In August 2005 Hurricane Katrina wreaked havoc onNew Orleans, claiming 1300 lives and causing $125bnof damage. Just two months later, the most intenseAtlantic hurricane ever recorded – Hurricane Wilma– devastated parts of Mexico, Cuba and Florida. In-deed, there were a record 27 tropical storms in theAtlantic during 2005, 15 of which developed into hur-ricanes. For the first time since it was established in1953, the alphabetical list of names assigned annuallyby the World Meteorological Organization (WMO)was exhausted. This brought us our first ever Greek-letter-named hurricanes, Beta and Epsilon, and theWMO recently announced that five names (yet anotherrecord) would be permanently retired from the rotatinglist – a fate reserved for those hurricanes causing themost devastation.

To reduce the catastrophic loss of life and materialdamage caused by hurricanes we need better forecastsboth of their paths and intensities. Currently forecastsof path are too error-prone to be of much practical usebeyond three days in advance, and predictions of inten-sity change are even less developed. Furthermore, lastyear’s record-breaking Atlantic hurricane season hasfuelled fears that global warming may be responsiblefor increasing the frequency and intensity of hurri-canes. Although controversial, such a link would be ofvital importance to the hundreds of millions of peopleliving in hurricane-prone areas.

Many features of hurricanes can be explained interms of classical physics – such as Newton’s second lawand the thermodynamics of moist air. By understand-ing the basic physics behind the growth and progress ofhurricanes, physicists are contributing to a global effortto obtain better hurricane forecast models.

Spirals and eyesHurricanes are rotating low-pressure weather systemsthat develop mostly over the warm tropical oceans.Strictly speaking, the term hurricane applies only tostorms that occur over the Atlantic Ocean, CaribbeanSea, Gulf of Mexico and Eastern Pacific Ocean, whilestorms elsewhere are called typhoons or tropical cyc-lones. On average 80 such storms form globally eachyear, mostly in the summer months, and are classifiedby average wind speeds in excess of 33 m s–1 at 10 mabove the ocean surface.

Most readers will be familiar with satellite images ofhurricanes (figure 1). They consist of a characteristicspiral formed by dense cirrus clouds, which surround acloud-free eye that can vary from a few kilometres toover 100 km across. Around the eye there is an annu-lar region of deep convective clouds. These are calledthe eyewall clouds since their inner edge forms an out-ward-sloping “wall” to the eye. The eyewall has thefastest wind speeds and heaviest precipitation, makingit the most-feared part of the hurricane.

Hurricane formation is a complex process that is notcompletely understood, but certain conditions do needto be met. Hurricanes usually form over ocean waterthat is warmer than 26.5 °C to a depth of about 50m andthey are seeded by a pre-existing low-pressure disturb-ance. The air above the ocean also needs to be veryhumid and the wind speed fairly constant with height.Almost all hurricanes form at a latitude of greater than5° from the equator due to the Coriolis force, whichcauses the air to rotate. The rotation strengthens thehurricane and leads to the spiral shape.

The Coriolis force, which appears in the equationsof motion when Newton’s second law is formulated ina rotating reference frame like the Earth, tries to de-flect moving air to the right of its direction of travel inthe northern hemisphere and to the left in the south-ern hemisphere. As a result, hurricanes rotate anti-clockwise in the northern hemisphere and clockwise inthe southern hemisphere. The Coriolis force is zero atthe equator and increases with latitude, explaining whyhurricanes rarely form close to the equator.

The physics of hurricanesThe dynamics of hurricanes are quite subtle, but it isconvenient to think of the motion of air in terms of twocoupled components of flow. The first of these is the“primary circulation”, which describes the tangentialmotion of the air about the central rotation axis. Thesecond component, which is more complex, is the “sec-

Roger Smith is in thePhysics Department,University of Munich,Germany, e-mailroger.smith@physik.uni-muenchen.de

� A record number of intense hurricanes in the North Atlantic in 2005 highlighted theneed to improve hurricane forecasting

� The basic dynamics of hurricanes can be explained in terms of classical physicssuch as mechanics and thermodynamics, and understanding the physics ofhurricanes can help improve forecasting models

� Forecasters need to predict both the path a hurricane will take and how its intensitywill change. Track forecasts are reasonably accurate in the short term, butintensity forecasts are much less developed

� Researchers have recently linked the increased frequency and intensity ofhurricanes to global temperature rises, although this remains to be confirmed

� A concerted effort to improve the historical database of hurricanes has begun, in order to better establish whether global warming is linked to hurricane incidence

At a Glance: Hurricanes

Hurricane forceUnderstanding the physics of hurricanes can help scientists make better forecasts ofthese devastating phenomena and determine whether the recent increase in the numberof intense storms is linked to global warming, as Roger Smith explains

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Destructive spiralHurricane Katrina just before it made landfall in New Orleans inSeptember 2005.

