Hurricane Threats

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Science  20 Jul 2001:
Vol. 293, Issue 5529, pp. 440-441
DOI: 10.1126/science.1062047

Tropical cyclones are among the most devastating natural disasters, frequently causing loss of human lives and serious economic damage through ocean storm surges in coastal regions, destructive winds, and flash flooding due to excessive precipitation. The annual costs for the United States, presently estimated at some $5 billion (1), are expected to rise as a result of growing population and increasing wealth in coastal areas. Other regions of the world can be even more exposed to tropical cyclones. In 1998, Hurricane Mitch [HN1] killed at least 10,000 people in Central America and caused enormous economic damage [HN2]. The average annual economic loss in the Philippines is estimated at some 5% of the gross national income.

Tropical cyclones are low-pressure systems that originate over tropical or subtropical oceans (2) and have organized convection [HN3] and a well-defined cyclonic circulation at the surface. At maximum sustained surface wind velocities of 17 m/s, they are called tropical storms or tropical cyclones. At 33 m/s or more, they are referred to as hurricanes (North Atlantic, northeast Pacific), typhoons (northwest Pacific), or severe tropical cyclones (southwest Pacific, Indian Ocean) [HN4].

A dangerous storm.

(Top) Hurricane Floyd [HN17] on 14 September 1999 at 1244 UTC (Universal Time Coordinated) over the Bahamas on a northwesterly course toward the mainland United States. This multispectral false color image was taken by the NOAA-15 polar orbiting satellite. (Bottom) The same hurricane 7 hours later as seen by the GOES-8 geostationary weather satellite [HN18] in a colorized infrared image. The dark red colors reflect areas of intense convective activity. Because of its size and intensity, Floyd constituted a very serious threat to the East Coast of the United States. It struck the coast of North Carolina (fortunately substantially below its maximum speed) and moved up into New England. River flooding caused 57 deaths (56 in the United States). The total damage has been estimated at $3 billion to over $6 billion. See


On page 474 of this issue, Goldenberg et al. (3) [HN5] report an analysis of tropical cyclones in the Atlantic and Caribbean during much of the 20th century. Their results suggest that there may be long-term variations in the number of hurricanes. If true, this would have important implications for those regions within the storm track of these Atlantic storms.

The initial dynamics leading to a tropical cyclone are not well understood because data are limited and complex interactions between many scales of motion are involved. Once a weak cyclonic circulation exists, however, it may intensify into a hurricane as follows: Near the sea surface, friction causes the air to spiral inward toward the storm center. Clouds near the center become organized into spiral rainbands and eventually into an eye wall by the strong rotation in the vortex. As the winds strengthen and surface pressure decreases, increasing amounts of water are extracted from the warm ocean. The air rises and cools and water vapor condenses, releasing latent heat. The heating of the center of the storm leads to its intensification, thereby further increasing the surface wind and evaporation. The storm will continue to intensify in this way until the energy input by surface evaporation is balanced by the frictional dissipation.

Tropical cyclones thus derive energy primarily from the evaporation of seawater and the associated condensation in convective clouds concentrated near the center of the storm. A well-developed tropical cyclone (hurricane) converts ocean heat energy into the mechanical energy of the winds, like a heat engine or Carnot engine (46) [HN6]. In contrast, extratropical cyclones [HN7] primarily obtain energy from the redistribution of air masses at different potential temperatures.

Observations indicate (7) that the empirical relations based on the concept of a Carnot engine developed in (6) provide a good measure of the upper bound on the possible wind speed and intensity of a hurricane as can be determined from the sea surface temperature and the state of the atmosphere. However, the conditions responsible for the development of tropical cyclones are poorly understood because of a lack of good observations in areas where they develop. Empirical assessment (3) and results from comprehensive climate models (8) [HN8] are in broad agreement that the following key conditions must be met: First, tropical storms will only develop over ocean areas where the sea surface temperature is ∼26°C or more because a minimum amount of ocean heat supply is required. Second, low vertical wind shear [HN9] is required, presumably because the convective cloud cells that provide the energy for the storm can only do so if their vertical structure is maintained. A strong wind shear will distort the structure of the convective cells and prevent them from systematically driving the storm.

