The Useful Pursuit of Shadows The study of clouds has profoundly influenced science and human culture and stands poised to lead climate science forward again Graeme L. Stephens n a December evening in 1802, Luke fixed laws.…” Howard’s ideas about the OHoward, a London pharmacist and ama- physics of clouds were generally sound despite teur meteorologist, aired his ideas about the the poor understanding of the physics of air classification of clouds. These ideas were pre- and water vapor in his time. sented to a small gathering of young science- By contrast, for the past 50 years the modern minded intellectuals who called themselves science of meteorology has fixated on the ever- The Askesian Society. Howard’s lecture on that expanding capability of computer technology evening was titled “On the Modification of and the numerical prediction of the movements Clouds” and opened as follows: of invisible air. To the nonmeteorologist this must appear most odd. Clouds, after all, are the most My talk this evening concerns itself with visible manifestation of weather in all its forms, what may strike some as an uncharacteristi- and their prediction should be more than an ob- cally impractical subject: it is concerned ject of curiosity. However, even the task of nu- with the modification of clouds. Since the merically integrating forward in time the increased attention which has been given to Navier-Stokes equations governing the behav- meteorology, the studies of various appear- ior of invisible air turned out to be substantially ances of water suspended in the atmos- more complex than was originally conceived. phere has become an interesting and even Today, 200 years after Howard’s lecture, we necessary branch of that pursuit. If clouds enter a period when we return in earnest to pur- were the mere result of condensation of suing the subject of shadows. Although the focus vapour in the masses of the atmosphere on weather prediction remains, we now embrace which they occupy, if their variations were more fully the broader problem of predicting the produced by the movements of the atmos- evolving, moist atmosphere as foretold some 30 phere alone, then indeed might the study years ago by Edward N. Lorenz: be deemed a useless pursuit of shadows.… The previous generation was greatly con- This was a historic lecture for many reasons. cerned with the dynamics of pressure sys- Most importantly, it heralded the beginning of tems and talked about highs and lows. To- meteorology, a previously unrecognized area day we have not lost interest in these Graeme Stephens is a professor of natural science. This lecture was published systems but we tend to look upon them as in the Department of the following year as an essay and appeared in circulation systems. This change in atti- Atmospheric Sciences at subsequent publications over a span of almost tude has led to a deeper understanding of Colorado State University. His 20 years. It is a remarkable testimony that research interests include the their dynamics. Perhaps the next genera- Howard’s classification, with minor changes, use of remote sensing to study tion will be talking about the dynamics of remains in use today by practicing meteorolo- the Earth’s climate with empha- water systems. sis on clouds and radiation. gists. His classification was a revelation, bring- Currently he is the principal ing a sense of order and understanding to a Concerns about inadvertent climate change investigator of NASA’s subject that had lacked coordinated thought— and our desire to predict changes associated with CloudSat mission, which is let alone any documented theories as to how the build up of greenhouse gases in the atmos- designed to probe clouds pro- pressure, temperature, rainfall and clouds phere return the subject of clouds to the forefront viding the kinds of information might be related. Perhaps even more impres- of the atmospheric and climate sciences. The end- illustrated in Figure 5 of this sive than Howard’s classification of clouds, or less, compensating flows of water between the article. Address: Department of “modifications” as he referred to them, was his earth and sky fundamentally govern how the cli- Atmospheric Sciences, Colorado State University, Ft. intuition, inspired by the earlier ideas of his mate of Earth evolves and how it will change in Collins, CO 80253. Internet: close acquaintance John Dalton that clouds the future. There can be no credible climate pre- [email protected] must be considered as “subjects of grave theo- diction without a proper quantitative account of http://cloudsat.atmos.colostate.edu ry and practical research … governed by … the atmospheric water systems that nurture the © 2003 Sigma Xi, The Scientific Research Society. Reproduction 442 American Scientist, Volume 91 with permission only. Contact [email protected]. Figure 1. Clouds were first described systematically by Luke Howard, a London pharmacist and amateur