Aerosols An aerosol consists of billions of minute droplets of liquid or particles of solid suspended in a gas. Pollen, sea salt and soot from combustion can all make aerosols in the atmosphere. Pollution and volcanic eruptions can lead to aerosols high in the stratosphere that consist of tiny droplets of sulphuric acid. Some aerosols (e.g. soot) absorb incoming solar radiation and so act to warm the Earth; others (e.g. sulphuric acid droplets) can reflect solar radiation back into space and so have a cooling effect. Aerosols can act as the seeds around which cloud droplets form. The size of aerosol particles can range from 1 nanometre (one billionth of a metre) to 100 micrometres (one ten-thousandth of a metre).
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.
Long-wave radiation The heat radiated by the warm surface of the Earth and warm areas in the atmosphere. It has a longer wavelength than the visible and ultraviolet light from the Sun which is known as short-wave radiation. Long-wave radiation is infrared and invisible but it is a form of electromagnetic radiation just like light and radio waves.
Pressure gradients The change of atmospheric pressure with distance in a given direction. This gradient results in a force that acts on the air in a direction that is at right angles to the isobars – the lines of equal air pressure seen on weather maps. This force tries to push the air from areas of high atmospheric pressure towards areas of lower pressures and it is a source of wind. The steeper the pressure gradient, the more tightly packed the isobars, and the stronger the resulting wind. In meteorology, the idea of pressure gradients is applied to the behaviour of the atmosphere and is usually measured in millibars per kilometre (mb/km) – a millibar being a unit of atmospheric pressure. The nominal atmospheric pressure of the Earth at sea level is 1,000 millibars or 1 bar.
Saturation The state of the atmosphere in which the air contains the maximum amount of water vapour that it can hold at that particular temperature and air pressure. At saturation, relative humidity – the amount of water vapour in the air compared to the amount that the air could hold – is 100 per cent and further evaporation of water vapour into the air cannot occur. The capacity of the air to hold water vapour grows with increasing temperature and declines with decreasing temperature. This is why warmer climates experience greater humidity and why warm humid air forms clouds as the air rises and cools.
Solstice An astronomical event that occurs twice yearly around 21 June and 21 December due to the tilt of the axis of the Earth’s rotation to the plane of its orbit around the Sun. A solstice occurs in the summer and winter. In the northern hemisphere the summer solstice occurs in June and the winter solstice in December, and vice versa in the southern hemisphere. At solstice, the amount of daylight is at an annual maximum in one hemisphere and at an annual minimum in the other.
Supercooling/supercooled water Supercooling occurs when a liquid is cooled below its normal freezing point but does not solidify. Supercooled water droplets are found in high-altitude clouds where the temperature of the air is below the freezing point of water. This supercooled state can only be achieved in droplets that do not contain impurities or aerosols that would otherwise act as seeds to trigger crystallization. Research suggests that the phenomenon of supercooling may be due to the molecules of water arranging themselves in a way that is incompatible with crystallization.
Temperature inversion In the troposphere (the lowest layer of the Earth’s atmosphere) air temperature usually falls with increasing altitude but sometimes it can increase, resulting in a blanket of warm air sitting above a layer of cooler air, and this is known as a temperature inversion. Rain falling through a temperature inversion can freeze. If the air below the inversion is sufficiently humid then fog can form. Over populated areas, temperature inversions can act as a lid that traps pollution near the ground.
Vortex/vortices In meteorology, a vortex refers to a rotating mass of air, often circulating around a low-pressure system. A hurricane or typhoon is an example of a vortex of air circulating around a centre of low pressure. Larger and more persistent atmospheric vortices are found circulating around low-pressure regions over each pole – and the so-called Polar Vortex over the North Pole has been famously associated with severe winters in North America and Eurasia.
The Earth is thousands of kilometres in diameter, whereas the air we breathe sits in a thin skin just 100 kilometres thick: a distance you could drive in under an hour. Air is a mixture of different gases, primarily nitrogen (78%) and oxygen (21%). The last 1% is inert argon, carbon dioxide (CO2) and tiny amounts of other gases like ozone. There is also water vapour, around 1% at the surface but this depends where you are. The restless weather systems in the troposphere constantly stir these gases with a dash of pollutants and other chemicals, so that most of the air is well-mixed. It takes under a year for mixing to occur globally, which is why increasing carbon dioxide from cities and industrial centres can be measured almost anywhere. Although a tiny fraction of the air, carbon dioxide affects Earth’s temperature and its concentration is increasing rapidly through human activities, driving global warming. These changes are happening fast but the balance of gases was not always the same. The distant geological past contains very long periods with far less oxygen and other periods with more, which had remarkable effects, including enabling insects to grow to many times their present size – one example of how the make-up of the air is intimately linked with life on Earth.
