The standard unit for Beaufort’s scale is knots, derived from an earlier system of calculating speed at sea by throwing a “chip log” overboard with a line attached to it. The line was knotted at uniform, measured distances and was played out for a fixed amount of time measured by a small hourglass. The number of knots that played out during that interval calculated the speed of the boat. Each knot is equal in speed to one nautical mile per hour, or 1.15 miles per hour.
Beaufort’s knots measured wind speed in a series from 0 (dead calm) to 12 (hurricane force winds). A Beaufort number of 2 is a light breeze or a vessel underway at one to two knots, while a moderate breeze with a Beaufort number of 4 is equivalent to five to six knots. In the original Beaufort scale, anything over 4 was not measured by knots but by the effect of the wind on sails. A “moderate gale” (Beaufort 6) required double-reefed topsails and jibs, whereas a “strong gale” (Beaufort 9) required close-reefed topsails and courses.
Today’s Beaufort scale has undergone a few modifications. In the early twentieth century, Antarctic explorers encountered such extraordinarily fierce winds in the blizzards howling around the South Pole that they added six more calibrations, bringing the scale up to 18. Those must have been some desperate measurements, because 12 on the original scale was at least 75 miles per hour, the strength of a category 1 hurricane. In Antarctica, the highest wind speed recorded was 200 miles per hour.
The Jet Stream
The Second World War was the first conflict in which meteorologists played a strategic role. So much so that, in England, in a measure that inconvenienced many citizens, public weather broadcasts were halted for the duration of the conflict. At the same time that meteorology was becoming a hard science, the war was becoming a crucible for forecasting skills, and some of the best and brightest young meteorologists were enlisted by the U.S. military to provide accurate forecasts for battles in the Pacific arena.
One of them was Reid Bryson, a geologist and meteorologist stationed in Guam. Unlike many of his peers, Bryson was familiar with the work of Heinrich Seilkopf, the German meteorologist who, in 1939, claimed that he had discovered high-altitude winds, which he called “jet currents.” Bryson was quick to adopt and use Seilkopf’s theories. Bryson predicted that bombers flying toward Japan might encounter high-altitude westerly headwinds that would slow their progress. Anecdotal reports from pilots crossing the Atlantic had already mentioned high-altitude westerly tailwinds that boosted their ground speed by up to 100 miles per hour.
Bryson was right. A secret U.S. Air Force mission deployed to strike Honshu, an industrial target near Tokyo, flew into a jet stream in late November 1944. One hundred and eleven B-29 bombers with specially trained crews on their way to make the first high-altitude bombing run of the Second World War ran into disastrous winds. Everything went smoothly until, approaching their target, they turned and began their west-to-east run above Honshu at 33,000 feet. As they began to release their bombs, the whole fleet lurched forward as if pushed by a giant hand. They had flown into the path of a 402-mile-per-hour gale and most of their bombs missed the target and fell harmlessly into the sea.
Later, other high-altitude bombers on their way to targets in Japan ran into similar mysterious headwinds so strong that one of the pilots recounted how “islands that they should have passed long ago remained stationary below them as though the whole scene had frozen.” Some missions were forced to turn back before they ran out of fuel.
These aviation hazards had to be understood and mapped out. Over the next few years, meteorologists discovered that there were four tubes (wiggly planetary hoops, actually) of high-speed wind snaking along the top of the global tropopause like sidewinders in the sand — two in the northern hemisphere and two in the southern. Similar to rivers, their velocity was slower at the edges and highest in the core, where winds reached 310 miles per hour.
If wind is air on its way elsewhere, then the jet stream is a high-level express lane for the westerlies. Like serpentine free-floating wind tunnels, they separate the three major convection cells of the northern and southern hemispheres: the polar cell, the Ferrel cell and the Hadley cell.
