to the south of the high there is a low pressure and there are easterly winds between the high and the low. The high-low pressure pattern is even stronger at greater altitude, and the westerly jet stream winds split into branches going around the block far to the north and to the south. When blocking occurs and westerly winds are replaced by easterlies, the climate of western Europe is strongly influenced by the rest of the Eurasian continent, rather than by the Atlantic. In winter this produces cold dry weather and in summer hot dry weather. Because blocking often lasts for a week or more, it gives cold snaps in the winter and heatwaves in the summer. Europe is situated at the downstream end of the North Atlantic jet stream and storm track where blocking often occurs. Winter blocking also occurs near western North America at the end of the North Pacific jet stream, and west of New Zealand at the end of the Australian jet.
Under a blocking high, parts of the world that have a maritime climate become distinctly continental – cold in winter and hot in summer.
Blocking keeps the weather systems away, but these weather systems are important for its existence. Blocking is usually initiated by a deep cyclone that slows down and moves subtropical air far polewards. This subtropical air is spinning less rapidly than the air usually in the region, and so it forms an anticyclone, or high-pressure system. After this, the weather systems approaching the block move more air polewards, which reinforces it.
Brian Hoskins
When a blocking high sits over a region the same kind of weather will persist for a long period – perhaps several weeks – which can be either extremely hot or distinctly chilly.
During the northern hemisphere summer, there is heavy rainfall in the northern tropics, much of it associated with monsoons. This heavy rainfall is associated with large regions of thunderstorms and generally rising air. At this time in the southern tropics and subtropics the air is descending and it is generally very dry. The rising and descending air must go somewhere, and to complete the circulation, on average there is motion from north to south high in the atmosphere and from south to north at low levels. The low-level flow is turned by the Coriolis Force and becomes the strong winter Southeast Trade Winds in the southern hemisphere. The situation is reversed during the southern hemisphere summer, which experiences rainfall and ascent, high-level motion towards the northern hemisphere, followed by descent and low-level motion back towards the southern hemisphere in the Northeast Trade Winds. In the equinoctial seasons the ascent is nearer the equator and air descends in the subtropics of both hemispheres. These average circulations in height and latitude are called Hadley Cells. At about 20-35 degrees in both hemispheres, descent and lack of rainfall dominate at most times of the year, which is why most of the world’s deserts are located in these latitudes.
What goes up must come down – the up is in the moist tropics and the down is in the subtropical desert regions.
The Hadley Cell is a picture of the average flow. However, there is much variation around the Earth. The Indian summer monsoon ascent is partially compensated by descent in the Mediterranean, resulting in its dry, hot summers. The rainfall regions in the tropical Atlantic and East Pacific stay north of the equator even in the southern hemisphere summer and the motions across the equator are in the opposite directions to the Hadley Cells!
GEORGE HADLEY
1685–1768
English scientist who proposed the model for the Earth’s atmospheric circulation in each hemisphere, which explained the Trade Winds
Brian Hoskins
The Hadley Cells are huge circulations of air in the tropics. They move moisture from the subtropics, creating most of the arid regions of the Earth, to the tropical regions of heavy rainfall.
There are three bands of wind currents in both the northern hemisphere and the southern hemisphere: in the middle latitudes, the near-surface winds blow from the west, while in the polar regions and in the tropics, the near-surface winds blow from the east. In the tropics, these steady breezes are called the Trade Winds. The trades exist out to about 30 degrees latitude in each hemisphere, and flow from the east and towards the equator, from the northeast in the northern hemisphere and from the southeast in the southern hemisphere. Even Christopher Columbus knew about the Trade Winds, as he used them to speed his journey to the New World. The Trade Winds take their strength from the rotation of the Earth. Hot air rises near the equator, and winds near the surface converge to feed the rising motion. As the winds flow towards the equator, they are bent to the right in the northern hemisphere (and to the left in the southern hemisphere) by the Coriolis force. This process gives the Trade Winds their easterly flow. The Trade Winds are strongest in winter in each hemisphere, and are characterized by a relative steadiness, compared to the disruption from weather systems that punctuate the westerly flow of the middle latitudes.
The steady breezes of the Trade Winds provided a reliable route for mariners to cross from Europe to the Americas.
In the middle of the Trade Winds, near the equator, exist the ‘doldrums’, a narrow band of low pressure where the winds are calm. Sailors dreaded their passage through the doldrums, because ships can drift aimlessly there for weeks, often with a dwindling supply of water and food. On the outside edge of the tropics, around 30 degrees latitude, are the ‘horse latitudes’, another band of relative calmness.
MATTHEW FONTAINE MAURY
1806–73
American oceanographer who made detailed maps of the Trade Winds and other air and ocean currents
Dargan M. W. Frierson
In the age of sail power the Trade Winds were a seafarer’s ally but the stillness of the doldrums was the maritime equivalent of being up a creek without a paddle.