The air is a thin layer of different gases constantly stirred by the weather, which mixes it up on a timescale of months.
Air is gradually circulating between the lower atmosphere and the stratosphere where the ozone layer sits. It rises in the tropics and sinks over the poles but this process is very slow and it takes years for air molecules to complete the circuit. This slow circulation is important as it sets the time for cleansing the air of ozone-depleting chemicals.
GLOBAL WARMING & THE GREENHOUSE EFFECT
JOHN TYNDALL
1820–93
Irish physicist who discovered numerous physical properties of the air including how molecules interact with infrared heat radiation to warm and cool the atmosphere
Adam A. Scaife
The composition of air is vital in ensuring there is life on Earth, blocking the deadly rays of the Sun, trapping heat to keep the environment comfortable, and, crucially, providing the oxygen we breathe.
Let’s take a journey up through the atmosphere. If you have ever climbed a mountain you will know that the air gets colder the higher you go, by about 1°C every 150 metres. This is because sunlight is absorbed by the ground, making it warmer there. Energy spreads upwards by reradiating heat and by warm moist air rising up through the turbulent overturning troposphere. The rising air causes much of our weather, with clouds regularly reaching 15 kilometres in the tropics. But what happens if we go higher? The air can’t just keep getting colder. As we ascend further and the air thins to just a tenth of its density at the surface, it starts to warm again; we have entered the stratosphere. Here the ozone layer warms the atmosphere by absorbing ultraviolet light from the Sun. As warmer air is now sitting atop cooler air, everything is stable – there is no weather here. Eventually the ozone starts to thin and temperatures begin to drop again: we are in the mesosphere. Here the air is 10,000 times thinner than at the surface and it is turbulent again. Ripples travel up from distant weather systems below and drive winds that pull air upwards in summer to create the coldest point in the atmosphere, more than 100 degrees below zero.
All of our weather occurs in the lowest layer of the atmosphere – the troposphere – above which sits the quiescent stratosphere and the tenuous mesosphere.
Other planets also have a troposphere and a stratosphere and the boundary between them often occurs at about the same pressure as in Earth’s atmosphere. Jupiter is a clear example but the balance of heating is different in the atmosphere of this gas giant, as much of its energy comes from a mysterious heat source beyond observation, deep in the Jovian atmosphere.
ARISTOTLE
384-322 BCE
Greek polymath who wrote the first book about meteorology in c. 350 bce in which he outlined the hydrological cycle and discussed numerous weather phenomena
Adam A. Scaife
Our atmosphere has four layers, based on temperature. The layer weather is confined to – the troposphere, closest to Earth – is the warmest. Highest lies the thermosphere, the realm of meteors and auroras.
The axis upon which the Earth spins each day is tilted at 23.4 degrees to the Earth’s orbit around the Sun. It has a fixed direction in space, so as the Earth makes its annual journey around the Sun, the northern hemisphere is tilted towards the Sun for half of the year, and the southern hemisphere is tilted towards the Sun for the other half. This causes the amount of daylight to increase and decrease, and makes the Sun rise higher or lower in the sky, changing its ability to warm the surface. Outside of the tropics, this creates a cycle of temperatures associated with four seasons – spring, summer, autumn and winter. In the tropics, the midday Sun is always high in the sky, and temperature varies little through the year. Here, the seasons are defined by changes in rainfall rather than temperature. The tropical rainfall zone tracks the shifting latitude where the Sun is overhead, which typically results in a short wet season and longer dry season, as in many parts of India. However, some locations, such as East Africa, experience two wet seasons corresponding to the northward and southward passage of the overhead Sun.
It is the tilt of the Earth’s axis that produces the seasons rather than the distance of the Earth from the Sun.
Astronomers can precisely measure the seasons by the solstices and equinoxes, which are key moments in the Earth’s annual journey around the Sun. But in terms of the weather, the seasons tend to change more gradually. Meteorologists’ seasons are therefore always sequences of calendar months, which are the building blocks of climate statistics. The sets of months are chosen for the region in question, such as the four three-month seasons used in mid-latitudes.