An atmospheric cell is a giant toroidal vortex, like a toeless sock being perpetually turned inside out or, better still, like a smoke ring. If you could somehow get a smoke ring to hover horizontally and then very carefully inserted a small globe up into the rolling ring, you would have a small-scale version of the Hadley cell. (Sure, a cross section of the smoke ring would be circular, while a cross section of any of the atmospheric cells would be more elliptical, but the analogy is pretty close.)
The Hadley cell is formed by air rising straight up from the equator, moving north in the upper atmosphere and sinking at about 30°N latitude, the same approximate latitude as Morocco, Georgia, Cairo and Shanghai. (This sinking air is dry; it’s the reason that most of the great deserts — Sahara, Sonora, Gobi — are also smack-dab on 30°N latitude.) Once it hits the Earth’s surface, the air moves south toward the equator, completing the Hadley circle. The northeast trade winds are a result of that southward-moving air being deflected by the Coriolis effect.
The Ferrel cell is a circular conveyer of air exactly like the Hadley cell except it rolls in the opposite direction. The Ferrel rises from about 60°N — think Stockholm, Whitehorse and the Shetland Islands — moves south in the upper atmosphere and sinks at 30°N where the Hadley cell is also descending. The last of the three cells, the polar cell, rolls in the opposite direction to the Ferrel (the same direction as the Hadley), rising up from 60°N to move north and sinking at the North Pole. From there, it spreads out and moves south in the lower atmosphere. The Hadley, Ferrel and polar cells are mirrored in the southern hemisphere with the same names.
In a sense, these bands of rolling air are similar to a milkshake in a blender: the fluid rises up the side of the blender, then moves across the top to the center where it sinks down into the blades in a single vortex. This metaphor works perfectly for the polar cell but not quite as well for the other cells, which are bands. Each of their vortices becomes an elongated groove running the length of the Hadley/Ferrel milkshake — in this case, right around the world. Wherever that groove occurs, whether between the polar cell and the Ferrel cell or between the Ferrel cell and the Hadley cell, a jet stream sits directly on that border. That’s why there are two of them in each hemisphere.
Now it gets a little more complicated, so we need to introduce another scientist: Carl-Gustaf Rossby. Born in Sweden in 1898, Rossby moved in 1925 to the U.S. to teach in the aeronautics department of MIT. Fifteen years later, at the beginning of the Second World War, he was appointed chair of the meteorology department at the University of Chicago (where Fujita later became a faculty member). Like the great American meteorologist Cleveland Abbe before him, Rossby was a fluid dynamics guy; he applied the same mathematical principles that described fluid dynamics to the atmosphere. Just before the war, in the same year that Heinrich Seilkopf discovered the jet stream, Rossby had made a momentous observation, one that forever changed forecasting and meteorology. He had discovered his eponymous waves.
The figure/ground relationship of jet stream and Rossby waves are central to modern forecasting. To explain what Rossby waves are, we have to return to the Coriolis effect: the Earth’s rotation causes storms to move clockwise in the southern hemisphere and counterclockwise in the northern hemisphere, an effect that gets stronger as you approach the poles. The closer to the pole something is, the more aligned it is with the axis of rotation (the invisible rod through the center of the Earth that connects the North and South Poles). The stronger the Coriolis effect, the stronger the destabilizing shear boundary between the polar cell and the Ferrel cell becomes, introducing points and bays like a shoreline. These points and bays are the Rossby waves. They move slowly, sometimes in a westerly direction and sometimes in an easterly direction. The jet stream follows their contours, which is why they are so impo
rtant to forecasting weather.
Many a meteorological student has wrestled with the complex math of Rossby waves, not to mention the labyrinthine mathematics of fluid dynamics. But forecasting the weather doesn’t need to be that complicated all the time. It can be as simple as standing out in your backyard or the street on a windy day.