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ondary circulation”. This describes the motion of air inthe radial direction, whereby it flows inwards nearerthe ocean surface and outwards at higher altitudes.These two components are not independent, and theycombine to form a picture of air parcels spirallinginwards, upwards and outwards (see box opposite).

The primary circulation is governed by competingforces in the inward and outward radial directions.Since the centre of the storm has a lower pressure thanthe surrounding region, the resulting pressure gradi-ent tries to make air parcels move towards the centre.As they do so, however, two additional forces comeinto play: the Coriolis force deflects the parcels per-pendicularly to their motion; while a centrifugal force

begins to kick in that drives them radially outwards.The net result is that the air rotates in a circular fash-ion, with the inward pressure-gradient force approxi-mately in balance with the outward centrifugal andCoriolis forces.

The secondary circulation is thought to be importantfor the intensification of the hurricane. In order for thisto happen, air must move inwards and upwards, allow-ing moisture within it to condense and release its latentheat energy. This inward and upward motion arisesbecause the pressure-gradient force, Coriolis force andcentrifugal force are not quite in balance: in particular,friction between the moving air and the ocean surfaceslows the winds flowing close to the ocean. Since theCoriolis force is proportional to the wind speed and thecentrifugal force to the square of the wind speed, theseforces weaken. But because the inward force providedby the pressure gradient remains approximately un-changed, air spirals inwards to the centre of the stormand is forced to rise. Furthermore, conservation of an-gular momentum dictates that as air moves to a smallerradius its velocity must increase, so the hurricane be-gins to rotate faster.

Now thermodynamics plays its part. The inflowingair progressively becomes more moist as it flows in-wards because of evaporation from the warm oceansurface. When it rises and cools, the water vapour con-denses to produce clouds and rain. The highest mois-ture content occurs at small radii, where the air risesto form the thick eyewall clouds. The temperature ofthe rising air increases with its moisture contentthrough the release of latent heat. Thus the moisturegradient at low levels sets up a negative radial tempera-ture gradient in the clouds, causing air at inner radii tobecome more buoyant and therefore rise relative to airfurther out. As the wind speed increases and the sur-face pressure falls, the evaporation rate and the mois-ture gradient increase, and hence so does the negativeradial temperature gradient. This positive-feedbackcycle is what turns a storm into a hurricane, althoughthere is a brake to the process: evaporation at theocean surface ceases when the relative humidity of theair reaches 100%.

The cloudless eye of the hurricane arises from sub-siding air in the centre of the storm, where the declinein tangential wind speed with height leads to a small netdownward force. This subsidence compresses airparcels, resulting in an increase in temperature thatcauses any water drops or ice crystals to rapidly evap-orate and stops clouds from forming.

Forecasting hurricanesIf we wish to reduce the devastation caused by stormslike Katrina, we need to be able to predict how a hur-ricane will develop. In particular, forecasters need toknow where a storm is heading and how intense it willbe when it gets there. Thanks to extensive research intothe dynamics of hurricane motion during the late 1980sand early 1990s, we are now able to predict the path ofhurricanes with reasonable short-term accuracy.

To a first approximation, hurricanes are carried alongby the larger-scale airflow in which they are embedded.Thus at low latitudes, storms tend to move westwardswith the trade winds, though they also drift polewards

1 Bird’s eye view

Close-up A satellite image shows the cloud-free eye of a hurricane indetail (top). Photographs have also been taken inside hurricanes byresearch aircraft, such as this one of Hurricane Isabel in 2003 thatshows the sloping eyewall clouds (bottom).

The wind damage produced by hurricanes rises roughly as the cube of the wind speed

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due to the increasing influence of the Earth’s rotationwith latitude. Often this poleward motion is accentu-ated by the flow of air around larger-scale weather sys-tems such as subtropical high-pressure systems, whichrotate in the opposite direction to hurricanes.

