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Of Heat Engines . . .

“The rain was torrential,” Herbie Riehl would later recall, as the U.S. Navy reconnaissance plane penetrated a tropical depression to the south of Guam on August 29, 1947. In the Northwest Pacific ocean basin, which produces the most intense tropical cyclones on Earth, this storms winds of 30 to 35 miles per hour were hardly remarkable. But in the context of Riehl’s current research they didn’t have to be. The thirty-two-year-old hurricane scientist—a German expatriate who liked to smoke cigars and play bridge and climb mountains, and who was sometimes a bit hard to get along with—had traveled to Guam along with several other University of Chicago meteorologists seeking to unravel the process by which typhoons form. This weak depression would teach Riehl a great deal about just how knotty and complex that problem truly is.

In the opinion of the naval aerologist on board the plane, who’d flown into many past Pacific storms, the rainfall in the depression seemed just as intense as what’s found in a full-blown typhoon. So Riehl expected such a storm to spin up shortly. But nothing happened, and the depression slowly meandered westward, unchanged, all the way to the China Sea.

Riehl had witnessed an enigma that confronts every hurricane forecaster. Not every storm with the apparent potential to do so grows into a hurricane or typhoon, and not even the best forecasters can always predict which will and which won’t. To Riehl, the experience of flying into this weak but very rainy depression had an important implication: “Condensation energy alone cannot create intense tropical storms,” he wrote. Instead, the release of latent heat, leading to clouds and rainfall, constituted a “necessary, but not sufficient, condition for both inception and growth” of tropical cyclones. It was just one of many important insights to arise from Riehl’s eagerness to fly onboard midcentury military planes and survey hurricanes from the inside.

“I desired very much to do something,” Riehl would later comment of his storm-flying ventures, “which produced a bit of life in an otherwise perhaps rather dreary research environment.” He still felt the same way in 1960, at age forty-five, and was incensed when the navy made him get a physical before allowing him to go on his final flight, admonishing him that “at your age you’re supposed to sit behind a desk and not up in front on airplanes.”

Riehl epitomized a group of scientists who, for reasons having as much to do with the national interest as basic curiosity, began to unravel the mysteries of the tropics. Born in Munich in 1915 to a Jewish mother and a German father, he’d emigrated to the United States in 1933 as Hitler rose to power. Not long after receiving his master’s degree in meteorology from New York University in 1942, Riehl joined his new country’s war effort against his old one. In 1943, the U.S. Army Air Corps (later the Air Force) teamed up with the University of Chicago to establish an Institute of Tropical Meteorology at the University of Puerto Rico, and Riehl moved down as part of the team. He became head of the institute a few years later, commuting back and forth between the Midwest and the Caribbean even though he hadn’t officially earned his Ph.D. yet.

The U.S. military’s sudden interest in tropical meteorology reflected a strategic necessity. There was a war on, much of it set in the Pacific theater, where new meteorological risks confronted U.S. military forces. The point became painfully apparent in late 1944, when three U.S. destroyers under the command of Admiral William F. “Bull” Halsey were sunk by a powerful December typhoon in the Pacific ocean, and 790 of the Third Fleet’s sailors perished beneath the waves.

And so World War II and its aftermath gave meteorology a tremendous boost in the United States. At a time when aviation was far more vulnerable to weather conditions than today, the war effort needed forecasts, and obligingly, the University of Chicago and four other American universities (MIT, UCLA, Caltech, and New York University) trained an entire generation of weather experts. Many of these scientists then traveled to the tropics and found themselves in an entirely new world. Inspired by the discoveries of the Bergen School, early twentieth-century meteorologists had focused most of their energies on understanding the cyclonic storms that occur over the middle and higher latitudes. In the Pacific, however, a large number of relatively unschooled forecasters confronted a very different problem—predicting cyclones in a place where the Norwegian theory of “fronts” didn’t apply. They had catching up to do, and they had to do it quickly.

Riehl led the way, both through his research in the U.S. tropical territories of Puerto Rico and Guam and through the University of Chicago’s training program for military meteorologists. He served as mentor to a large number of newly minted tropical scientists, many of whom took Riehl’s courses at the University of Chicago and, once it had been written, studied from his foundational 1954 textbook, Tropical Meteorology. In the process, Riehl helped establish two key traditions in hurricane science: storm-flying as a means of getting data, and empiricism as a broad means of learning about tropical weather. Both would be carried forward by William Gray, one of Riehl’s most distinguished students (and surely the most famous of them).

