Atmosphere of Earth

• 1. The Thin Blue Shield

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  Every second, the Earth loses about 3 kilograms of hydrogen, 50 grams of helium, and much smaller amounts of other constituents, bleeding away into space. This quiet leak happens in the outermost reaches of a layer of mixed gas that gravity holds against the planet's surface. We call that gas air. Three quarters of its entire mass sits within about 11 kilometers of the ground. Above that, it thins toward nothing, with no definite boundary marking where the planet ends and the void begins. By international convention, the line is drawn at 100 kilometers, the Kármán line. Cross it, and travelers are considered astronauts. How does so thin a film of gas shield a whole world from meteoroids and ultraviolet radiation? Why does it grow colder and then hotter as you climb? And how did air that once held no free oxygen at all come to carry the 21 percent that animals now breathe? The answers begin with what the air is actually made of.

• 2. The Recipe Of Air

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  By mole fraction, dry air is 78.08 percent nitrogen, 20.95 percent oxygen, and 0.93 percent argon, with carbon dioxide trailing at 0.04 percent. Everything else is a trace. Neon, helium, methane, krypton, and xenon all appear in vanishingly small amounts, alongside compounds like nitrous oxide and ozone. Water vapor refuses to hold a fixed figure. It averages around 1 percent at sea level and about 0.4 percent across the whole atmosphere. In the coldest regions it drops to around 10 parts per million by mole fraction. In hot, humid air masses it climbs to as much as 5 percent. Because of this, scientists usually quote gas concentrations in terms of dry air. The average molecular weight of dry air comes to about 28.946 or 28.964 grams per mole, a number that drops when the air turns humid. Up to roughly 100 kilometers, turbulence keeps the gases stirred so their proportions stay constant. A transition zone between about 80 and 120 kilometers hands that stirring over to molecular diffusion. Above it, the lighter gases begin to rise to the top, sorting themselves by weight in a region whose architecture we will climb through next.

• 3. Climbing Through The Layers

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  The troposphere starts at the ground and reaches an average height of about 12 kilometers, stretching from roughly 9 kilometers at the poles to 17 kilometers at the Equator. It holds roughly 80 percent of the atmosphere's mass and nearly all its water vapor, which is why almost all weather happens here. Its name comes from the Greek word tropos, meaning turn, a nod to the vertical mixing driven by a surface that warms the air from below. The stratosphere rises above it, from about 12 kilometers to a stratopause near 50 to 55 kilometers, and it does something strange: temperature climbs with altitude. The ozone layer absorbs ultraviolet light here, warming the upper reaches to just below 0 degrees Celsius and damping turbulence so completely that the layer is nearly cloudless. This stratosphere is unique to Earth, since neither Mars nor Venus has one. The mesosphere, from about 50 to 80 or 85 kilometers, holds the coldest place on the planet, averaging around minus 85 degrees Celsius. Above it, the thermosphere can reach 1,500 degrees Celsius, though the gas is so sparse that the figure barely means anything to a human body. Beyond all of these lies the exosphere, so tenuous that some scientists count it as interplanetary space rather than atmosphere at all.

• 4. Lights, Clouds, And Vanishing Sound

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  Just below the mesopause, the very scarce water vapor freezes into noctilucent clouds of ice particles, the highest clouds in the atmosphere. They glow when sunlight catches them about an hour or two after sunset, most visible when the Sun sits 4 to 16 degrees below the horizon. Higher up, around 100 kilometers, the aurora borealis and aurora australis appear in the thermosphere. The most common green aurora comes from atomic oxygen in the 1S state, glowing at altitudes from 120 to 400 kilometers. Sound behaves strangely along this climb. The atmosphere absorbs sound waves at a rate proportional to the square of their frequency, so audible noise from the ground never reaches the mesosphere. Only infrasonic waves can travel that high, and they are hard to produce at strong power. The mesosphere is also where most meteors and satellites burn up on entry, too high for jet aircraft and balloons yet too low for orbital spacecraft, reachable mainly by sounding rockets. Lightning-induced transient luminous events flicker here above thunderclouds far below. These displays are reminders that the atmosphere does more than hold gas. It scatters and bends the very light that reaches our eyes.

• 5. Why The Sky Is Blue

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  At sea level the speed of sound is 340 meters per second, but in the stratosphere's cold of minus 60 degrees Celsius it slows to 290 meters per second, because sound speed tracks temperature rather than pressure. Light, meanwhile, plays its own tricks. Through Rayleigh scattering, shorter blue wavelengths scatter more easily than longer red ones, which is why the sky looks blue and why sunsets burn red as the Sun's low rays pass through extra atmosphere. Different molecules drink different wavelengths. Oxygen and ozone absorb almost all radiation shorter than 300 nanometers, while water absorbs at many wavelengths above 700 nanometers. The combined absorption leaves windows of low opacity. The optical window runs from around 300 nanometers up through the visible spectrum near 400 to 700 nanometers and on to about 1100 nanometers. There is even a radio window spanning roughly one centimeter to eleven-meter waves. Warmth shapes emission too. The Sun, at about 6000 kelvin, peaks near 500 nanometers in visible light, while Earth at about 290 kelvin radiates near 10,000 nanometers, far too long for human eyes. That infrared exchange is the heart of the greenhouse effect, and without those gases the planet's surface would freeze rather than rest at its present average of 15 degrees Celsius.

• 6. The Engine Of Weather

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  Earth's rotation rate and the gap in solar radiation between equator and poles drive the large-scale movement of air through the troposphere. The flow divides into three convection cells by latitude. The Hadley cell rises along the equator and sends air poleward in the upper atmosphere. At mid latitudes the Ferrel cell reverses that pattern, and in the high latitudes the Polar cell lifts air again. Where these cells meet, jet streams form, narrow fast bands that run west to east at an elevation of around 9,100 meters. They blow strongest in winter, when the boundaries between hot and cold air sharpen, and their instabilities steer the weather systems that cross the middle latitudes. The atmosphere also carries waves and tides, triggered by uneven heating from the Sun and by the daily solar cycle. These range from small gravity waves that push momentum upward to vast planetary Rossby waves. The axial tilt of the planet keeps shifting where heat lands, producing the seasons, while the uneven scatter of land and water further breaks the flow. This restless circulation is recent compared with the air's deep history, which began with a planet that had no free oxygen at all.

• 7. An Atmosphere Reinvented

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  During the Hadean eon, Earth's first atmosphere was made of gases from the solar nebula, mostly hydrogen with simple hydrides like water vapor, methane, and ammonia. The collision with Theia melted and ejected much of the mantle and crust, outgassing steam that later cooled into ocean water. As the crust solidified, the air cooled, water vapor rained into a superocean, and volcanism built an Archean atmosphere rich in nitrogen plus carbon dioxide and methane. About 3.4 billion years ago, nitrogen became the major component of a stable second atmosphere, even as the early Sun shone 30 percent fainter, a puzzle called the faint young Sun paradox. Around 2.4 billion years ago, free oxygen finally began to accumulate during the Great Oxygenation Event, after a billion years of cyanobacterial photosynthesis recorded in stromatolite fossils from 2.7 billion years ago. Oxygen could not build up until its production outpaced the iron, sulfur, and methane that consumed it. The level swung wildly, peaking near 35 percent around 280 million years ago during the Carboniferous, far above today's 21 percent. Since 1750, human activity has pushed up carbon dioxide, methane, and nitrous oxide, leaving the 2011 to 2020 decade 1.1 degrees warmer than 1850, and threatening the same ozone shield that lets life endure.