When the majority of the annual rainfall for a region regularly occurs over a well-defined period of up to a few months, this period is known as the rainy season. Tropical regions, for example West Africa and Southeast Asia, tend to experience rainy seasons – usually in the summer months as monsoons – but in some regions they occur twice yearly. Tropical rainy seasons are linked to a band of clouds and thunderstorms that circle the Earth near the equator. This band, called the inter-tropical convergence zone, or ITCZ, moves, following the maximum of solar heating on the Earth’s surface where the Sun’s path across the sky is at its highest. This means that the ITCZ tracks around 800 kilometres into the northern tropics during the northern hemisphere summer and drifts into the southern tropics during the southern hemisphere summer. The heat from the Sun warms the oceans and these warm waters heat the atmosphere above. This causes the evaporation of moisture from the sea surface and strong rising motion to form clouds and thunderstorms that make up the ITCZ. The rainy season occur when the band of ITCZ clouds and storms moves over a land mass and therefore occurs twice annually in many regions as the ITCZ shifts northwards and southwards.
In some tropical regions for months it almost never rains, then it pours – the rainy seasons.
The location of the migrating band of clouds and convective storms known as the ITCZ generally lags behind the relative position of the overhead Sun by one or two months. Apart from the drastic effect of the ITCZ on the frequency and intensity of rainfall over many equatorial land masses, the cumulus and cumulonimbus clouds associated with it can billow to heights of 16 kilometres, presenting a formidable barrier to aircraft at high altitude.
EDMOND HALLEY
1656–1742
English astronomer who, in 1686, suggested that solar heating of the oceans was the main driver of tropical weather
Leon Clifford
Temperatures build up far more rapidly during the day over large land masses than oceans. As a consequence, heavy downpours in the afternoon are a characteristic feature of tropical weather.
Monsoons are caused by seasonal winds that maintain their direction for months at a time. Monsoon winds are driven by the Sun, which warms the Earth’s surface. They always blow from a relatively cooler region to a relatively warmer region where the Sun-heated surface heats the air above, causing it to rise, thus drawing in more cooler air and so maintaining the wind pattern. The summer monsoon in the Indian subcontinent lasts from May to September and blows northeastwards from the sea onto the hot summer land, bringing with it moist air from the southwestern Indian Ocean, which results in heavy rainfall. The Indian summer monsoon is particularly strong due to the intense heating of the land made possible, in part, because the Himalayas block cooling air from heading southwards. India also experiences a weaker winter monsoon, between October and March, which sends dry air from inland China heading southwest across the subcontinent although the Himalayas act to block much of this wind from reaching the coast. The winter monsoon in Southeast Asia brings moist air from the South China Sea across Indonesia and Malaysia, causing significant rainfall. Similar wind systems occur in North and South America, northern Australia and West Africa.
Monsoons are seasonal winds driven by solar heating of the Earth’s surface; they are often linked with the onset of rainy seasons.
Asia’s monsoons provide an example of how geological processes deep within the Earth millions of years ago help to shape our weather today. Evidence of past climate conditions found in ground and ocean-floor sediments together with computer model experiments all suggest that the evolution of Asia’s monsoons is inextricably linked with the formation of the Himalayan Mountains and the uplift of the Tibetan Plateau which began around 50 million years ago.
HIPPALUS
fl. 1st century BCE
Greek explorer and navigator, credited by the Roman writer Pliny the Elder as being the first to document the Indian Ocean monsoon path
HENRY FRANCIS BLANDFORD
1834-93
British meteorologist who studied India’s monsoons. He successfully predicted a monsoon rainfall failure in 1885 which caused a drought
Leon Clifford
Monsoon winds always blow from cold regions to warm, which, in the case of most of India and Southeast Asia, has shaped their climates.
The fastest global-scale winds in the atmosphere are not associated with oceanic storms or America’s Tornado Alley, but are high in the stratosphere between 10 and 50 kilometres altitude. Winds here regularly exceed 250 km/h – similar to wind speeds in the strongest hurricanes. Continually whirling around the poles in wintertime, they form a gigantic cyclone known as the stratospheric polar vortex. The origin of the vortex relates to the fact that the stratosphere contains ozone, which absorbs heat from the Sun. In the polar winter, however, the Sun does not rise for months. This allows the polar stratosphere to become intensely cold – down to -85T – far colder than the sunlit stratosphere. This temperature difference is the cause of strong winds around the polar vortex and means that the polar vortex only forms in winter. Rapid winds around the edge of the vortex also isolate air in its interior, a fact that is instrumental in the formation of the Antarctic ozone hole. In the Arctic, however, the vortex is less strong than in the Antarctic, and in some winters it is distorted or suddenly breaks down, exerting an influence on surface weather over northern polar and mid-latitudes.
In winter a powerful circulation of air around the poles in the stratosphere of each hemisphere influences weather and the formation of the ozone hole.
The stratosphere is the layer of the atmosphere above the Earth’s weather; the air there is extremely dry, and most of the stratosphere has no clouds – certainly no rain. The polar vortex is so cold, however, that the tiny amount of water vapour present sometimes condenses into so-called polar stratospheric clouds. These tenuous clouds are also known as nacreous clouds, owing to their pearly, iridescent appearance near sunset or sunrise.
LÉON PHILIPPE TEISSERENC DE BORT
1855-1913
French meteorologist and physicist who pioneered the use of unmanned balloons and discovered the stratosphere
Jeff Knight
Powerful stratospheric vortices that form over the poles in winter play a critical role in the depletion of the polar ozone layer.
