Rain Read online

  Global rain patterns generally follow those of the prevailing winds, such as the trade winds or the fast-flowing currents of air known as jet streams. Rain is most abundant where air rises, and skimpiest where it sinks. In the tropics, the trade winds converge and the heat pushes air upward, building the dark-castle clouds known as cumulonimbus, heavy with rain. So regions around the planet’s equator-middle are typically wet. As air moves away from the equator it cools and sinks, creating two dry bands around the globe at the subtropics. These are home to many of the great deserts, from North Africa’s Sahara to America’s Mojave. Meteorologists commonly describe the subtropical climates as a belt, not a particularly helpful image because they wrap around the planet in two belts, one above and one below the equator. I like to think of the subtropics as Mother Earth’s bikini.

  Here in the Pacific Northwest, the winds generally travel from west to east across the ocean, blowing strongest along a jet stream a few hundred miles across and a few miles deep. As they speed thousands of miles from Japan over just a few days, the winds scoop up the evaporating water and sun’s warmth from the sea. After this warm, wet air roars past the sea stacks and the pebbled beach, past the driftwood logs and coastal cliffs, it runs smack into the Olympic range.

  When air meets a mountain, it has nowhere to go but up. Remember rain’s MO: abundant where air rises, lightest where air sinks. So the world’s rainiest places are usually found on the windward side of mountains, looking to the sea. The most extreme rain in the United States occurs in the North Pacific on Hawaii’s emerald-green Mount Waialeale on the island of Kauai, soaked with 460 inches a year. Slightly higher than that, the worldwide rainfall records are set in the northeast corner of India in the state of Meghalaya overlooking the Bay of Bengal. In 1860, a village there called Cherrapunji logged the greatest rainfall in recorded history—1,042 inches in one year. Every rain-obsessed scientist I interviewed seemed to dream of visiting there.

  On the Pacific side of the Olympics, the moisture-laden air drizzles between 140 and 170 inches of rainfall a year onto the Hoh Rain Forest. As the spent clouds continue over to the east side of the range and descend into the lowlands, rainfall decreases as well—creating what’s known as a rain shadow. This is why cacti thrive in the town of Sequim, Washington, east of the range and less than thirty miles from white-crowned Mount Olympus. A fast-growing haven for Puget Sound retirees, Sequim sees only 15 inches of rainfall a year—typical of Southern California.

  The mountains likewise keep Seattle from having much rainfall. You read correctly. Notwithstanding the deluges and lightning that pound outside Frasier Crane’s apartment on the TV sitcom Frasier, Seattle is not one of America’s rainier or more lightning-struck cities.

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  Rather than flashy thunderstorms, Seattle is known for salmon-silver clouds that drizzle through winter, just the way I found the skies on the morning I drove to the University of Washington to interview the weather pasha of the Pacific Northwest, atmospheric sciences professor Cliff Mass. I hoped Mass could help me clear up some common misconceptions about rain. I live in Florida in the waterlogged South, the rainiest region in the United States. Seattle gets little rain in comparison. In the global swirl of the atmosphere, what makes the rain so different on the same continent? And why does Seattle, nicknamed “City of Rain,” have such a soggy reputation?

  A native New Yorker, Mass was drawn to meteorology as an undergraduate at Cornell, where he worked in Carl Sagan’s planetary studies lab. He published his first academic paper—a model of the Martian atmosphere—with Sagan. They found the circulation of weather on Mars “strikingly tied to topography.” So, too, is Seattle’s weather.

  Mass, who has a popular book, blog (cliffmass.​blogspot.​com/), and public radio program on the weather of the Pacific Northwest, spends a lot of his time debunking his adopted city’s rain refrain. To its west, Seattle is sheltered by the Olympics. To the east, the rugged Cascades stretch in a startling line of volcanoes from British Columbia all the way to California. As air crosses the coastal mountains and descends into the lowlands of western Washington and Oregon, annual rainfall decreases, from more than 100 inches a year to between 25 and 45 inches. This rain shadow covers an urban corridor from the Canadian border all the way to Eugene, Oregon—making Seattle and Portland drier than any major city on the U.S. Eastern Seaboard, with 20 inches less rainfall than Miami, 5 less than New York and Boston.

