Earth’s atmosphere is composed mostly of oxygen and nitrogen, with some carbon dioxide and other trace gases like argon and hydrogen thrown in for variety. Meteorologists divide the atmosphere into several layers, each of which blends seamlessly into the next. Nitrogen makes up around 78 percent of the air we breathe at the surface, with oxygen taking up about 21 percent. Unless you’re an astronaut, you spend most of your time in the bottom layer of the atmosphere, called the troposphere, which extends anywhere from 5 to 10 miles up depending on how much of the Sun’s energy is reaching the earth at the time.
In the troposphere, the temperature falls an average of 4°F for every 1,000 feet you climb, a phenomenon called the lapse rate. Eventually the temperature stops falling, meaning you’ve reached the tropopause and the beginning of the next layer, the stratosphere. Really, you wouldn’t want the temperature to fall much lower anyway: at the tropopause, it can dip as low as –70°F. You’d think the temperature would just keep on falling as you leave the troposphere and gain more altitude, but that’s not the case.
Instead, as you climb up into the stratosphere, the temperature begins to rise again, up to a high of around 40°F. One reason for that is because the stratosphere contains the ozone layer, which acts as a protective blanket to prevent harmful amounts of ultraviolet (UV) solar radiation from reaching the earth’s surface (and the people on it) and helps to warm the stratosphere. Even where the amount of ozone is greatest—around 16 miles up—you’ll find only about twelve ozone molecules for every million molecules of air, but that’s still enough to block out the worst of the UV rays. That’s a good thing, because UV radiation is known to cause skin cancer, and can even induce genetic mutations in DNA.
Climbing even higher, you finally reach the edge of the stratosphere, or stratopause, at around 30 miles above the earth’s surface. Now you’re really getting into nosebleed territory: at this height, the air is much too thin to breathe, and atmospheric pressure is only about one millibar (the metric equivalent to mercury). By contrast, air pressure at sea level is about 1,013 millibars. You’re now above most of the atmosphere.
The mesosphere is the next layer, extending from 30–50 miles high. With very little ozone to provide warmth, the temperature begins to fall again, to a low of about –130°F. It continues to decrease until you reach the mesopause, then begins to rise again as you enter the thermosphere, which extends from 50 to more than 120 miles above the earth.
Perhaps “rise” isn’t the right word—temperatures in the thermosphere can reach a blistering 2,700°F. The thermosphere gets that hot because it’s the first layer of air the Sun’s rays hit as they zoom toward Earth. A space shuttle must pass through the thermosphere on its way to and from orbit, so the obvious question is: why doesn’t it burn up? Fortunately, at that height there are so few air molecules that the net amount of heat energy hitting the shuttle isn’t enough to destroy it.
Some are just too big or dense for the thermosphere to handle. Thousands of rocks from the size of pebbles down to grains of sand burn up each day, but space rocks larger than about 33 feet in diameter can usually make it to the ground (most often in pieces).
The lack of air molecules would actually make it feel downright cold if you could somehow sit out in the thermosphere for a few moments. It sounds crazy, but there just wouldn’t be enough air molecules to heat up your skin. It’s a good thing for us that the number of molecules in the thermosphere is still great enough to intercept and destroy most incoming meteorites, however.
The thermosphere also contains most of the ionosphere, so-called because energy from the Sun smacks into molecules at that height and separates them into ions, which carry a positive charge, and free electrons, which are negatively charged. Many years ago, it was discovered that this layer reflects radio waves, especially at night, allowing the transmission of signals beyond the curvature of the earth for hundreds of miles or more. This principle allows ham radio operators to receive broadcasts from faraway countries, although the effect is not always predictable.
As an example of uncontrolled warming, scientists point to Venus, a planet nearly the same size as Earth but with a much more hostile atmosphere. On Venus the “air” is about 96 percent carbon dioxide with a temperature hot enough to melt lead. Scientists say the same conditions may occur on Earth if pollution isn’t controlled.
What lies above the thermosphere? If you think the answer is air, think again. The layer above 120 miles of altitude—the exosphere—contains so few molecules that many of them are actually able to escape Earth’s gravity and fly off into space. The exosphere is the domain of satellites and space shuttles, a transitional zone between Earth’s atmosphere and interplanetary space. The exosphere has no real upper boundary; it just becomes more and more diffuse until it’s no longer detectable.