Forecasting by Wind
The direction of the wind tells us a lot about what will happen to the weather. It has to do with an insight of a Dutch physicist in 1857. Christophorus Henricus Diedericus Buys Ballot, known to posterity as Buys Ballot, taught at the University of Utrecht in Holland. In 1845, he famously confirmed the Doppler effect (the change in pitch created when a moving source of sound, say an ambulance siren, approaches or moves away from the listener) with a group of musicians playing in an open train car. His interests turned to meteorology shortly thereafter. The first weather maps showing barometric pressure gradients, or isobars (regions of equal pressure), were just beginning to be published, and Buys Ballot was fascinated. What caught his attention was not the wiggly bull’s-eyes of high- and low-pressure areas, but the wind-direction arrows between the isobar lines. He noticed a pattern.
The wind always blew along these isobars, not across them, and if you factored in the rotation of high- and low-pressure systems, Buys Ballot realized you could predict the weather by standing with your back to the wind. So in the northern hemisphere, the area of low pressure would always be to your left and the high pressure to your right. (The reverse would be true in the southern hemisphere.) Given the fact that most weather in the mid-latitudes moves west to east, it becomes a simple matter to make a rudimentary, fairly accurate forecast.
For instance, if the wind is from the north, you’re between a low-pressure area rotating counterclockwise to your left and a high-pressure region rotating clockwise to your right. As the high-pressure area moves over you, the weather will most likely change from rainy to clear and cooler. Buys Ballot intended his shorthand forecasting law to be a means to save lives at sea, which it was. Sailors were able to apply his general rule and avoid storms and hurricanes well in advance. For long-range forecasting, however, sailors had to wait another hundred years.
8
Which Way the Wind Blows
The Story of Weather Forecasting
“The Book of Nature is written in the language of mathematics.”
Galileo Galilei
Our inability to see into the future has frustrated humans for thousands of years. Along with our evolutionary leap into self-consciousness came the realization that we were enslaved by time’s arrow, that we’re blindly groping toward the future unaware of the imminent catastrophes that might ambush us there. This is indeed a limit to our assumed omniscience. You might call it a state of temporal claustrophobia, and once it was an anxiety that could only be remedied by prophets and soothsayers. Every ancient civilization honored citizens who claimed they could predict the shape of things to come and it was on these few seers we laid our hopes.
The early Romans turned to augers for advice. The appearance and flight of birds were central to these prognostications. An eagle that landed on Augustus’s imperial tent while he was on campaign foretold his victory at Actium. Far north of Rome, in Scandinavia, legend says that Odin sent out his two ravens, Huginn (Old Norse for “thought”) and Muninn (Old Norse for “mind”), into the world every day to bring back knowledge. Odin was obsessed with soothsaying, and he ultimately sacrificed an eye to the Norns for the ability to see into the future. Actually, Odin got a deal. In Greek mythology, poor Tireseas lost sight in both eyes for the gift of prophecy, and in his case the trade-off was not voluntary. Athena blinded him but then took pity and granted him the ability to understand bird songs and their divinations. Even today, palm readers do a brisk trade and futurist speakers are hot tickets at investment seminars. But none of them hold a candle to meteorologists for accuracy, at least in the short term.
The seventeenth century was a golden age of prognostication. Once Galileo and Kepler had figured out the orbits of the planets, it was child’s play to determine when alignments and conjunctions would occur. That’s why astronomers can take the cake for long-term predictions, able as they are to predict eclipses hundreds of years into the future. But their divinations are based on the clockwork orbits of planets and stars, which is, ultimately, not that much different from glancing at a wristwatch and noting that six o’clock will follow five o’clock.
Predicting the turbulent and erratic vagaries of the atmosphere is of another order of magnitude altogether. A modern weather forecast is like pinning a will-o’-the-wisp to a wall, more akin to augury than astronomy. Yet even here, in the world of turbulence and butterfly effects, mathematics underpins the works. The ephemeral clouds, the fickle winds and variable ocean currents are all funneled through the analytical machinery of hydrodynamic and thermodynamic equations. But I’m getting ahead of myself.