A few other patterns have been found in connection with the development of tropical cyclones. Large-scale cyclonic circulation systems in the lower troposphere (onset vortices) and an unstable moist stratification through the depth of the atmosphere commonly occur over the ocean areas where tropical storms develop. Observations (9) and modeling results (10) indicate that El Niño events influence the frequency of hurricanes in the Atlantic [HN10]. The suggested mechanism is that during an El Niño warm event, vertical wind shear is increased over the tropical Atlantic through changes in the large-scale tropical circulation. Other remote factors have been suggested, such as rainfall variability over the western Sahara (11) or influences by the quasi-biennial circulation in the stratosphere (12), but these are empirical and lack a clear physical understanding.

Why some disturbances intensify to a hurricane while others do not is not well understood although they can be simulated and predicted reasonably realistically with numerical models (8) [HN11]. Neither is it clear why some tropical cyclones almost reach their maximum potential and others do not. It is the major hurricanes, reaching wind speeds above 50 m/s, that produce 80 to 90% of the damage in the United States, although they account for only 20% of all land-falling tropical cyclones (1).

On average, 45 tropical storms reach hurricane strength each year, 30% of them in the western North Pacific. Because of the short period of reliable observations—about 60 years in the Atlantic and the western North Pacific and only about 30 years elsewhere—it is not yet feasible to determine a trend or reliable low-frequency variations. Goldenberg et al. (3) suggest that there is evidence of long-term (multidecadal) shifts in the number of major hurricanes in the Atlantic and the Caribbean. A high level of activity from 1920 to 1960 is followed by reduced activity from the mid-1960s to the early 1990s. Thereafter, the authors report a return to a more active period. Superimposed on this slow variability are substantial variations from year to year, often influenced by El Niño-Southern Oscillation (ENSO) events.

Other tropical storm areas show no conclusive trend or variation (13). In some regions, the numbers of tropical storms have increased, in other regions they have decreased, and in some regions they are unchanged. The lack of long reliable records and a systematic classification of the storms in previous years makes it impossible to identify trends or clearly defined fluctuations.

Tropical cyclones usually form and spend most of their time over remote ocean areas. It was therefore not until the advent of space observations in the 1970s that the detection and systematic monitoring of the storms became possible [HN12]. Additional observations come from reconnaissance aircraft, coastal radar, ships, buoys, and land stations. The combined use of better observations and advanced numerical modeling and data assimilation has gradually improved the forecasting of tropical storms (14) [HN13], which is now increasingly done with comprehensive high-resolution atmospheric models (1), with some success even on the time scale of a week (15).

Predictions of seasonal hurricane activity have demonstrated modest forecast skill. Since the mid-1980s, these empirically based forecasts (1) have been able to anticipate nearly 20% of the variance of hurricane frequencies. The phase of ENSO, vertical wind shear, and Atlantic sea surface temperatures are the most important factors. The most promising approach for future development is the use of advanced climate models, which a priori incorporate these different factors.

How tropical cyclone frequency and intensity might respond to climate change is still a very open question (13) [HN14]. The above discussion on the mechanisms for hurricane development suggests that ocean warming would enhance tropical cyclone development. From this, one may be led to infer that if the area enclosed by the 26°C sea surface temperature isotherm increases, so too would the area experiencing tropical cyclogenesis. However, this is incorrect, as has been shown in (16): Cyclone development in a warmer climate occurs at higher oceanic temperatures, particularly in the case of intense tropical cyclones, because upper atmosphere warming compensates to some extent for the increased energy potential from the warmer ocean. This result is supported by modeling studies (17).

The broad geographical regions affected by tropical cyclones are thus not expected to change substantially. In particular, there is no reason to expect that the region of cyclone development will expand with the 26°C isoterm. However, although the number of cyclones may not increase substantially in the near future, this does not necessarily mean that the strength of the most powerful and dangerous cyclones will remain the same. Given optimum conditions in a future warmer climate, with an atmosphere potentially holding more moisture, the development of more intense cyclones cannot be excluded. This notion is supported by a high-resolution climate modeling study (18) [HN15].

The societal vulnerability to hurricanes has increased substantially in recent decades, mainly because of (19) increased population in hurricane-exposed areas all around the world and, in some areas such as the U.S. coastal regions, increased wealth and advanced infrastructure. It has been estimated that if the hurricanes of 1925 had occurred in the late 1990s, the damage would have cost some $75 billion instead of a few billion if normalized for inflation, coastal county population changes, and changes in wealth (20) [HN16].