When a storm is swept round the western side of sucha system, it may “recurve” and begin moving towardsthe east as it is carried by the predominantly westerlywinds at higher latitudes. In fact, about 40% of Atlantichurricanes do precisely this as they move out of thetropics; some of them becoming intense extra-tropicalcyclones, while others decay over the cooler seas. Ver-tical variations in wind speed can also have an effect onhurricane motion and cause the storm centre to wob-ble about its mean track.

Over the last decade, improvements in the numer-ical weather-forecasting models that predict the globalairflow have led to more accurate short-term (24–48 hours) track forecasts. But the accuracy is still oftenunacceptably large beyond about 72 hours, which canbe a problem when trying to prepare heavily populatedareas such as the cities on the Gulf Coast for a hur-ricane impact. In the Atlantic during 2004 the USNational Hurricane Center’s mean track error (the dif-ference between the forecast position and the actualpath of a hurricane) was 107 km after 24 hours, increas-ing to 187 km after 48 hours, 280 km after 72 hours,395 km after 96 hours, and 546 km after 120 hours.

The intensity of a hurricane can also change dramat-ically over its lifecycle, but forecasts of intensity changeare still much less developed than track forecasts. Inorder to make better intensity forecasts – which arecritically important when storms are close to makinglandfall – we need to improve the way physical pro-cesses are represented in forecast models.

One example is the problem of concentric eyewallcycles. In strong hurricanes a new eyewall can form out-side the initial eyewall, for reasons which are not wellunderstood. The new outer eyewall then moves radiallyinwards and the inner eyewall disappears. These cyclesof eyewall formation and contraction correspond tochanges in the intensity of the hurricane: the hurricaneweakens when the new eyewall forms but re-intensifiesas it contracts. One possible explanation is that the sub-sidence associated with the outer eyewall weakens theconvection in the inner eyewall, allowing the inner-coreregion to spin down because of friction. Another pos-sibility is that air converging into the inner eyewall isredirected to the outer eyewall, reducing the supply ofangular momentum and moisture to the inner eyewall.

The ocean is another factor. The supply of moisturethat gives the storm its energy depends strongly on thetemperature of the ocean surface. But deeper, coolerwater can be drawn up to the surface by the turbulencecaused by the hurricane-force winds, reducing the sur-face temperature and weakening the hurricane. Theamount of cooling depends on the depth of the warmlayer before the storm hits and on the length of timethe storm lingers over a particular spot. It is thereforegreatest for slow-moving storms.

A major challenge is to accurately quantify the rate ofmoisture supply at the high wind speeds found near thecentre of a hurricane. Making measurements in theseextreme conditions is very difficult: it requires an air-

craft to fly several journeys through turbulent air at lessthan 100 m above a rough sea (figure 2).

During the last few Atlantic hurricane seasons,researchers at the National Oceanic and AtmosphericAdministration’s Hurricane Research Laboratory inMiami carried out such measurements in order todetermine the “exchange coefficients” that representthe moisture supply and surface exchange of momen-tum in hurricane models. Meanwhile, researchers atthe Hurricane Research Laboratory led by Peter Blackand at the University of Miami led by Lynn Shay havebeen gathering ocean data by dropping instrumentedocean probes at regular intervals along the flight pathbefore and after the passage of a hurricane.

Researchers at the Geophysical Fluid DynamicsLaboratory in Princeton, led until recently by YoshioKurihara, have developed a sophisticated hurricaneforecast model that includes the changes in oceanstructure brought about by the storm. The ocean datagathered by aircraft are therefore important forverifying the predictions of this model. This model is currently one of the most accurate for short-term

A mature hurricane consists of a largely cloud-free eye surrounded by deep “eyewall”clouds and then further out by spiral “rainbands”. These clouds are regions of veryheavy rain and strong gusts of wind. The strongest winds are found under the inneredge of the eyewall. Warm, moist air spirals inwards at low levels and rises in theeyewall and the rainbands. Most of the air spirals out in the upper troposphere andthe circulation eventually reverses direction, but a little air slowly subsides in the eye.The outward-flowing air forms dense cirrus clouds – the characteristic spiral seen insatellite images.

The inflow near the ocean surface is caused by friction between the air and theocean. This effect can be demonstrated by placing tea leaves in a beaker of waterand vigorously stirring the water to set it in rotation: the leaves gradually congregateat the bottom of the beaker near the axis, where they are swept by the inflow in thethin (~1 mm) layer of friction between the water and the beaker. As the water movesoutwards above the friction layer, it conserves angular momentum and spins moreslowly, so the rotation in the beaker gradually declines.