Jeff Knight
Antonio Vivaldi would not have been inspired to compose his Four Seasons had he lived near the equator. The tilt of the Earth’s axis explains why seasons are more apparent in the mid-latitudes in contrast to the tropics where seasonal change is less marked.
Water vapour is a gas, invisible but present nearly everywhere in the atmosphere in varying concentrations. Cooling air reduces its capacity to contain water vapour and, if it is chilled sufficiently, saturation occurs. At this point water starts changing from its gaseous to its liquid or ice phases. The commonest cooling mechanism is lifting, for instance as warm air rises above a wedge of cold air at a front, or when bubbles of air rise over ground heated by the Sun. Decreasing pressure causes a rising parcel of air to expand, the work done using up heat energy on much the same principle that a fridge uses. Liquid water produced by chilling collects in minute droplets on surfaces, apparent as misting on a glass of iced water. In the atmosphere, the surfaces required for condensation are provided by specks of material known as aerosols. They have many sources, including salt particles released by breaking ocean waves, and industrial pollution. All cloud droplets contain microscopic nuclei of this sort and grow by condensation (or deposition in the case of ice) to a size of the order 1-10 microns (millionths of a metre). Being tiny, they have negligible fall-speeds, and remain in effect suspended, despite bulk cloud weights of millions of tonnes.
Clouds are composed of tiny water droplets or ice particles, each of which forms around an aerosol, a minuscule, solid particle.
In 1802 Luke Howard, an amateur meteorologist, turned a lifelong interest in staring out of the window at the sky to good use and published a cloud classification, On the modification of clouds. Cloud types today are named after the terms he proposed, such as cumulus, for vertically extensive clouds, cirrus for wispy clouds, stratus for layered clouds and nimbus for rain clouds.
LUKE HOWARD
1772–1864
British pharmacist and amateur meteorologist who proposed a nomenclature of clouds in 1802
Edward Carroll
Luke Howard’s original cloud classification was further refined by combining types and including reference to three height forms, giving rise to such denominations as altocumulus, cirrostratus and cumulonimbus.
To become a raindrop, a cloud droplet has to increase in mass about a million fold. Droplets of differing sizes gradually settle out at different speeds and can grow by colliding and coalescing. This is generally a slow process, but a small proportion of droplets have a high enough number of chance collisions to grow and acquire significant fall-speeds, allowing them to sweep up an increasing number of smaller droplets – an accelerating process which can produce a raindrop within 20 minutes. Outside the tropics, another process dominates. Here, most rain-bearing clouds are below 0oC, but unless very cold (below -20oC), just a few of them will freeze. Instead, the bulk of the rain cloud consists of droplets of supercooled water below 0oC. Deposition of water vapour onto ice takes place more readily than condensation into water, so the few ice particles grow quickly, drawing water vapour from the air and causing the many supercooled water droplets to shrink by evaporation. Rapidly growing in size at the expense of cloud water, ice particles fall, collecting supercooled water droplets on their way, which freeze onto them. As they descend to warmer levels they melt, and all of this is forgotten as they reach the ground as rain.
Most extra-tropical raindrops start life as ice particles in the cold, upper reaches of clouds, sweeping out cloud droplets as they fall and melt.
In diameter, typical cloud droplets are about 1-10 microns (millionths of a metre), drizzle drops 100-500 microns and raindrops 500-5,000 microns. Small cloud droplets, being closer in size to the wavelength of visible light, scatter it back more efficiently than large cloud droplets or raindrops, which absorb more of the light. Clouds therefore often look darker and more menacing as droplets approach raindrop-size.
ACID RAIN & ATMOSPHERIC POLLUTION
Edward Carroll
Without clouds there is no rain but the droplets that form clouds are too small to fall – until they coalesce with other droplets and become heavier. When those drops are 0.5mm in diameter or larger, the heavens open, making us grateful for a roof over our heads.