The situation in the coastal United States is particularly precarious because the population increased substantially between 1960 and the 1990s. During this time, major hurricanes were rather rare, and this may have created a sense of false security. A change to what was typical, say, for 1920 to 1960 would create a potentially serious situation requiring most urgent attention (3).

The high hurricane activity during the last couple of years is as typical as the previous quiescent period, but the records are too short and incomplete to claim that the coastal United States may be in for a longer period of higher hurricane activity. Neither are there any indications that the climate warming may increase the frequencies of hurricanes in the area although the risk of very powerful storms may slowly mount. The risk of human losses is likely to remain low, however, because of a well-established warning and rescue system and ongoing improvements in hurricane prediction. A main concern is the risks of high damage costs (up to $100 billion in a single event) because of ongoing population increases in coastal areas and increasing investment in buildings and extensive infrastructure in general.

HyperNotes Related Resources on the World Wide Web

General Hypernotes

S. Baum, Department of Oceanography, Texas A&M University, provides a glossary of physical oceanography and related disciplines.

The WWW Virtual Library of Meteorology is maintained by T. Owens, Data Assimilation Office, NASA Goddard Space Flight Center.

WINDandSEA is a collection of links to Internet resources prepared by the Central Library of the National Oceanic and Atmospheric Administration (NOAA). NOAA's National Climatic Data Center (NCDC) is an archive of weather data; collections of climate resources and Internet links are provided.

The Google Web Directory provides links to Internet resources related to meteorology. A section on hurricanes is included.

R. H. Cummins, Interdisciplinary Studies, Miami University, provides a collection of Internet resources on tropical weather.

The Interactive Weather Information Network is provided by the U.S. National Weather Service.

UM Weather from the University of Michigan provides a collection of links to Internet weather information resources. The Weather Underground offers links to tropical weather information.

The Encyclopedia of the Atmospheric Environment is presented by the Atmosphere, Climate and Environment Information Programme, Manchester Metropolitan University, UK. Weather and climate topics are included.

The Weather Channel offers a storm encyclopedia with a collection of articles on tropical storms.

The Canadian Hurricane Centre presents All About Hurricanes. A glossary and Internet links are included.

NOAA's Hurricanes: Nature's Greatest Storms Web site offers a presentation on the basics of hurricanes and links to hurricane resources on the Internet. NOAA's National Hurricane Center (NHC) issues watches, warnings, forecasts, and analyses of hazardous weather conditions in the tropics; a glossary and links to other tropical cyclone Web sites are provided.

The Hurricane Research Division of NOAA's Atlantic Oceanographic and Meteorological Laboratory (AOML) provides a research overview on hurricanes and tropical meteorology; links to Internet resources concerning hurricanes, tropical weather, and storms; and a FAQ by C. Landsea on hurricanes, typhoons, and tropical cyclones.

The MetEd (Meteorology Education and Training) Web site is a joint effort of the National Weather Service Training Center, the NOAA Warning Decision Training Branch, and the Cooperative Program for Operational Meteorology, Education and Training. Web-based educational modules and links to Internet resources are provided.

Weather World 2010 (WW2010), presented by the Department of Atmospheric Sciences, University of Illinois, provides a guide to meteorology.

Fundamentals of Physical Geography, a Web textbook by M. Pidwirny, Department of Geography, Okanagan University, Kelowna, BC, Canada, includes a section on climatology and meteorology.

N. Atkins, Department of Meteorology, Lyndon State College, Lyndonville, VT, provides lecture notes and other resources for a course on meteorology.

H. Grissino-Mayer, Department of Physics, Astronomy and Geosciences, Valdosta State University, GA, offers lecture notes for a course on meteorology and climatology.

H. Shirer, Department of Meteorology, Pennsylvania State University, offers lecture notes for a course on weather analysis and forecasting.

G. Lash, Department of Geosciences, Fredonia State University College, NY, offers lecture notes for a course on catastrophic weather and climatic change.

K. Droegemeier, Center for Analysis and Prediction of Storms, University of Oklahoma, provides lecture notes for a course on severe and unusual weather.

J. Evans, Department of Meteorology, Pennsylvania State University, offers lecture outlines and definitions for a course on tropical meteorology. An animation of a tropical cyclone is presented.