Riehl was more than a mentor, however. Working closely with his onetime student Joanne Malkus (later Joanne Simpson), he helped to develop the basic thermodynamic understanding of hurricanes that’s accepted today. And not unlike Redfield or Loomis, Riehl built up this edifice from a vast reservoir of scientific data. In the words of Richard Anthes, a hurricane specialist who is president of the University Corporation for Atmospheric Research in Boulder, Colorado, in the 1950s and early 1960s Riehl and Malkus led an “observational assault on the mysteries of the tropics.”

That’s not to say that Riehl couldn’t transform an equation; he certainly could. But he was first and foremost an observational scientist. When Science magazine glowingly reviewed Tropical Meteorology, the writer observed: “The over-all treatment is empirical or synoptic rather than dynamic; the mathematics is simple and unobtrusive.” The foundation for Riehl’s advances centrally lay in the unprecedented compilation of hurricane data that the war and postwar era made possible.

 

One data source was the balloon-borne “radiosonde,” which allowed for vastly improved measurements of the upper atmosphere. As they float up into the air, radiosondes provide readings of temperature, pressure, and relative humidity at different elevations. Later, Riehl also drew upon measurements from “rawinsondes,” a special type of radiosonde that can be tracked using radio or radar, thus allowing for measurements of the direction and strength of upper-level winds. Such devices gave scientists a three-dimensional glimpse of the structure of the atmosphere at any given place or point in time. World War II spurred the establishment of radiosonde and rawinsonde networks, and scientists like Riehl first made sense of the new data.

Cobbling together radiosonde measurements taken over San Juan, Swan Island in the western Caribbean, and other locations in the tropics, Riehl took a major step forward in our understanding of hurricane formation. In 1945 he published a pioneering report on the meteorological perturbations now known as African easterly waves—or as Riehl then called them, “waves in the easterlies.” These aren’t the sort of waves that surfers catch; they undulate in the fluid of the atmosphere rather than the fluid of the oceans. Preceded by a drop in sea-level air pressure and often followed by intense thunderstorm clusters, the waves originate over sub-Saharan Africa and ripple through the tropical atmosphere from east to west, passing by once or twice per week at 10 to 15 miles per hour during the summer season in the Atlantic. Sometimes, for reasons that remain poorly understood, a wave can spark the development of a closed air circulation—the first step in the growth of a hurricane.

Riehl’s radiosonde studies of tropical waves pushed him toward another important insight. Hurricanes don’t just spring into existence spontaneously. Rather, these most destructive of storms always develop “within preexisting disturbances,” such as easterly waves. It was the earliest of many data-induced revelations that would emerge from Riehl’s research career.

In addition to radiosondes, Riehl and other mid-century hurricane scientists drew upon a glut of other newly available observational tools. Radar, a wartime technology quickly converted to civilian use, greatly expanded knowledge of hurricanes. The first radar images of these storms—showing their unmistakable spiraling outer rain bands—were taken in the 1940s, and by the 1950s, the groundwork had been laid for establishing a national weather radar network in the United States. Then in 1960, the first weather satellite was launched into space, so that before long scientists peering down into hurricane eyes as glimpsed from above could see the storms peering right back at them. By the 1970s, satellite imagery, combined with cloud-pattern recognition techniques, meant even hurricanes never penetrated by aircraft or seen on radar could be detected and described. No longer would a devastating storm go entirely unnoticed until landfall, or be observed only by ships that had the misfortune of coming near it.

But perhaps the most significant of the new postwar data-gathering technologies, and one of the techniques most exploited by Riehl, was the use of instrument-equipped aircraft that could take scientists straight into the heart of hurricanes. No other data-gathering technique could rival these storm flights. They allowed for targeted forays into specific regions, at specific elevations, and at different points in a storm’s life cycle. And they provided a life-transforming experience to boot.

The first deliberate hurricane flight took place during the war, in 1943. Despite occasional crashes in the early years, they quickly became common. While it would be suicide to maneuver a ship into such a storm, flying into them isn’t particularly dangerous so long as you stay away from the sea surface, know how to navigate using instruments rather than the naked eye, and have a strong stomach.