Climatologically, a frost is the occurrence of a temperature below 0oC, the melting point of ice. Temperature can vary strongly with height, and standard recording practice is to measure it around 1.5 metres above ground level. Night-time cooling of the ground by long-wave radiation on clear, calm nights results in an inversion of the usual tendency for temperature to fall with height; in these circumstances ground temperature can be 5oC below that at 1.5 metres. So ground frost forms more easily than air frost, especially over grassy surfaces, where air trapped between grass blades provides insulation from heat stored in the ground. A late spring frost can nip tender, low-growing plants in the bud because the freezing of water causes expansion, rupturing cell walls. Hoar frost is a visible manifestation of frost, being ice crystals deposited directly from water vapour onto subzero vegetation and other surfaces. With high humidity and a breeze, fresh supplies of water vapour are constantly brought into contact with cold surfaces and a thick coating of hoar frost can accumulate, occasionally producing a magical, wintry landscape. Clearer ice results when dew freezes, or when supercooled fog droplets spontaneously freeze on contact with surfaces, in the latter case known as rime.
When the temperature falls below 0oC, a frost is said to occur, and if conditions are right, it is marked by the formation of ice.
From the sixteenth to the nineteenth century, during a period known as the Little Ice Age, winters in London were sometimes severe enough to freeze the Thames. The opening up of this wide thoroughfare through the city stirred excitement in the populace and resulted in spontaneous ‘frost fairs’ during which stalls and entertainments were set up, oxen were roasted and even, on one occasion, an elephant was led across the river.
Edward Carroll
The crystalline structure of ice manifests itself in many different guises, from delicate, lacy and ephemeral forms to a lattice of immense rigidity – strong enough to bear the weight of man and beast.
Cloud ice particles form on aerosols known as freezing nuclei, which have a similar, hexagonal, small-scale structure to ice crystals. Deposition from vapour into the ice phase can result in the growth of astoundingly complex, geometric, even organic-looking structures – basically hexagonal, but needle-like, plate-like or intricate, fern-like branching shapes, depending on temperature and humidity. The branching forms easily interlock and aggregate as they fall, reaching the ground as classic, feathery-looking snowf lakes if the temperature is close to or below 0oC. Falling ice particles can also grow by gathering supercooled water droplets, which freeze on contact and stick to give a more irregular shape, masking the original crystalline form. This process is called accretion and the resulting precipitation graupel or, when consisting of small particles, snow grains. In practice, snow is often composed of a mixed type resulting from combined aggregation and accretion. Due to trapped air, snow accumulates to depths 10-15 times greater than the same mass of liquid water – for example, 10 millimetres rainfall equivalent of snow lies 10-15 centimetres deep. A blizzard occurs when falling or lying snow is whipped up by strong winds. It swirls around, piling in hollows and against obstacles into drifts, burying cars and livestock.
Tiny, often complex, hexagonal ice crystals grow when water vapour deposits as ice onto aerosols, forming snow as they collide and interlock.
Snow can be difficult to forecast in marginal winter climates such as northwest Europe’s, since an error of 1°C in temperature can make the difference between 10 millimetres of rain with little impact, and traffic chaos from 15 centimetres of snow. The challenge is made harder because temperature falls more with increased intensity of rain, so that what is forecast to be moderate rain can turn to disruptive snow if it is heavier than expected.
Edward Carroll
A farmer from Vermont, USA, Wilson Bentley, began taking snowflake photographs in 1885 and captured more than 5,000 over his lifetime using a microscope rigged to a camera. He died of pneumonia after walking home through a blizzard.
If a parcel of air is raised, it expands, cools and normally finds itself colder and denser than its environment, so sinks to its original position. However, when the atmosphere is cooled aloft, or warmed from below, it can become unstable. Its temperature falls with height faster than that of a rising parcel, which is therefore lighter than its surroundings and continues to ascend as a buoyant bubble – a process called convection. Water condenses and releases heat, further adding to buoyancy, and when ice forms the cloud becomes a cumulonimbus. Towering up to 10-15 kilometres tall, bulging cauliflower-like protuberances at its boundaries mark the edges of individual rising currents, and more fuzzy edges aloft signify that ice particles predominate. These hailstone ‘seeds’ grow fast and soon start to fall, supercooled water droplets freezing onto them. If horizontal wind varies strongly with height, warm updraughts can remain separated from cold, precipitation-induced downdraughts which would otherwise eliminate them. Powerful updraughts support rapid growth of the hailstone from ice precipitation, which can recirculate as it exits the top of, then re-enters, a sloping updraught. Fresh layers of ice form, sometimes alternating clear and opaque due to varying water droplet concentrations in different regions of the cloud, before the hailstone finally plummets to Earth.
