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The Wild Blue Yonder
The Layers of the Atmosphere
When I was a child, I would sometimes climb up onto my parents’ garage roof on late summer afternoons and lie down on the warm asphalt shingles. There was a feeling I was after, a sort of giddy, floating sensation when I stared straight up. I’d imagine the garage was gone and that I was alone, suspended in the endless blue sky that surrounded me. No neighborhood, no city, no planet Earth, just blue, blue atmosphere and the sun. Well, there were birds winging by me in my azure world, but they were fellow travelers.
I would sometimes invite my best friend up to share my fantasy, though he would get vertigo if he got too deeply into the vision. In fact, it was the vertiginous sense of endlessly falling upward into an infinite, warm universe of blue sky that I was secretly addicted to. The sky was a sea of warm air in which I hovered. And so it is. We live at the bottom of an ocean of air, 5,200 million million tons to be exact, or 25 million tons piled on every square mile of the planet. If the atmosphere were compressed into a slab of granite, it would be 2,000 miles long, 1,000 miles wide and half a mile thick. It sounds like a lot but Earth’s atmosphere is pasted in an alarmingly thin layer — 99 percent of it lies within 18 miles of the surface. Earth’s transparent skin. Without it we couldn’t exist.
Earth is not unique in the solar system in terms of having an atmosphere. All the planets have atmospheres except Mercury, which has the misfortune of being too close to the sun. A few of the 173 moons that orbit other planets in our solar system also have atmospheres, but most moons, including our own, do not. Not a whiff, not a puff, not a wisp of gas. The moon’s airless surface is completely exposed to the extreme temperature swings of outer space. At lunar noon, the thermometer often reaches 123°C, so if you were to set a glass of water on the regolith (the lunar soil), it would boil away in a few minutes. At night, the temperature plunges to -181°C. If you were an astronaut there in the starry darkness and if you had the suicidal inclination to remove your helmet, the carbon dioxide in your last exhalations would freeze and fall as snowflakes of dry ice.
We’re lucky here on Earth not only to have an atmosphere but a relatively benign one. In fact, we can get down on our knees and thank our atmosphere for protecting us — and not just from such temperature extremes. It also intercepts about 99 percent of meteorites and a lot of downright hostile subatomic particles. And if that isn’t enough, Earth’s atmosphere has just the right balance of oxygen and nitrogen for us to breathe comfortably.
But it is a thin layer At 19 miles high, the windows of ascending spacecraft darken and stars begin to appear. The unofficial border of outer space, or at least the airless vacuum part of it, is only the distance of a big-city crosstown excursion, albeit a vertical excursion that consumes an Olympic swimming pool’s worth of rocket fuel rather than a dollar of gasoline.
In horizontal terms, we could walk the distance in about six hours. Climbing would be a whole other matter. The peak of Mount Everest, at 5 1/2 miles (29,029 feet), only gets about a third of the way. The lucky few that summit Everest, exhausted and light-headed, must get a dizzy sense of what space feels like. At the mountain’s peak, the sky is a dark sapphire blue and there’s barely enough oxygen for a human to survive. It’s not because oxygen levels drop disproportionately compared to the rest of the atmospheric gases up there — oxygen remains at 21 percent — it’s because the air itself is thinning out. With less atmospheric pressure, the lungs cannot extract the same amount of oxygen with each breath.
Airliners normally fly one and a half miles above the height of Everest, cruising at 35,000 to 40,000 feet, or 7 miles high. They’re a little closer to the edge of the atmosphere, which is why passengers breathe a pressurized atmosphere. It might also be why a few cocktails imbibed en route suddenly gang up on unwary drinkers when the plane lands and the cabin is flushed with normal atmospheric pressure.
The early aeronauts of the nineteenth century didn’t have in-flight service during their daring ascents. And they certainly had no onboard oxygen or even a closed cabin when they piloted their balloons into the upper atmosphere. They hadn’t a clue what they were going to encounter up there — they ascended literally into the unknown. Scientists at the time didn’t how high the atmosphere extended or if its composition changed with altitude. From mountaineering accounts, early aeronauts knew that temperature dropped the higher you went and that oxygen levels also fell, but that was all. There was nothing for it but to send someone up, and the only way was to hang from a fragile balloon filled with coal gas.