Originally hurricane flights were conducted by the military and used strictly for forecasting, but soon scientists started hitching rides on board and occasionally even got their own research expeditions. Robert Simpson, a colleague of Riehl’s who would later direct the National Hurricane Research Lab and the National Hurricane Center (the Saffir-Simpson scale is named after him), was among the first to exploit this opportunity. One notable mission, involving several 1947 flights into the Great Atlantic Hurricane, allowed Simpson to observe the anticyclonic (or clockwise) rotation of the storm’s upper outflow levels, where spiraling winds reversed direction. Meanwhile, following a 1951 flight into Typhoon Marge in the Pacific—the most intense storm ever penetrated at the time, with a central pressure of 895 millibars^* or 26.42 inches as measured from the aircraft (the average at sea level is about 1,013 millibars or 29.92 inches)—Simpson wrote of his experience in the eye of the storm:

 

Around us was an awesome display. Marge’s eye was a clear space 40 miles in diameter surrounded by a coliseum of clouds whose walls on one side rose vertically and on the other were banked like galleries in a great opera house. The upper rim, about 35,000 feet high, was rounded off smoothly against a background of blue sky. Below us was a floor of smooth clouds rising to a dome 8,000 feet above sea level in the center. There were breaks in it which gave us glimpses of the surface of the ocean. In the vortex around the eye the sea was a scene of unimaginably violent, churning water.

 

Soon even journalists started flying on hurricane-hunting missions. In 1954, legendary newsman Edward R. Murrow rode on board a B-29 out of Bermuda as it set out to penetrate Hurricane Edna, later broadcasting stunning footage from inside the storm. As Murrow put it: “In the eye of a hurricane, you learn things other than of a scientific nature. You feel the puniness of man and his works. If a true definition of humility is ever written, it might well be written in the eye of a hurricane.”

As these passages show, the perception of hurricanes, by scientists as well as the public, would be forever altered by the opportunity to safely experience them from the inside. Suddenly being an observa tionally inclined hurricane researcher like Riehl or Simpson meant regularly going on adventurous, quasi-military missions. Like North Pole or Arctic exploration, hurricane research became romantic, dangerous, and heroic, and those who flew into storms justifiably came to view themselves as possessed of special insights by virtue of their unique experiences. The research also became well funded: In August 1956, specialized flights began after elected representatives created the National Hurricane Research Project (NHRP). As is often the case in hurricane science, the dollars followed the destruction: In 1954, hurricanes Carol, Edna, and Hazel had pummeled the U.S.’s eastern coastline in quick succession.

Much like the 1821 storm that Redfield studied, Hurricane Carol struck Long Island with 100-mile-per-hour winds on August 31 and then swept through. New England. Hurricane Edna followed eleven days later, cutting across Cape Cod and making landfall along the eastern coast of Maine in the midst of transitioning into an extra-tropical cyclone, as northward traveling hurricanes sometimes do.^* Both storms also brushed the Outer Banks of North Carolina. But on October 15 the year’s most devastating storm, Hazel—which had already killed hundreds in Haiti through landslides and flooding—made a full landfall at the South Carolina-North Carolina border at Category 4 strength. Along the Carolinas the storm surge reached 18 feet; “every pier in a distance of 170 miles of coastline was demolished and whole lines of beach homes literally disappeared,” noted a report from the time. Still considered the worst hurricane in North Carolina history, Hazel blew north over land so quickly that it delivered hurricane-force winds to Washington, D.C., and, after undergoing another extra-tropical transition, killed 81 people in Toronto from massive flooding. No wonder Hazel helped kick off a new era in hurricane research. It didn’t hurt that North Carolina saw three more destructive hurricane landfalls (Connie, Diane, and lone) in 1955.

As the dean of tropical meteorology, Riehl was angered not to be named director of the newly launched National Hurricane Research Project. Whatever his scientific strengths, apparently he wasn’t a good enough politician. In fact, Riehl was almost as well known for his abrasive personality as for his scientific influence. Offered a variety of more subordinate roles with the NHRP, he rejected all of them and refused to participate—a stance that, had it continued, would have been a severe blow to the project. Later, however, Riehl came around and flew on a large number of NHRP hurricane flights as they traced their standard “cloverleaf” paths through any Atlantic storm within range, allowing Riehl and his fellow scientists to gather unprecedented observations.

Riehl also passed the storm-flying experience on to his students. In 1958, he took a young William Gray down to the NHRP’s headquarters in West Palm Beach, Florida, and got him on board two B-50 flights into Hurricane Helene, a Category 4 storm at its peak. Helene looked as if it might threaten Charleston, South Carolina, and later skirted the North Carolina coast, but ultimately its path recurved and it remained at sea. Flying into the storm, the scientists bent the rules a little. “Riehl talked the pilot into staying down at 1,500 feet when we had 120 knot winds”—almost 140 miles per hour—Gray remembers. “You were supposed to go up to 5,000 if you had winds above hurricane force.” If a hurricane’s strongest winds could be found at the lower altitude, so could great data. Not surprisingly, the experience piqued Gray’s interest in hurricanes.

“He used to ask me as a graduate student, what makes a hurricane form?” Gray recalls of Riehl.