Cumulonimbus clouds with strong, vertical currents can support lumps of ice, exceptionally over 10 centimetres in diameter and weighing a kilogram.
Large hail results in significant annual crop damage in some areas. In urban locations in the developed world, the biggest financial costs occur, with over US$1 billion attributed to individual storms in the USA, Europe and Australia. Elsewhere, for example rural India, Bangladesh and China, where people are more likely to be caught without shelter, there are records of multiple fatalities in hailstorms.
Edward Carroll
A hailstone accumulates layer upon layer of ice as it is recirculated through the cloud, supported against falling by updraughts of between 25 and 50 metres per second.
Fog is cloud at ground or sea level, dense enough to reduce horizontal visibility to below 1,000 metres, and frequently to less than 200 metres. When low cloud intersects high ground, hill fog results, though in general fog is unlike other clouds, which form by cooling through ascent. The principal mechanism over land is night-time emission of long-wave (heat) radiation, which is most effective in the absence of cloud to radiate back. Light winds prevent warmer air mixing down from above and the resulting chilling reduces the capacity of air to contain water vapour, so water droplets condense onto tiny cloud condensation nuclei. Fog forms initially just above the ground, but deepens upwards as the fog top itself becomes the radiating surface. The resulting radiation fog is commonest in valleys, where colder, denser air collects by drainage and watercourses provide moisture. Advection fog forms when moist air is chilled by flowing over a cold surface. Sea fog is a type of advection fog, commonest in spring and early summer when the sea is still cold but the air is warming up. In some dry, or seasonally dry, parts of the world, sea or hill fog is a vital source of water for trees, such as California’s Coast Redwoods. Intercepted droplets coalesce on leaves or needles and drip to the ground, providing moisture to their roots.
To be in fog is to enjoy a first-hand experience of having one’s head in the clouds.
Sooty particles from burning coal act as cloud condensation nuclei and promote fog. The combination can result in a particularly dense and persistent smoky fog – smog – common in nineteenth- to mid-twentieth-century London. Episodes of very poor visibility -sometimes only a few metres – known as pea-soupers were responsible for severely disrupting travel and causing respiratory problems which killed thousands.
Edward Carroll
The concentration of water droplets in fog determines the visibility in foggy conditions. Cool ocean temperatures mean California’s coasts are often subject to sea fogs which sometimes shroud the Golden Gate Bridge but also sustain forests of huge Coast Redwoods.
Religious belief and an interest in science shaped the life of the man who invented modern weather forecasting. Born into a Quaker family, Lewis Fry Richardson developed an aptitude for science which took him to Cambridge where his studies included a mix of mathematics, physics and earth sciences – an ideal background for meteorology.
Early in his career, Richardson used the same mathematics that he would later apply to the weather in a very practical scientific problem involving the flow of water through peat. This approach, based on hydrodynamics, can calculate the evolution of constantly changing systems using the mathematics of finite differences.
Richardson was introduced to the challenges of forecasting on joining the Meteorological Office in 1913 to run the Eskdalemuir Observatory in Scotland. He recognized that weather could, in principle, be forecast using the differential equations of hydrodynamics and he set about trying to do this.
In the First World War Richardson was a conscientious objector to military service on religious grounds and left the Meteorological Office in 1916 to work in a battlefield ambulance unit. Nevertheless, he continued to develop his ideas and, in a remarkable feat of mathematics, he produced the world’s first-ever numerical weather forecast by hand calculating changes in the pressure and winds at two points in central Europe over a six-hour period. Unfortunately, this forecast proved inaccurate due to the way the equations behaved.
Richardson included details of this pioneering calculation in a book published in 1922 and followed it with a series of papers establishing the theoretical basis of numerical weather forecasting. He realized that the sheer volume of calculations required for numerical weather forecasting was a major challenge. ‘Perhaps some day in the dim future it will be possible to advance the computations faster than the weather advances .... But that is a dream,’ he concluded. More graphically, describing the effect of atmospheric turbulence – another breakthrough aspect of his research – he wrote:
‘Big whirls have little whirls,
That feed on their velocity;
And little whirls have lesser whirls,
And so on, to viscosity ...’
Richardson rejoined the Meteorological Office after the war but his pacifism led him to resign shortly after when, in 1920, it was amalgamated into the Air Ministry – a part of the government military establishment. In later life, he shifted his research focus to other areas as he saw the military taking a greater interest in meteorology.