Up, Up and Away
The grand exploration of the atmosphere began in 1783 when Jean-François Pilâtre de Rozier and François Laurent, marquis d’Arlandes, first rose above France in a hot air balloon. The ballooning craze, for those who could afford it, spread quickly throughout Europe, and by the beginning of the nineteenth century, ballooning had become a science, and the race to probe the atmosphere was on.
The first scientist to go aloft was the great French chemist Joseph-Louis Gay-Lussac, whose specialty was gases, particularly how the volume of a gas changes with temperature and, as he would soon find out, with height. It was during his second flight in a hydrogen balloon in September 1804 that he set an altitude record, reaching a height of 23,018 feet (4.3 miles). Gay-Lussac carried a barometer, thermometer, hydroscope, compass and flasks to capture samples of air. From air samples taken during the ascent, he discovered that the composition of the high-altitude atmosphere was the same as that at the surface; it was just thinner. One of his most useful discoveries was the confirmation that the temperature dropped by almost 1°C for every 328 feet in elevation. He was also the first human to experience the beginnings of decompression — suffering a tremendous headache at the peak of his ascent. It would be more than 50 years before Gay-Lussac’s altitude record was broken by English aeronauts James Glaisher and Henry Coxwell on the afternoon of August 18, 1862.
James Glaisher was a self-taught mathematician and meteorologist who had practically grown up in the Royal Observatory at Greenwich Park. Years later, when the appropriately named Sir George Biddell Airy was appointed Astronomer Royal in 1835, one of his first acts was to make Glaisher superintendent of the new magnetical and meteorological department. Glaisher was an excellent scientist and had a knack for reducing almost any phenomenon into numbers. (He once devised a formula for what he insisted was the perfect cup of tea.) But above all, he was a dedicated meteorologist.
He was instrumental in founding the British Meteorological Society in 1850 and then on August 8, 1851, as part of the Crystal Palace of Industry exhibition, he published the very first British daily weather map from telegraphically collected data. It was a breathtaking miracle of scientific technology, and the public was enthralled.
Glaisher was fascinated by the dew point, the temperature at which condensation forms out of atmospheric water vapor and he hoped, with this voyage into the upper atmosphere, to find out how the dew point might change with altitude.
The pilot of their vessel, Henry Coxwell, was an experienced balloonist, a veteran of vertical ascents. They were an optimal crew for such a mission. On a hot August day, they clambered aboard and sat amid Glaisher’s scientific equipment in a basket (which balloonists of the time referred to as a car) suspended below the largest balloon that had ever been inflated. It was called the Mammoth, and it was filled to bursting with 90,000 cubic feet of coal gas. Everything they needed was stored aboard, including a first-aid compartment containing only a pint of brandy.
Once the balloon was straining at the tethers, all that Coxwell and Glaisher had to do was cut them. The balloon rose quickly, and in a little more than 20 minutes, it had risen three miles above the Earth’s surface. To get even more elevation, they threw some ballast overboard. (I wonder what happened to those sandbags. Dropped from that elevation, their terminal velocity would be around 330 miles per hour, yet there seem to
be no contemporary accounts of holes smashed through the roofs of vicarages or complaints about flattened livestock.)
During their ascent, Glaisher took readings from his instruments, including a barometer that also acted as an altimeter. About 30 inches of mercury is normal for sea-level air pressure. Previous balloon ascents and experiments that involved transporting barometers up mountains had proven that air pressure drops two inches for every half mile of elevation. That gave our Victorian balloonists a fairly accurate idea of how high they’d ascended. By the time the barometer had dropped to 11.5 inches, or about five miles high (26,400 feet), the temperature inside their car was -23°C, and the sky had turned a dark, Prussian blue.
At this point, Glaisher began to pass out, losing the ability to move one of his arms. He later wrote in his diary, “I then tried to move the other arm, but found it powerless also. I next tried to shake myself, and succeeded in shaking my body. I seemed to have no legs. I could only shake my body. I then looked at the barometer, and whilst I was doing so my head fell upon my left shoulder . . . and then I fell backwards, my back resting against the side of the car, and my head on its edge. In that position, my eyes were directed to Mr. Coxwell in the ring.” And what he saw momentarily before he completely lost consciousness panicked him.