 

Storm flights allowed scientists like Riehl to do with tropical cyclones what Loomis had done with mid-latitude storms: Take a wide range of observations of the same storm and analyze the picture that emerged. In general, Riehl’s mode of science relied heavily upon data-gathering, though it didn’t end there. “He wasn’t entirely an observationalist, I wasn’t entirely an observationalist,” remembers Riehl’s frequent co-author Joanne Malkus. Rather, data-gathering gradually merged with theory.

First, either Riehl or another storm-flying scientist—not Malkus, because at first women weren’t allowed on the research flights—would take a range of measurements inside hurricanes. Then came attempts to create storm models that could explain at least some of the observations successfully. The messiest part was gathering the data: Not all storms were equally approachable by aircraft; not all storm regions could be equally accessed (the “boundary layer” between air and sea was pretty strictly off-limits, for obvious reasons); and instrument failures sometimes wrecked the best-planned missions. But over time, the approach led to considerable progress.

Perhaps Riehl’s most significant contribution to hurricane science lay in his discovery of the thermodynamic nature of these storms. Once again, this “theory” grew out of his observational approach. It has also stood the test of time and, despite having been largely eclipsed for several decades, prevails today.

In the late 1940s and early 1950s, Riehl and a small group of other scientists identified the key characteristic of hurricanes that would subsequently inspire concerns that global warming might intensify them—namely, that a hurricane’s central energy source lay in the evaporation of warm seawater, a process that releases heat from the ocean up into the air. This discovery was related to, and yet represented a great advance over, the previous thermal theory of cyclones espoused by Espy, Ferrel, and their followers. That theory credited latent heat with the central role in driving a cyclone, whether extra-tropical or otherwise. Riehl and his scientific compatriots also understood the importance of the release of latent heat from condensation in rising currents of moist air. But they recognized that for hurricanes, this meant little except in the context of a deep reservoir of energy delivered by the sun to the tropics and stored in tropical oceans.

In a sense, it all went back to Riehl’s 1947 experience inside the very rainy Pacific depression that never intensified into a typhoon, which had cautioned him not to ascribe everything important about hurricanes to condensation. It’s trivially true that clouds require moisture to form. But the towering cumulonimbus clouds and powerful winds of hurricanes could not exist without an ocean heat source, and that’s what makes them unique—and so dangerous.

The emphasis on ocean heat began with a seminal 1948 paper, in which the Finnish scientist Erik Palmén—who spent time at the University of Chicago—compiled data suggesting that hurricanes formed only in regions where temperatures at the sea surface exceeded 26 or 27 degrees Celsius (about 80 degrees Fahrenheit). In other words, these storms form in the warmest parts of the ocean during the warmest seasons of the year. Thanks to Palmén’s paper, scientists now knew of the importance of easterly waves and had identified two further conditions for hurricane formation in a particular region. First, due to the Coriolis force, the area had to be north or south of (but not too close to) the equator. Second, the region apparently had to have sea-surface temperatures that exceeded the threshold of 26 or 27 degrees Celsius during at least part of the year (although hurricanes can move out of such regions once formed).

Before long, Riehl and a scientist still working in Germany, Ernst Kleinschmidt, expanded upon Palmén’s emphasis on ocean heat. In a 1950 paper drawing on a glut of upper-air observations from the previous decade, Riehl described the hurricane as a “heat engine”: a thermodynamic system that converts heat energy into mechanical energy (the ability to do work or, in the case of hurricanes, lift air and drive winds) by processing it from an area of high heat to one of lower heat. For hurricanes the central heat reservoir is the warm tropical ocean, and its energy gets transferred up into the atmosphere through the processes of evaporation and condensation, which first store heat in the air and then release it. Finally, energy not used to power the storm gets released in the hurricane’s outflow region, high in the freezing upper troposphere (or lower stratosphere), as a kind of exhaust stream. “Atmospheric machines do not differ in principle from the machines known in physics and engineering,” Riehl wrote in Tropical Meteorology. Later he continued with the same metaphor: “If the machinery is faulty, a hurricane will no more form than a man-made machine will run.”

Thanks to Riehl’s heat-engine theory, we can now give a much fuller picture of how hurricanes work, merging the important but in complete discoveries of men like Redfield, Espy, and Ferrel with knowledge gained during and after World War II. It’s important to understand the basic mechanisms driving these storms to see why global climate change could intensify or otherwise change them. By making this intuition more plausible, Riehl’s work provided an early eddy that would swirl into the modern hurricane-global warming debate.