E. Takle, Department of Geological and Atmospheric Sciences, Iowa State University, provides lecture notes and Internet links for a course on global change.

The Monterey Marine Meteorology Division of the Naval Research Laboratory makes available the Tropical Cyclone Forecasters' Reference Guide. A collection of tropical cyclone related links and web sites is provided.

The June 2000 issue of the Bulletin of the American Meteorological Society had a policy statement (in Adobe Acrobat format) by the American Meteorological Society on hurricane research and forecasting (1).

C. Landsea of the AOML Hurricane Research Division makes available a book chapter titled “Climate variability of tropical cyclones: Past, present and future.”

The 22 September 2000 issue of Science had a review by D. Easterling et al. titled “Climate extremes: Observations, modeling, and impacts.”

Numbered Hypernotes

1. The NOAA Operational Significant Event Imagery Server makes available a collection of Hurricane Mitch images. The Tropical Cyclones information page from the UK Met Office provides a fact sheet on Hurricane Mitch. NHC makes available a preliminary report on Hurricane Mitch. NOAA's NCDC provides an information page on Hurricane Mitch. USA Today provides information and news stories about Hurricane Mitch. The Atlantic Hurricane Track Maps and Images Web site, maintained by S. Babin, Space Department, Applied Physics Laboratory, Johns Hopkins University, provides information on Hurricane Mitch.

2. The U.S. Agency for International Development issued a fact sheet on 19 November 1998 about the impact of Hurricane Mitch on Central American countries. The Pan American Health Organization provides an information page about dealing with the aftermath of Hurricane Mitch. ReliefWeb provides updates about Hurricane Mitch relief efforts. The Center for Integration of Natural Disaster Information of the U.S. Geological Survey offers a presentation on Hurricane Mitch titled “Disaster in Central America.” The United Nations System Honduras office provides a Hurricane Mitch Information Center.

3. Hyperphysics, maintained by C. R. Nave, Department of Physics and Astronomy, Georgia State University, defines heat convection. The glossary of Arctic meteorology and climatology defines convection and convection cloud. The Encyclopedia of the Atmospheric Environment has an entry on convection. WW2010 offers an introduction to convection. The York Educational Science Net offers an introduction to convection.

4. The Canadian Hurricane Centre offers presentations on the formation, structure, and movements of tropical cyclones. offers an Encyclopædia Britannica article on tropical cyclones; the article on the ocean includes a section on the formation of tropical cyclones. The online World Book encyclopedia offers a presentation on hurricanes. Resources in Atmospheric Sciences, provided by B. Geerts, Department of Atmospheric Sciences, University of Wyoming, includes a section of articles by B. Geerts and E. Linacre on tropical cyclones. WW2010 offers a presentation on hurricanes with a section on tropical cyclones around the world. USA Today provides a hurricane information Web page with guides to hurricane basics and hurricane science, as well as a glossary, a hurricane history page, and animations. NASA's Earth Observatory offers a reference feature titled “Hurricanes: The greatest storms on Earth.” The Department of Atmospheric Sciences, Texas A&M University, makes available lecture notes on hurricanes for a course on weather forecasting. G. Lash offers lecture notes on hurricanes for a course on catastrophic weather and climatic change. The Unisys Weather Web site provides tracking and other information on hurricanes and tropical storms.

5. S. B. Goldenberg and C. Landsea are in the AOML Hurricane Research Division, Miami. W. Gray is in the Department of Atmospheric Science, Colorado State University, Fort Collins. A. Mestas-Nuñez is at the Cooperative Institute for Marine and Atmospheric Studies, Rosenstiel School of Marine and Atmospheric Science, University of Miami. The AOML Hurricane Research Division provides information about Goldenberg and Landsea's project on climate variability of tropical cyclones around the world.

6. C. R. Nave's Hyperphysics has an introduction to the Carnot cycle in the section on heat engine concepts. G. Quirion, Department of Physics, Memorial University of Newfoundland, provides a series of slides about the Carnot engine. Start Your Engines, a ThinkQuest student Web project, includes a section on the Carnot cycle and biographical information on Nicolas Léonard Sadi Carnot. D. Davis, Physics Department, Eastern Illinois University, includes a presentation about ideal heat engines in lecture notes on thermodynamics and heat engines for a physics course. The NTNU Virtual Physics Laboratory, presented by F.-K. Hwang, Department of Physics, National Taiwan Normal University, offers an applet of a Carnot heat engine.