The same process would lead to the decay of a hurricane if the outflow of air was tooccur just above the friction layer. For a hurricane to intensify, air must flow inwardsabove the friction layer. This allows conservation of angular momentum to speed upthe air as it converges towards the axis. The mechanism for producing inflow abovethe friction layer is the (negative) radial gradient of the upward “buoyancy force”associated with the release of latent heat in the clouds (see text).

The ins and outs of intensification

eyewall clouds eye

cirrus outflow

warm, moist airhurricane winds

spiralrainbands

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hurricane forecasting. However, it is due to be replacedin the next year or so by a new model that is currentlybeing developed by a team at the National Centers forEnvironmental Prediction in Washington, DC led byNaomi Surgi.

Hurricanes always decay in intensity as they moveover land, not so much because the friction at the sur-face is increased, but because the lifeline to the mois-ture supply from the sea is cut off. This leads to coolingin the eyewall clouds, and hence reduces the radial gra-dient of buoyancy force that maintains the secondarycirculation. But even when the winds have slowed andthe surface-moisture supply has greatly diminished,hurricanes can still produce copious amounts of rainfalland flash-flooding. Also, because hurricanes constituteregions of air rich in angular momentum, they oftenspawn tornadoes on making landfall.

Worst-case scenarioBoth from a practical and theoretical standpoint, wewould like to know what sets the maximum intensity a storm can achieve in a given environment. In a series of papers, the first in 1988, Kerry Emanuel at the Massachusetts Institute of Technology has tried toanswer this question by likening a mature hurricane toa Carnot heat engine. The idea is that the hurricaneacquires heat energy (primarily in the form of latentheat) at the sea surface, where the temperature is typ-ically 26–30°C, and exports it to the upper troposphere,some 15 km above the sea, where the temperature istypically –60 to –70 °C. Using this analogy, Emanuelcalculated the maximum intensity that a storm canachieve at a given location and time.

Such calculations are important not only to fore-casters but also for making assessments of the impact of global warming on hurricane intensity. For example,they enable the maximum possible increase in intensityto be calculated for particular scenarios of a global

increase in temperature. The accuracy of Emanuel’stheory has recently been called into question byMichael Montgomery at Colorado State University onthe basis that numerically simulated hurricanes cansignificantly exceed the calculated maximum intensity.In reality, however, the majority of observed stormshave significantly lower intensities than the predictedmaximum. This suggests that there are frequently pro-cesses at work in the atmosphere that are detrimentalto intensification.

While the basic dynamics of hurricanes can be un-derstood in terms of processes that are symmetricabout the rotation axis, non-axisymmetric processesare very important as well. Large-scale asymmetriesarising from the interaction of storms with their en-vironment can have an important effect on stormmotion and perhaps also on intensity. Moreover, hur-ricanes are able to support different types of asym-metric waves in which air parcels move radially inwardsor outwards during a cycle.

One particular type of wave – the “vortex Rossbywave” – propagates in the opposite direction to the tan-gential wind. These waves are almost certainly excitedby moist convection, and also by external influencessuch as changes in the large-scale vertical wind-shear.In the last decade Montgomery and Wayne Schubertat Colorado State University have carried out pion-eering studies of these waves and the instabilities theyproduce. In particular, they have shown that vortexRossby waves can transport angular momentum radi-ally in a hurricane, which means that the waves can playan important role in the intensification of storms. Theteam also found that the waves may become unstable,which may be an important mechanism for transport-ing angular momentum and heat across the eyewallinto the eye itself.

During the last Atlantic hurricane season, research-ers at the Universities of Washington and Miami, and at

Hurricanesdecay inintensity asthey move overland becausetheir lifeline tothe moisturesupply from thesea is cut off

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Aircraft such as the US National Oceanic and AtmosphericAdministration’s P3 (top left) allow researchers to perform in situmeasurements of hurricanes. The underbelly of the aircraft has a radarthat detects the pattern of precipitation during the flight. Regions of highradar reflectivity correspond to heavy precipitation, which makes theeyewall clouds and the spiral rainbands that form at larger radii prominentfeatures of radar images (bottom left). The graph shows a typical profile offlight-level wind speed and temperature measured by the aircraft as ittraverses the storm at an altitude of 1.5–3 km. Winds are light in the eye,rising rapidly to a maximum and then declining steadily with radius.Maximum wind speeds are found at low levels under the eyewall clouds.

eye eyewall rainbands

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the National Center for Atmospheric Research, sup-ported by the Hurricane Research Division, carried outa major programme to document the asymmetries inhurricanes, and especially the structure of the spiralrainbands. The researchers, led by Robert Houze at theUniversity of Washington, obtained in situ measure-ments in hurricanes Katrina, Rita and Ophelia usingmultiple research aircraft. The first results from thewealth of data collected are just beginning to emergeand should provide a deeper insight into the role ofspiral rainbands in the intensity change of hurricanes.