Richardson lived to see the first-ever weather forecast generated by computer and, thanks to modern supercomputers, his dream of using mathematics to create weather forecasts is now an everyday reality.
Leon Clifford
11 October 1881
Born in Newcastle upon Tyne, England
1903
Graduates from Cambridge with a first-class degree in the Natural Science Tripos
1907
Solves a problem involving water flow through peat using approximate mathematical methods applied to the differential equations of hydrodynamics
1913
Joins the UK Meteorological Office for the first time and investigates the use of mathematics in weather forecasting
1916
Conscientious objector to military service. Works in an ambulance unit in France
1919
Returns to work for the Meteorological Office
1920
Resigns from the Meteorological Office when it becomes part of the Air Ministry
1920s
In his spare time, conducts research into the relationship between wind and heat that produces turbulence. His equations identified what is now called the Richardson number used to predict where turbulence will occur in the atmosphere and the oceans.
1922
Publishes a groundbreaking work, Weather Prediction by Numerical Process, which includes details of his pioneering hand-calculated mathematical weather forecast and his research into turbulence
1926
Elected to the Royal Society in recognition of his work
1929
Awarded a BSc in Psychology from University College London
1940
Retires to focus on research into areas including the application of mathematics in psychology and international conflict
1950
Hears news of the first 24-hour numerical weather forecast made by computer
30 September 1953
Dies in Kilmun, Scotland
Although pressure comes in many forms, in fluid dynamics and meteorology it means something quite specific: it is the force per unit area exerted by a fluid either on its container or on another part of the fluid. Which is why, for example, having too high blood pressure can cause blood vessels to burst and is thus not recommended! In the atmosphere the pressure at a point is equal, to a very good approximation, to the weight of air above that point, and so pressure decreases with altitude. Pressure varies in the horizontal too, caused by contrasts in temperature in conjunction with the Earth’s rotation, leading to a never-ending pattern of pressure variations across the globe. Regions of low and high pressure are called cyclones and anticyclones, respectively. Air tends to spiral in towards cyclones and then rise, causing water vapour to condense and rain to fall. Particularly strong cyclones in low latitudes can evolve into hurricanes, with intense rain and powerful winds. In contrast, air tends to spiral outwards and is replaced by sinking within an anticyclone, preventing cooling and condensation into rain and therefore giving rise to fine, clear weather.
Pressure is, indeed, a force of nature: it produces the winds and sometimes the rain, and if we know the pressure we (almost) know the weather.
Weather is caused by the variations of global pressure patterns, for the wind and the rain can be largely inferred from these patterns: gradients of pressure produce strong winds, rain comes with cyclones and sunshine comes with anticyclones. But these variations are chaotic – that is to say fickle and tempestuous -and the difficult science of weather forecasting lies in trying to predict what those patterns will be a day or a week or even longer into the future.
EVANGELISTA TORRICELLI
1608–47
Italian physicist who invented the barometer, essentially by measuring the weight of air by seeing how much mercury it displaced
VILHELM BJERKNES
1862–1951
Norwegian physicist, founder of the Bergen School of Meteorology, which attempted to understand and predict atmospheric motion with the aid of maps of surface pressure
Geoffrey K. Vallis
In the late Renaissance Evangelista Torricelli invented the barometer, enabling the study of the atmosphere to become truly scientific.
The Coriolis Force (CF) is an apparent force on the air (and the ocean) due to viewing it from an Earth that is rotating. The CF is proportional to the strength of the wind and in the northern hemisphere it acts to the right of the wind. It is the CF that leads to air moving around a low-pressure system rather than being accelerated inwards. In anti-clockwise motion around the low, the CF to the right of the wind is outwards and balances the pressure force inwards. In the southern hemisphere the CF is to the left of the wind and there is clockwise motion around a low. To understand the nature of the CF, imagine a ball that is stationary on a spinning roulette wheel. If the slope of the wheel is just right, the outwards centrifugal force is balanced by gravity pulling the ball inwards down the slope. If the ball is given extra speed around the wheel, motion in a straight line would appear to curve outwards when viewed from the spinning wheel. In addition, the outward centrifugal force is increased by the extra speed and is no longer balanced by gravity. The two effects give an apparent force outwards – the CF.