As air flows into a hurricane—moving toward a central low pressure region and spiraling due to the Earth’s rotation—it draws up water vapor from the evaporating sea surface. This vapor contains energy in the form of latent heat. The air also takes up warmth from the ocean, a factor that turns out to be very important. As the air spirals inward near the surface, it experiences a pressure drop, which (according to very basic physics) should cause it to expand and its temperature to decrease. Yet “such decreases never occur,” wrote Riehl in 1950. Instead, “rapid transfer of heat from ocean to atmosphere” helps to keep the air warm. This means the inflowing air cannot disrupt the hurricane by cooling it off. The air is, however, slowed down by friction as it drags along the sea surface—a central constraint upon the intensity that hurricanes can achieve.

Before long, the warm, humid, and buoyant air converges near the storm center, where it begins to rise rapidly upward. (Rising also occurs in the hurricane rainbands, threads of cloud organized in spiral arrays farther from the storm center.) The ring-shaped eye wall of the hurricane is, in essence, an air elevator, sloping slightly outward with height, whose lifts are thick towers of cumulonimbus clouds. The air never reaches the storm center because it is rotating, thanks to the Coriolis force. That’s why hurricanes can have calm eyes, where air is slowly sinking, surrounded by eye walls in which air is rushing upward, winds are howling, and torrents of rain fall almost horizontally—a phenomenon that would surely have mystified Espy, with his centripetal theory of winds.

As the air rises in the hurricane eye wall, it cools, and eventually the water vapor it contains condenses into clouds or rain. As this occurs, the once-latent heat gets released back into the atmosphere as sensible heat, warming the air further. Eventually, the air will be expelled at the top of the hurricane, miles above the Earths surface, in anticyclonic outflow jets that spiral clockwise in the Northern Hemisphere. Here in this exhaust region, the ejected air joins cooler upper-atmospheric winds and travels many miles away, radiating some of its remaining heat to space.

 

Image of a hurricane’s heat engine in cross section. Image credit: NOAA.

 

At any time, however, more air is being pulled into the hurricane at lower levels, and with it more heat. And in an intensifying hurricane, air gets pulled in, and rotates, faster and faster. That’s why scientists measure hurricane strength by estimating the storm’s maximum sustained wind speed and minimum sea level pressure (hurricanes create the most dramatic pressure falls ever recorded at the Earth’s surface). As pressure decreases due to rising heated air, winds spiral inward more quickly. The winds also blow harder as the radius of spiral rotation shrinks toward the center of the storm due to a law of physics known as the “conservation of angular momentum”—the same reason that figure skaters twirl faster when they tuck in their arms.

The storm’s powerful winds, in turn, dramatically roil the ocean, generating gigantic waves and then lopping off their tops, sending sheets of sea spray through the air. Through processes not yet completely understood (and difficult to study), sea-spray effects at high wind speeds apparently create more evaporation, leading to still more heat being carried into the center of the storm. The state of the ocean in a full-fledged hurricane—the final sight of many an unfortunate sailor—must be terrifying to behold. As Riehl put it in his textbook: “It is hard to say where the ocean ends and where the atmosphere begins!”

If not disturbed by other environmental factors, a hurricane will continue to strengthen as long as its central energy source, the warm ocean, can sustain it. It’s simple thermodynamics. If the storm turns over land, it will quickly weaken, having lost its power source. If it veers over a patch of cooler water—perhaps stirred up from the ocean depths by a previous hurricane—it will also weaken. Finally, hurricanes weaken themselves by mixing the ocean and drawing up cool water from below. Should a hurricane plow over a deep layer of warm water, however, it can rapidly intensify to Category 4 or 5 strength. One of the most dramatic cases was 2005’s Hurricane Wilma, which had the lowest central pressure—882 millibars, or 26.05 inches—of any known Atlantic hurricane. Wilma strengthened from a mere tropical storm into a Category 5 monster in just twenty-four hours before slamming the Yucatán and, later, southern Florida.

As we’ll see, many environmental factors besides ocean heat also influence hurricane strength and regions of formation. But by the 1950s, the essential linkage between hurricanes and heat—specifically ocean temperatures—had been clearly established. Riehl could describe these storms as being “sensitive to slight temperature and moisture variations.” He could even suggest—and did—that given reliable measurements of tropical sea surface temperatures, it ought to be possible to predict seasonal hurricane activity in advance. All of this strongly foreshadows not only future forecasting schemes, but the present-day global warming-hurricane debate.

More generally, thanks to the work of Riehl and his colleagues, it became possible to say, for the first time, that scientists on some level actually understood how hurricanes amass their deadly power. The “observational assault on the mysteries of the tropics” had borne fruit—and Riehl’s student, William Gray, would carry it still further.