  Mass pulls up the National Weather Service’s precipitation map—rainfall pulsing in electric blue—to show me how rain moves across the continent. During winter, the jet stream sends decent rainfall to both sides of North America; the Pacific Northwest gets more than half its rain between November and February. But come summer, the Pacific jet stream weakens. The Pacific Northwest gets hardly any rain. As Melville’s sea stays cool, the Atlantic Ocean and the Gulf of Mexico grow warmer and warmer. The eastern half of the country endures lightning strikes, washed-out picnics, tropical storms, and hurricanes through summer, while Professor Mass hosts a “Dry Sky Party” in Seattle the last week of July. Almost always a drought week, it is also the answer to his most-frequent question from the public: “What date should we choose for our wedding?”

  Mass can put grooms and brides at ease about the rain, but makes no promises of azure skies. The sea still sends its clouds during summer. But, idling low between the mountain ranges, they don’t do much more than block the sun. The real reason for Seattle’s rainy character is the cloud cover. Mass explains that Seattle and Portland are blanketed by clouds about 230 days a year, compared with 160 in Boston and 120 in sunny Miami. The number of days with just a trace of rain is also considerably greater over the Northwest than back east.

  While Seattle experiences thunderstorms only about 7 days a year, the U.S. record-holder, Lakeland, Florida, watches thunderstorms roll in over its slate-colored lakes about 100 days. Lake Victoria in Uganda, Africa, is the world’s thunderstorm capital, rumbling an average of 242 days a year.

  Thunderstorms need convection—warm, moist air currents that rise quickly to form the dark-tower cumulonimbus. This is the same convection we know from cooking; the little eruptions caused by rising heat in a pot of, say, seafood gumbo simmering on the stove. The Gulf of Mexico cooks up the most memorable of both.

  The ten rainiest metropolitan areas in the United States are all in the Southeast, most doused by storms brewed in the warm waters of the Gulf. Its surface temperatures hover in the 80s. Tropical storm season, between June and November, spins off far more rain than Seattle sees all year. Gulf-front Mobile, Alabama, is drenched with more rain than any other metro in the nation, 65 inches a year. New Orleans and West Palm Beach come in second and third, both just over 62 inches, followed by Miami and Pensacola. As it moves up and out of the Southeast, this summertime air, heavy with moisture and warmed by the Gulf and Atlantic, continues across the eastern half of the continent, where it is further heated by the sun. This turns on the convection, roiling up thunderstorms across the eastern United States. The storms flash summer rains and lightning from the Atlantic Seaboard all the way to the Rockies. That’s where the action slows. Air meets the mountains and rises. The ascending clouds release their rain, this time on the east side—creating a rain shadow at the gateway to the arid West.

  To the west and south of the Rockies lie America’s driest places, which lose more water to evaporation and transpiration from plants than they receive from the skies. (Transpiration is the way moisture moves through plants, from the roots up to small pores on the underside of leaves, where it changes back to water vapor. An acre of corn can send 4,000 gallons of water back into the atmosphere each day.)

  Those suffering from ombrophobia—fear of rain—might consider settling in Yuma, Arizona. It seems unfair to label Yuma the driest city, straddling as it does a bucolic little bend of the Colorado River. But Yuma is the rain-scarcest city in the nation, averaging a three-inch trickle in a year. Not infrequently in the wide skies
over Yuma and other parts of the arid Southwest, residents watch sheets of rain begin to unfurl from auspicious purple storm clouds, backlit by the sun. But the rain stops halfway, hanging mid-horizon like a magician’s trick. Known as rain streamers or by their scientific name virga, the half-sheets evaporate into the dry air before the rain can reach the ground. “Torture by tantalizing, hope without fulfillment,” Edward Abbey called the withering curtains in Desert Solitaire.