meteorologist, in an 1802 address to the Aske- sian Society. Howard’s Linnean cloud-classification system remains largely intact today, and its development thus marks the birth of the science of me- teorology. During the last half of the 20th century, however, atmospheric science fixated on the numerical modeling of the movement of invisible air masses, only very recently returning to the daunting but crucial subject of water transport in the atmosphere. Meanwhile, the practice of classifying clouds by type has largely disappeared from mainstream research in the atmosphere. The author argues that as effective as modern modeling and imaging techniques may be, something is lost when clouds become only data. These ethereal assemblages of water vapor still hold the potential to in- spire quantitative science to new heights. (Maynard Dixon’s 1926 Mesas in Shadow courtesy of the Brigham Young University Museum of Art.) planetary water cycle. Yet as we focus anew on I pass through the pores of the ocean and the physics of water within the giant circulating shores; eddies of air, it is sobering to realize that our cur- I change, but I cannot die. rent ability to measure the amounts within these massive systems is rudimentary. Not only does Shelley express a deep apprecia- tion of the intimate role of clouds in what is now The Classification of Clouds known as the water cycle, but he also conveys In the 18th century, scientists seemed to be pre- one of the truly complex properties of clouds occupied with naming and classifying objects that thwarted early attempts at classification— of nature. The language used to describe the their ability to mutate rapidly from one form to natural world at that time was evolving and another in a smooth, fluid continuum within an rapidly becoming standardized. Before evolving chaotic world of vapor. How could any Howard, however, attempts to standardize the classification, which by its nature suggests per- nomenclature for clouds had failed. The un- manence, capture a sense of endless mutability? derlying reason for this failure can be drawn Howard’s contribution made a leap forward by from a stanza taken from Percy Bysshe Shel- proposing that the myriad of cloud forms we see ley’s famous poem “The Cloud”: in the sky are characterized by a fixed yet small number of basic cloud types that evolve into hy- I am the daughter of Earth and Water, brid forms as they transform from one type to And the nursling of the Sky; another. In his essay, Howard notes: © 2003 Sigma Xi, The Scientific Research Society. Reproduction www.americanscientist.org with permission only. Contact [email protected]. 2003 September–October 443 Figure 2. Cloud systems display their structure well when viewed from space. At left, cumulonimbi have organized into supercell thunderstorms ob- served over Canada during the Space Shuttle mission STS064 in September 1994. The image shows the bubbling up of cumulonimbi and the spew- ing out of layers of high anvil clouds. The outflow of air associated with individual cumulonimbus or clusters of cumulonimbi often affect neighboring cumulonimbi, organizing them into a squall line. The photograph at right shows such a squall line observed over the Atlantic Ocean from the Shut- tle during mission STS51G in June 1985. These squall lines are often massive, extending thousands of kilometers. (Photographs courtesy of NASA.) There are three simple and distinct modifi- • Cirro-Stratus: “Horizontal or slightly inclined cations in any one of which the aggregate of masses attenuated towards a part or the whole minute drops called a cloud may be formed, of their circumference, bent downward, or un- increase to its greatest extent and finally de- dulated, separate or in groups consisting of crease and disappear … but the same ag- small clouds having these characteristics.” gregate which has been formed in one mod- • Cumulo-Stratus: “The cirro-stratus blended ification, upon a change in attendant with the cumulus, and either appearing inter- circumstances, may pass into another.… mixed with the heaps of the latter or super- adding a wide spread structure to its base.” The organizational model Howard adopted This classification scheme, although simple, for cloud classification was based on the system granted the naming of clouds a sense of free- introduced by Swedish botanist Carl von Linne. dom. Howard also made one notable observa- The Linnean system employs a binomial nomen- tion about clouds that has only recently been clature designated by a pair of Latin names; one used as an organizational principle in cloud re- defines the cloud genus, and the second indi- search. He realized that clouds could be cates cloud species. The names Howard chose thought of as “systems”—a concept brought for his three major types of clouds conveyed a clearly to view by technological advances that sense of their outward characteristics: deliver global images of clouds from space.