The Coriolis Force is perverse: whichever direction the wind is heading, the CF tries to deflect it.
The Coriolis Force is important on any scale larger than the wind speed divided by the Earth’s rotation about the local vertical. For a 10 ms-1 wind this is about 200 kilometres. Contrary to popular belief, a bath tub is too small for the vortex above the plughole to be affected by Coriolis forces. The force was first discussed in the seventeenth century in terms of the displacement of cannonballs.
GASPARD-GUSTAVE CORIOLIS
1792–1843
French mathematician and mechanical engineer who discussed the forces relative to rotating waterwheels and derived an equation for CF in 1835
Brian Hoskins
The air appears to want to turn to the right in the northern hemisphere under the action of a fictitious force, the CF.
A wind is a flow of air, commonly referring to flow on a relatively large scale. Winds blow around Earth from west to east in mid-latitudes, and from east to west and towards the equator in the tropics, where they are called Trade Winds. Confusingly, winds are known by the direction whence they came, so Westerlies blow towards the east. Winds blow as they do because air moves – in fact accelerates – when acted upon by a force, and pressure forces commonly arise in the atmosphere when part of the globe is heated, causing air to expand and pressure to fall. The immediate reaction of air is to flow from high pressure to low. However, Earth is rotating so a Coriolis Force comes into play, bending air to the right in the northern hemisphere and to the left in the southern. The upshot is that winds do not actually travel from high pressure to low; rather, they travel around regions of low pressure (cyclones) and high pressure (anticyclones) with, in the northern hemisphere, low pressure on the left and high pressure on the right.
Winds are like politics: to go in a straight line you have to withstand pressure from the right.
The balance between winds and pressure is known as geostrophic balance, and both mid-latitude Westerlies and low-latitude Trade Winds are in this balance. Geographical changes in temperature are a prime cause of pressure variations, and there is a related balance between temperature gradients and the wind aloft, whereby a horizontal temperature gradient is associated with wind that increases with height. The temperature gradient between low and high latitudes causes Westerlies that increase with height, hence flying from Europe to America (against the wind) takes longer than vice versa.
C. H. D. BUYS BALLOT
1817–90
Dutch meteorologist who proposed Buys Ballot’s law, which states that if a person stands with his back to the wind the atmospheric pressure is low to the left, high to the right, a precursor of the notion of geostrophic balance
WILLIAM FERREL
1817–91
American meteorologist who may have preceded Buys Ballot in understanding geostrophic balance, and increased our insight into atmospheric circulation
Geoffrey K. Vallis
As in life, balance is key in science. The Coriolis Force is almost balanced by the pressure force; this balance lies at the very core of meteorology.
The large-scale circulation systems which drive the weather in the extra-tropics are often weak or absent at low latitudes. Instead, winds are caused by topographic contrasts. Land surfaces warm and cool quickly with the day-night cycle whereas sea temperatures remain more constant. Air heated over land becomes less dense, causing pressure to fall. This sucks denser sea air well inland on a sea breeze, causing a wind shift, temperature drop and humidity rise on its passage. The leading edge is made visible by cloud forming where air is undercut and lifted, sometimes causing showers. In the tropics it can lead to a highly predictable daily sequence. In mid-latitudes it occurs less regularly but can be well marked in slack summertime settings, such as anticyclones. A similar mechanism causes anabatic winds, blowing up slopes warmed by the Sun, and katabatic winds blowing down slopes cooled at night. The Fohn is a class of local wind in which air forced over a mountain range is warmed and dried through condensation and rain-out of moisture. Named after an Alpine wind in southern Germany, it includes the Chinook of the North American Rockies, which in winter can cause temperature rises of 30°C in just a few hours.
Winds are often influenced by local topography, and can be largely driven by it, especially where large-scale pressure systems are weak.
The Mediterranean has many local winds due to the complex terrain surrounding it. One such is the French Mistral, which is funnelled and accelerated down the Rhone valley and out into the Golfe du Lyon, typically when a low-pressure system forms near Genoa, a lee effect of the Alps. It can howl relentlessly for days on end and is said to affect people’s state of mind, causing depression and headaches.
Edward Carroll
Sea breezes and up-slope, anabatic winds result from temperature, and hence pressure, differences brought about by rapid solar heating of land surfaces. Such winds focus clouds inland from the coast and over mountain tops.