George Coxwell, realizing they’d gone too high, was also suffering from extreme oxygen deprivation, but he would prove his balloonist mettle. While Glaisher was passing out, Coxwell had clambered out of the car and up into the lattice of ropes suspending the car from the balloon. The valve that let the air out of the balloon (so they could descend) had been twisted by one of the guy ropes and was out of reach. Coxwell managed to snare it and pull it back into the car, but by then he’d also lost the use of his arms. Holding the valve with his teeth, he gave it three tugs and the gas began to escape. Then he collapsed on the floor of the car as they started their descent. In a few minutes, Glaisher regained consciousness and resumed his observations until they landed.
Glaisher wrote, “When we dropped it was in a country where no accommodation of any kind could be obtained (Shropshire), so that we had to walk between seven and eight miles.” Not exactly a hero’s welcome. Although they didn’t know it at the time, they had sailed up through the entire troposphere to the edge of the stratosphere.
The troposphere, about seven miles thick, is where all weather happens: clouds, jet streams, rain and hurricanes. The stratosphere is a much more tenuous layer of atmosphere. Its lowest regions, right where it meets the troposphere, are very cold, around -60°C. Glaisher and Coxwell could have told you that. It’s why when you are in an airplane and your flight has reached cruising altitude, you can sometimes see frost forming in your cabin window. Jets cruise the lower levels of the stratosphere, which goes up quite a way. In fact, the stratosphere includes the 18-mile “it’s beginning to look like space to me” limit, and then also extends another 13 miles to the edge of the mesosphere. When you see a bright falling star, it’s burning up in the upper stratosphere.
The Ozone Layer and Beyond
The ozone layer occupies the lower portion of the stratosphere, generally between 12 and 19 miles above Earth, depending on the time of year. Not only does it provide protection from ultraviolet light, but it also provides a thermal lid to the troposphere.
So what happens in the ozone layer? When UV light strikes oxygen molecules in the lower stratosphere, it converts some of them into ozone molecules — a kind of hybrid oxygen, with three molecules instead of two. (Lightning strikes also produce ozone, which is why you can smell it in the air after storms.) These ozone molecules then absorb even more UV radiation from the sunlight, which splits them back to plain oxygen and releases a bit of heat at the same time. This cycle, called the Chapman cycle, is continuous, with oxygen molecules reacting with UV to create ozone molecules that split into oxygen molecules. The ozone layer is therefore considerably warmer than the air below and above it, sitting at about 0°C.
It acts as a thermal barrier, an inversion layer if you like, separating the frigid stratosphere from the equally frigid troposphere. That means that the ozone layer is the highest point that atmospheric convection currents can reach, and because convection is the engine of weather, driving everything from evening zephyrs to hurricanes, there is no weather in the stratosphere. The ozone layer’s double duty is to protect life from unmitigated UV radiation and to put a vertical limit on weather.
The degradation of the ozone layer, which began in the late twentieth century, had such dire consequences for life on the planet that, in an unprecedented act of global cooperation, the world immediately banned the fluorocarbons that were destroying the ozone layer. But dangerously high levels of UV were not all that meteorologists were worried about. There was also the unknown consequences of losing the lid of the troposphere. What would have happened to climate worldwide? Hopefully we’ll never know.
From the ozone layer to the top of the stratosphere is another 14 miles. (The stratosphere in total is about 20 miles thick.) The temperature gradually increases with height, so that by the time you get to the top of the stratosphere and the beginning of the next layer, the mesosphere, the temperature has reached a relatively warm -3°C. The mesosphere is a remnant atmosphere, more like a sprinkling of atoms, that extends from the top of the stratosphere, 31 miles above the Earth’s surface, to 50 miles in altitude. All in all, it is about 19 miles thick, and it had to wait until the early 1960s before explorers reached it.
On July 17, 1962, almost exactly a century after Glaisher and Coxwell’s precarious excursion to the beginning of the stratosphere, test pilot Robert M. White made a flight that put him on the cover of Life magazine and into the record books. White was a top gun. At age 20, he flew P-51 Mustangs over Germany in the closing stages of the Second World War. He was shot down, spent a year in a German prisoner-of-war camp and then, after being liberated, went right back to flying the fastest, hottest fighters the airforce could throw at him.