The mathematics that describes weather was a long time coming. The fluctuating cycles of weather and climate are deeply wired into all living creatures, an instinctual knowledge that gives them the ability to anticipate changes in the weather with a sort of climatic prescience — from birds flocking into a leafy tree just before a storm to green darner dragonflies migrating south in the fall. We humans too have a visceral, deeply wired relationship with the weather. Something primal that underlies our quotidian observations. Migraine sufferers know all too well when the barometer plunges. But ultimately, it is is our ability to observe, remember and see patterns that allows us to build our collective body of knowledge.
No one knows just when the first forecasts were made by humans, but it was likely very early on in our development. Chimpanzees have been observed gathering to watch particularly colorful sunsets, so it’s safe to assume we had an eye for certain regularities in the weather long before we ventured out of Africa, regularities that hinted what the skies held in store. When we transformed from hunter-gatherers to farmers, a foreknowledge of weather made the difference between survival and ruin, between wealth and poverty. A crop of mature wheat could be wiped out by too much rain, and knowing oncoming weather even a day or two ahead of time could save the lives of citizens in the coming winter. The color of the rising and setting sun was probably one of the first meteorological forecasts. We know it today as “red at night, sailors’ delight; red in the morning, sailors’ warning.” Similar, written observations have been found that date back to 650 BCE, when the Babylonians studied cloud patterns.
But it was the Greeks, a few centuries later, who initiated the next stage in forecasting. It might be argued that they invented the first daily weather forecasts, which were posted on columns in the agora of many of their cities. These were known as parapegmata, or peg almanacs, but they were more like nowcasts, often consisting of accounts of local conditions sometimes accompanied by astronomical details, “Rising of Arcturus; south wind, rain and thunder.” One of my favorites reads simply, “The weather will likely change.”
A parapegmata was still in use in Athens when Aristotle published his Meteorologica, a compendium of speculations about the four elements in 340 BCE: “Fire, air, water, earth, we assert, originate from one another, and each of them exists potentially in each, as all things do that can be resolved into a common and ultimate substrate.” He didn’t have the details we have today, but he more-or-less sketched out the carbon cycle — where air (carbon dioxide) is locked in earth and ultimately released by fire (volcanoes) into the atmosphere. He also theorizes about clouds, wind and the formation of dew, among other phenomena. Some of it sounds eccentric, such as “When there is a great quantity of exhalation and it is rare and is squeezed out in the cloud itself we get a thunderbolt,” but given that most of his contemporaries were convinced that Zeus was the one hurling lightning bolts, he was a radical logician.
Around the same time Aristotle published Meterologica, his pupil and eventual colleague Theophtra
stus of Eresus assembled a compendium of folk weather sayings in De Signis (“Book of Signs”). These arcana have a rural flavor and show how rooted Greek civilization was in the natural history of the Aegean area: “It is a sign of rain if ants in a hollow place carry their eggs up from the ant-hill to the high ground, a sign of fair weather if they carry them down”; “It is a sign of wind or rain when a heron utters his note at early morning: if, as he flies toward the sea, he utters his cry, it is a sign of rain rather than wind, and in general, if he makes a loud cry, it portends wind”; and “If a lamp burns quietly during a storm, it indicates fair weather.”
These adages were the Farmer’s Almanac of the times, and some of them were reliably predictive. But there were other issues that Aristotle grappled with. A theoretical abstraction was making the rounds: under certain circumstances, in the absence of any material substance, there might exist something called a vacuum. Well, not something at all, really nothing. Aristotle dismissed the notion of nothingness and issued an assertion that remained unchallenged for almost two millennia — that a vacuum was a “logical contradiction.”
Like so much else, the received wisdom of Greek folklore passed on to the Romans. Cicero, the great orator who could recite his speeches forward and backward, wrote, “Storms are often stirred up by some particular constellation.” Pliny the Elder (who perished in Pompeii after commandeering an imperial Roman navy ship to take him to the eruption and confidently reminding a fellow passenger that fortune favors the brave) wrote copiously about weather in his Natural History: “When clouds sweep over the sky in fine weather, wind is to be expected in whichever quarter the clouds come from” and “Jellyfish on the surface of the sea portend several days’ storm.”
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