7. The Encyclopædia Britannica offers an introduction to extratropical cyclones, as well as a section on extratropical cyclones in the article on climate. WW2010 offers a presentation on midlatitude cyclones. USA Today provides a feature on extratropical storms. The climatology and meteorology chapter of M. Pidwirny's Fundamentals of Physical Geography includes a section on mid-latitude cyclones. J. Cortinas, NOAA National Severe Storms Laboratory, Norman, OK, provides lecture notes on fronts and extratropical cyclones for a meteorology course.

8. The Climatic Research Unit at the University of East Anglia, Norwich, UK, offers an information sheet on global climate models. R. Myneni, Department of Geography, Boston University, offers lecture notes on global climate models for a course on global climate change and environmental impacts. E. Takle provides a series of lecture notes on climate models (lectures one, two, three, and four) for a course on global change. R. Gallop, School of Environmental Sciences and Land Management, University College Worcester, UK, offers lecture notes for a course on modeling Earth's climate. A chapter on climate models is included in the 2001 report Climate Change Science: An Analysis of Some Key Questions issued by the Committee on the Science of Climate Change of the National Research Council. The CLIVER Web site makes available a 1998 conference paper by L. Bengtsson titled “Climate modelling and prediction — Achievements and challenges.” The 16 June 2000 issue of Science had a review by H. Grassl titled “Status and improvements of coupled general circulation models.”

9. The glossary provided by the Canadian Hurricane Centre defines vertical wind shear. WW2010 provides an introduction to vertical wind shear and a section on the development of hurricanes and the effect of wind shear. The Regional and Mesoscale Meteorology Team at the Cooperative Institute for Research in the Atmosphere offers a presentation on vertical wind shear and tropical cyclones. The Tropical Cyclone Forecasters' Reference Guide includes a section on vertical wind shear.

10. The Center for Ocean-Atmosphere Prediction Studies, Florida State University, provides a resource page on El Niño and La Niña. NOAA offers an El Niño Page with links to El Niño resources. Other NOAA pages on El Niño and ENSO are provided by the Office of Global Programs, the Climate Prediction Center, and the Pacific Marine Environmental Laboratory, which offers a section on the impacts of El Niño. The El Niño Scenario from the Earth Space Research Group, University of California, Santa Barbara, provides an illustrated hyperlinked introduction to El Niño; sections on its mechanisms and effects are included. WW2010 includes a presentation on how hurricane frequency may be affected by El Niño. C. Landsea makes available a preprint of an article titled “El Niño-Southern Oscillation and the seasonal predictability of tropical cyclones,” as well as an article by R. Pielke Jr. and Landsea titled “La Niña, El Niño, and Atlantic hurricane damages in the United States” and an article by Landsea et al. titled “Atlantic basin hurricanes: Indices of climatic changes.” Consequences, an online journal provided by the U.S. Global Change Research Information Office, had an article (vol. 5, no. 1, 1999) by K. Trenberth titled “The extreme weather events of 1997 and 1998” and (in vol. 5, no. 2, 1999) an article by S. Zebiak titled “El Niño and the science of climate prediction” and an article by C. Ropelewski titled “The great El Niño of 1997 and 1998: Impacts on precipitation and temperature.”

11. WW2010 includes an introduction to numerical weather prediction. The Shodor Education Foundation makes available an introduction to numerical modeling for a course on air quality meteorology developed for the U.S. Environmental Protection Agency. H. Shirer provides lecture notes on numerical weather prediction models for a course on weather analysis and forecasting. The Numerical Modeling Laboratory at the Graduate School of Oceanography, University of Rhode Island, offers a presentation on the group's tropical cyclone research.

12. N. Atkins provides an introduction to the history of satellite meteorology for a course on remote sensing. Looking at Earth from Space, a presentation available from NASA's Earth Science Enterprise Web site, includes a history of the GOES program, as well as a section on NOAA missions. E. Takle provides lecture notes on satellite observations for a course on global change. WW2010 offers a module on satellite meteorology. The Remote Sensing Tutorial, made available by the Applied Information Sciences Branch, NASA Goddard Space Flight Center, includes an introduction to meteorological, oceanographic, and Earth system satellites and a section on meteorology from space that includes historical information.