On that July morning, he was the pilot chosen to climb into the cockpit of an X-15 rocket slung under the wing of a flying fortress B-52. The B-52 took White to an elevation of 45,000 feet and released him. After a moment of freefall, he lit the rocket engine and tipped the nose of the X-15 almost straight up. “The horizon disappeared and I could see nothing ahead of me but sky,” he later recounted. The G-force slammed him back in his seat, and, with the rocket engines on full, he watched the altimeter climb until he was at 120,000 feet. Then the engines cut off, their fuel exhausted. His X-15 was now ascending under pure momentum. In seconds, he was at 217,000 feet, his old record, but the X-15 was still climbing. “There was very little pressure on the control surfaces. The aircraft responded lazily,” he reported. At that altitude, there is definitely not enough atmosphere to steer an aircraft, so his X-15 had been fitted with thrusters to adjust pitch and yaw. He was still climbing. He couldn’t believe the altimeter — 300,000 feet, 310,000 feet, then finally 314,750 feet, a full 59 miles above the Earth’s surface. White had flown through the entire mesosphere and on into the thermosphere above it.
In an interview after the flight, he recalled the experience: “But most impressive was the color of the sky . . . it’s a very deep blue — not a night blue, just a deep, deep blue that’s hard to describe but marvellous to see.” And then the view beneath: “Wow! The Earth is really round . . . Looking to my left I felt I could spit into the Gulf of California; looking to my right I felt I could toss a dime into San Francisco Bay.”
White was the first person to fly a fixed-wing aircraft into outer space. Yes, Yuri Gagarin and Alan Shepard had rocketed into space the year before, but they were passengers, not pilots. Everyone called them astronauts. The U.S. Air Force decrees that 50 miles high is the official, aeronautical edge of space, and any pilot who has flown above this limit is given official astronaut status by the U.S. Air Force and a medal to prove it. So White got his wings and the bragging rights to call
himself an astronaut. (The Europeans set the bar higher. According to the Fédération Aéronautique Internationale, outer space lies on the other side of the Kármán line, 100 kilometers, or 62.1 miles, above our heads. White almost qualified for that designation too, though he wouldn’t have received any hardware; the Fédération Aéronautique Internationale doesn’t issue medals.)
The mesosphere is the realm, along with high-flying X-15s, of the ethereal, rare noctilucent clouds, made of meteor dust that can be seen in the summer months at higher latitudes. Beautiful but not really clouds at all. And if there is no weather in the stratosphere, there is certainly none in the mesosphere. At the mesosphere, meteorologists bail out. What goes on there doesn’t really affect them. This is where the next tier of scientists kicks in — the exo-physicists and high-altitude researchers.
Scientists love dividing ambiguous phenomena into discrete slices, and atmospheric researchers are no exceptions. Many have made their careers by defining and extending ever more tenuous layers of atmosphere. At about 50 miles above the Earth’s surface, the mesosphere gives way to the thermosphere, which stretches another 160 miles above the Earth’s surface. The thermosphere is very hot — 500 to 2,000°C, though that is almost an abstract reading. There are too few particles per square yard to transfer any of that heat to objects moving through it. It couldn’t melt a snowflake. Otherwise the space station, which circles our planet well inside the thermosphere on an orbit between 200 and 240 miles above the Earth, would burn up in a few seconds.
On the edge of space, the base of the thermosphere is the stage for Earth’s greatest light shows: the aurora borealis (northern lights) and its southern cousin, the aurora australis. The northern lights are the most famous of the two because Canada, the Scandinavian countries and Russia are much closer to the North Pole than Australia, Chile, Argentina and South Africa are to the South Pole and so are seen by more people. From the vantage of space, the auroras crown each pole with electric iridescence, their rays stretching up from the mesosphere and through the thermosphere. They are a multihued electric particle show fueled by high-energy electrons from solar flares that have been caught by the Earth’s magnetic field and corralled toward the poles. The eerie colors happen when these electrons collide with the sparse atmospheric atoms found at such high altitudes. Red and green hues are created by collisions with oxygen atoms, and the rarer violet hues are the result of solar electrons hitting nitrogen atoms. The shifting undulations in the “curtains” are caused by wavefronts of electrons moving at near light speed.
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