  The world’s dry places have their seasons of rain, often called monsoons, from mausim, the Arabic word for season. Come summer in the Southwest, North American monsoon clouds dollop white peaks atop the low-lying desert mountains, signaling the rains to come. When the showers finally kiss the parched desert land, they release a perfume of sagebrush and creosote befitting the special occasion. We may be losing our regional dishes and dialects, but rain is always vernacular—preserving indigenous scents, language, and customs wherever it travels.

  Many of the East Coast transplants who move west for the desert sun don’t realize how much they will miss the rain that was so annoying when it washed out Fourth of July fireworks back home. In droughty recent years, rain’s arrival roused applause in the streets of Los Angeles, sent Albuquerque residents into front yards to celebrate, and lit up social media across the West with squall-gray selfies, rain hashtags, and umbrella exclamation points.

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  Of all the rains that soak Earth, none are more celebrated than those that blow in with the Asian monsoon. Scientists define the monsoon as a great wind; it’s essentially Earth’s biggest sea breeze. But to most of humanity, the monsoon is a miracle of seasonal rain. Almost two-thirds of the world’s people live by the rhythms of the spectacular inundations that bring water for drinking, farming, and life’s other fundamentals. The annual drama makes monsoon season something of a holiday for people across south and east Asia, who throw colorful festivals, sing monsoon ragas, go for monsoon cures, hook up for monsoon romances (the Eastern version of the summer romance), and dance barefoot in deep street puddles to rejoice in its arrival.

  The monsoons can also bring peril; floodwaters can kill hundreds, sometimes thousands of people in China, India, Nepal, and surrounding regions, displacing millions of others. But few disasters have been worse than the intermittent failure of the Asian monsoon, source of some of history’s most grievous famines. Even today, because the crops and water supply of entire nations, including India, depend upon its strength, failure of the monsoon can crash markets, spike food prices, provoke suicides, trigger energy shortages, and swing national elections.

  Monsoon meteorology boils down to temperature differences between land and sea. Like other rains, monsoons are driven primarily by the sun, constantly drawing up all that moisture from the oceans. As summer approaches, old Helios beats directly down on the countries bordering the Indian Ocean and the South China Sea, making the lands much warmer than the waters. As air heats over the land, it becomes lighter and rises. This causes cooler, wet air originating at sea to change direction and rush to the land. When these fast and fluid atmospheric freight trains meet the highest mountains in the world, the Himalayas, they dump load after load of rain. This is how the villages of India’s state of Meghalaya—the name means “abode of clouds”—balanced on the steep Khasi Hills overlooking the Bay of Bengal, are drenched with the greatest rainfall in the world. I pined to see those hillsides during monsoon season.

  Monsoons are influenced by such a dynamic mix of sea, land, and atmospheric conditions that computer models are not yet good at predicting a strong, weak, or failed season. Even rain’s most predictable patterns can be knocked off kilter by what scientists call teleconnections—faraway shifts in the atmosphere that reverberate halfway around the world. The best-known is El Niño, an unusually warm sea-surface temperature in the middle of the tropical Pacific Ocean. Every three to seven years, El Niño arrives to massively shake up rainfall on every continent. It can weaken the Asian monsoon, bring torrential rains to the western United States and scorching drought to Australia, and tamp down hurricanes in my part of the world.

  Modern living, with its reservoirs, irrigated agriculture, and invisible water pipes that run for millions of miles underground, dulls us to our fragile dependence on rain. El Niño—named for the Christ Child in the 1600s by South American fishermen who first observed the warming that tends to come around Christmastime—is one dramatic reminder. Human-caused climate change is proving another. Scientists say a warming globe will double extreme El Niño events, the sort that send California mansions sliding into the sea and set bushfires raging in the Australian outback. Climate change is also causing an alarming worldwide spike in torrential rains. If there is one point of solace, it is that we have lived through them before.

  Scientists and historians are careful not to claim that climate was the dominant force in developments such as agriculture and the rise and fall of civilizations. But in the past few decades, proxy records reconstructed from tree rings, tiny pollen grains from ancient swamps, and cores drilled from glaciers, lake beds, and the ocean floors have revealed profound connections among human progress, aridity, and rain. The anthropologist Brian Fagan calls climate “a powerful catalyst in human history, a pebble cast in a pond whose ripples triggered all manner of economic, political, and social changes.”