Recently, Sang Nguyen in my group at the Universityof Munich carried out numerical calculations that sug-gest the intensification of the hurricane core is intrin-sically asymmetric. This is due to the irregular patternsof convective clouds that form, and the calculationssuggest that the core region is inherently unpredictable.

Global warmingOne of the most controversial topics at present is thepossible effect of global warming on the frequency andintensity of hurricanes. Since the Earth has warmedconsiderably in the last 50 years, it seems reasonable toexpect that warmer temperatures at the sea surface willprovide more-favourable conditions for hurricane for-mation. However, a particular problem of attributingchanges in hurricane frequency and intensity to globalwarming is the large natural variation in the frequencyof storms, which also follows long-term cycles.

Nevertheless, last year Peter Webster and co-workersat the Georgia Institute of Technology attempted suchan analysis of 35 years’ worth of storm records in allocean basins. They reported a large increase in thenumber of intense storms in most basins with, perhapssurprisingly, the smallest percentage increase being inthe North Atlantic Ocean. However, the number ofless-intense storms has decreased in all basins exceptthe North Atlantic during the last decade. The in-creased frequency of intense storms coincides with an average increase in the surface temperatures of0.5 °C for these basins in the same period.

In 2005 Kerry Emanuel pointed out that while thefrequency of hurricanes is an important scientific issue,it is not the best measure of the threat posed by suchstorms. He showed that the wind damage produced byhurricanes increases roughly as the cube of the windspeed, and therefore defined a “power dissipationindex” by integrating the cube of the maximum windspeed over the life of a storm. He showed that fluctu-ations in this index correlate rather well with the meansea-surface temperatures in the North Atlantic andNorth Pacific – the two basins that have the most reli-able data on storm intensities – suggesting that warm-ing of the oceans will increase the damage potential ofhurricanes. In view of the population growth in manycoastal areas prone to hurricanes, this suggests thatdevastating storms like hurricanes Katrina and Wilmacould become the norm in coming decades.

However, some researchers, including Chris Land-sea at the National Hurricane Center in Miami andWilliam Gray at Colorado State University, have ex-pressed scepticism about the quality of the data used to examine links between global warming and hurricanefrequency. The problem is that in most areas where

tropical cyclones occur there are virtually no in situ data,so intensities have to be inferred from satellite images.The methods for doing this have improved over theyears, but most have a significant subjective element.In fact, Gray claims that there has been no significantincrease in the number of intense hurricanes in allbasins except in the Atlantic over the last 20 years andthere has even been a slight decline in the NorthwestPacific. Such controversies highlight an urgent need to improve the historical tropical-cyclone database. Tothis end a major reanalysis project is now under way,coordinated by Greg Holland at the National Centerfor Atmospheric Research in Boulder, Colorado.

Our understanding of the physics of hurricanes hasgreatly improved over the past two decades and fore-casts of hurricanes have become ever more reliable.Even so, we still have much to learn about the basicphysical processes that are responsible for the intensityof storms. Such knowledge is important for developingthe next generation of hurricane forecast models, andalso for determining the limitations of such forecasts.The 2006 Atlantic hurricane season officially began on1 June, and the coming months will allow more data tobe gathered to test and improve our knowledge of thesedevastating storms.

More about: HurricanesR A Anthes 1982 Tropical Cyclones: Their Evolution, Structureand Effects (Boston, American Meteorological Society)R L Elsberry (ed) 1995 Global Perspectives on TropicalCyclones (Geneva, World Meteorological Organization)K Emanuel 2005 Divine Wind: The History and Science ofHurricanes (Oxford University Press)

3 Potential devastation

Hurricane Katrina caused $125bn of damage in New Orleans last year, and increasedspeculation about whether the growing incidence of intense storms is linked to global warming.Researchers have calculated the maximum possible intensity a hurricane can achieve in agiven environment, and can relate this to predicted climate-change scenarios.

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