13. B. Geerts's Resources in Atmospheric Sciences includes a section of articles on weather forecasting. The NOAA hurricanes Web site offers an introduction to forecasting. NHC provides forecasting information and a presentation on forecasting models; NHC's Hurricane Awareness Week Web site includes a presentation on the forecast process. NOAA makes available a 21 May 2001 press release titled “NOAA hurricane forecasters expect normal Atlantic storm activity in 2001: NOAA says 5 to 7 hurricanes could threaten.” The UK Met Office offers a presentation on tropical cyclone forecasting. C. Landsea's hurricane FAQ includes a section on tropical cyclone forecasting. W. Gray's Tropical Meteorology Project at the Department of Atmospheric Science, Colorado State University, provides seasonal Atlantic hurricane forecasts. The Geophysical Fluid Dynamics Laboratory offers a presentation titled “Hurricanes: Modeling nature's fury.” NASA's Earth Observatory offers a feature by J. Weier titled “Seeing into the heart of a hurricane” and a feature by L. Schmidt titled “Forecasting fury.” Making Climate Forecasts Matter is a 1999 report by the Commission on Behavioral and Social Sciences and Education of the National Research Council. USA Today makes available a 20 June 2001 Associated Press article titled “New techniques may help hurricane forecasters.” The 23 April 1999 issue of Science had a News of the Week article by R. Kerr titled “Forecasters learning to read a hurricane's mind.”

14. The Hurricane Dynamics Group at NOAA's Geophysical Fluid Dynamics Laboratory offers a presentation on global warming and hurricanes. The January 1998 issue of the Bulletin of the American Meteorological Society had an article (full text available in Adobe Acrobat format) by A. Henderson-Sellers et al. titled “Tropical cyclones and global climate change: A post-IPCC assessment” (13). NOAA's NCDC offers a FAQ on global warming. NASA's Earth Observing System Web site offers a fact sheet (in Adobe Acrobat format) on global warming. The U.S. Environmental Protection Agency provides a Global Warming Web site. The Warming of the Earth is a presentation offered by the Woods Hole Research Center.

15. The 13 February 1998 issue of Science had a report by T. Knutson, R. Tuleya, and Y. Kurihara titled “Simulated increase of hurricane intensities in a CO2-warmed climate” (18). T. Knutson, Geophysical Fluid Dynamics Laboratory, offers a presentation on global warming and hurricanes and a presentation on the climate impact of quadrupling atmospheric CO2.

16. C. Landsea makes available an article (published in Weather and Forecasting, vol. 13, p. 621, 1998) by R. Pielke Jr. and Landsea titled “Normalized U.S. hurricane damage, 1925–1995” (20). The Environmental and Societal Impacts Group at the National Center for Atmospheric Research provides a Societal Aspects of Weather Web site with a section on hurricane Internet resources. NHC makes available a presentation by E. Rappaport and J. Fernandez-Partagas titled “The deadliest Atlantic tropical cyclones, 1492-present” (updated through 1996). The 7 September 1998 issue of Time magazine had an article by E. Larson titled “Waiting for Hurricane X.” The U.S. Federal Emergency Management Agency provides a tropical storm watch Web page; a section with hurricane background and preparedness information is included.

17. NHC makes available a preliminary report on Hurricane Floyd. NOAA's NCDC provides information on Hurricane Floyd. NASA's Earth Observatory offers a feature by D. Herring titled “Hurricane Floyd's lasting legacy.” The Atlantic Hurricane Track Maps and Images Web site includes information on Hurricane Floyd. The Gallery of the Geophysical Fluid Dynamics Laboratory offers a 3D visualization of Hurricane Floyd and other hurricane images.

18. The NOAA Satellite Information System provides an overview of the NOAA geostationary and polar-orbiting satellite systems. NOAA's NCDC provides background information about Polar-orbiting Operational Environmental Satellites (POES) and Geostationary Operational Environmental Satellites (GOES). The Remote Sensing Tutorial includes sections on NOAA series and GOES satellites. NASA's Goddard Space Flight Center offers GOES and POES project Web sites. WW2010 provides information on GOES and POES satellites in the guide to remote sensing.

19. L. Bengtsson is in the Climate Modelling Group, Max-Planck-Institut für Meteorologie, Hamburg, and at the Environmental Systems Science Centre, University of Reading, UK.

References and Notes

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