  In this case, the ripples fan out from a drop of rain. The same one ruining a seaside wedding today could have fallen millions of years ago on our primate Adams and Eves.

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  In the course of geological time, rain can fall heavily for hundreds or thousands of years. Scientists call such a wet period a pluvial, from the Latin word for rain, pluvia. Before humans split off from their primate cousins, our earliest ancestors lived in pluvial times. To travel through the storms of history and imagine prehumans and their lives in the rain forests of East Africa, take a look at your fingers after a long bath or an afternoon spent swimming.

  We’ve all seen our fingers and toes shrivel up like alien extremities after a good soak. Popular belief long held that this prune effect is caused by osmosis—our digits must absorb water and swell up, rippling the skin into little folds. But a neurobiologist named Mark Changizi had another idea.

  In 2008, Changizi was researching the shape of human hands when he came across a surgical paper from the 1930s documenting that patients with damage to their arm nerves don’t get wet wrinkles on their fingers. It turns out that doctors who work with nerve-damaged patients already knew pruniness couldn’t be an accidental side effect of wetness. Instead, our water wrinkles are the work of the autonomic nervous system—a largely involuntary control panel in the lower brainstem that also prompts basics such as breathing, swallowing, and sexual arousal.

  Changizi put aside his question on the shape of hands for another within grasp. Other primates, such as macaques, get the finger and toe wrinkles, too. An automatic physical trigger to water suggested to Changizi that they must be some sort of adaptation. Primates would not have evolved to deal with porcelain bathtubs or Olympic-sized swimming pools. But they would have had to adapt to rain.

  Changizi had a new grad student, Romann Weber, working in his Boise, Idaho, lab and asked him: “Can you think of a good reason to get pruny in the rain?” Weber gave it some thought. “Rain treads?”

  In dry conditions, smooth tires like those on race cars provide the best grip. On wet roads, though, rain treads handle much better. Changizi and Weber hypothesized that smooth fingertips likewise give humans the best grip in dry times, but wrinkly ones might help us hang on when it’s wet. Their subsequent research indicates that’s true and, moreover, that human evolution appears to have gone a step further to best the tire.

  Magnified, our finger wrinkles look like drainage channels: rain chiseling a landscape for thousands of years. They are not perfectly circular, but flow downward like a river branching into an estuary. The channels move farther apart as they flow down, like mountain topography shaped by streams
flowing toward the sea. The channels are also pliable—so pressing your finger on a surface squeezes water out through them.

  The wrinkles appear nowhere else on our bodies but the hands and feet. They form quickly enough to gear up for rainy conditions, but not so fast that casual contact with water, such as eating fruit, shrivels us up. All in all, Changizi believes, these are clues to an ancient adaptation for gripping in the rainy forests where human ancestors lived some 10 million years ago.

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  On the family tree of human evolution, the branch between the fruit-noshing primates of East Africa and the chai-drinking moderns of today is represented by the hominids. Since Charles Darwin’s time, science textbooks have attributed their emergence to the “savanna hypothesis” of evolution: For millions of years, our primate ancestors stuck around their Eden-like forests. Why would they need to stand up and stretch out when plenty of fruits, berries, and other tasty nourishment hung so easily within reach? It was not until the rains tapered off, and the forests with them, that our primal foremothers and fathers went on the move. Their rain forest home gradually gave way to wide, open savannas. A new sort of primate began to walk on hind legs, and developed a long field of vision to look out over the grasslands.

  New fossil discoveries and advances in DNA research are proving our evolution from tree to tea was a lot more complicated. Emerging paleoclimate and fossil evidence still supports the theory that hominids developed their ability to walk on two legs as the region’s rains tapered and forests transitioned to savanna grasslands between 5 and 7 million years ago. But anthropologists now see that human evolution was not just survival of the fittest, but survival of the most adaptable to changing climate. The major leaps in our evolutionary history line up with dramatic changes in prehistoric climate, namely temperature and rainfall.