Layers of the atmosphere

The sky is the very definition of vastness. When we take a boat to sea or find ourselves on a treeless plain, were reminded just how all-encompassing the sky can feel. Its easy to believe the atmosphere must go on forever, stretching outward and upward.

Try this perspective, though: Earth as seen from the US space shuttle, over 160km/100 miles above sea level From this vantage point, the atmosphere looks paper-thin, a sheer layer that barely covers the globe, like cellophane stretched around a beach ball. In fact, the atmosphere is just about that skimpy relative to Earth. More than 90 percent of our atmosphere lies within 32km/20 miles of the planets surface, a mere sliver compared to Earth's diameter of nearly 13,000km/8000 miles.

This filmy coating is all it takes to produce the enormous variety of weather on the surface where we live. The “weather layer” of the atmosphere, in fact, is even thinner: only a few miles deep. Above that are several other atmospheric layers. Each one has its own temperature profile, cloud types, chemical make-up and impacts – some minor, some major – on the inhabitants of Earth.

The ground floor

The troposphere is where most of our weather action takes place. It's the bottom of the atmosphere, extending from ground and sea level up to around 10-16km/6-10 miles high, depending on the season and location. It's tallest wherever and whenever the air is warmest. Mount Everest stretches more than halfway up through the troposphere, and many other mountains are tall enough to affect how the air in this layer moves. And move it does. The Latin root of “troposphere” means “to turn”, and the air in this swirling realm is indeed turning and overturning all the time. In a turbulent thunderstorm it may rise at speeds of over 160km/100 miles per hr.

Usually, the upward and downward motions are far more gentle, typically less than 1.6kph/lmph. The average horizontal motions we experience as wind are quite a bit stronger. A typical breeze is 16-24kph/10- 15mph.

What keeps the troposphere moving are contrasts between warm and cold, moist and dry. The play of sunlight across Earth varies with the season, the time of day, and where it's cloudy and clear. The result is a patchwork of air masses that have either been heated by sunlight or cooled by its absence. These contrasts are greatest near the ground, where soil and ocean have a chance to add their stored heat to the Suns. Overall, the heating differences are most obvious between the poles and the tropics, a contrast that forms the source of the fronts and cyclones driving our daily weather.

Air that's warmed becomes less dense than its surroundings and rises as a result. With height, however, the air becomes thinner and thus chillier. Cool air goes through the opposite process, typically settling, compressing and wanning with time (except during calm nights, when the air can chill down as it pools in darkness and Earth radiates heat to space). The patchwork of the troposphere, then, is in a state of perpetual upheaval, seeking to equalize conditions through a never-ending series of highs, lows, fronts and storms. As any mountain climber knows, air gets colder the higher you go. The drop-off rate averages 6.0°C for every kilometre you climb (or 3.3°F for every 1000ft). At certain times and places – when moist layers are present, for example, or when there's a cold pool trapped near the ground – this relationship doesn't hold, but it's a good rule of thumb. In the upper troposphere, where most jets fly (around 10-13km/6-8 miles at mid-latitudes), it's always well below freezing.

Where the ozone is

Compared to our weather layer, the stratosphere – the layer above the troposphere, around 16-50km/10-30 miles – is a topsyturvy place. Temperatures plummet to as low as -80°C (-112°F) in the tropopause (the troposphere-stratosphere boundary), but then they actually rise as you ascend through the stratosphere. Near the top, you're again close to the freezing point.

This temperature flip-flop occurs mainly because of the stratosphere's role as a shield for harmful ultraviolet sunlight. The ozone layer, centred in the lower stratosphere, is the prime absorber of ultraviolet light. The top of the ozone layer takes the biggest hit from the incoming rays, which allows it to heat up the most. The sunlight itself actually helps generate ozone by splitting oxygen molecules. Together with the mixing of ozone downward into the troposphere, a natural cycle has kept the ozone layer in balance for millennia.

In recent decades, humans have disturbed the stratosphere from below Industrial chemicals have wafted upward and jeopardized our ultraviolet defence shield. Working in tandem with sunlight and stratospheric clouds, these chemicals (chlorofluorocarbons, or CFCs) have depleted the ozone layer by several percent worldwide. For a few weeks each spring above parts of the Antarctic, the loss is closer to 100 percent).

Only a small number of aircraft – mainly research planes – fly at stratospheric heights. There's long been concern, however, that more extensive flying at these altitudes, such as by a fleet of passenger planes, may cause additional damage to the ozone layer. Major studies, such as a 1998 review by the US National Academy of Sciences, point to a number of unresolved questions. Nobody knows, for example, exactly how long it takes for aircraft fumes to disperse in the poorly mixed stratosphere.

Since the air is so thin and cold in the stratosphere, very little moisture is present for cloud making. The only type found is the nacreous, or mother-of-pearl, cloud, which tends to form close to the poles and above mountain ranges. These clouds filter sunlight in a way that gives them a luminous spectrum of colour.

On the way to outer space

In the mesosphere (roughly 50-80km/30-50 miles high), the air's temperature once again drops with height, falling to readings well below -73°C/- 100°F. Clouds are absent except for rare noctilucent clouds, first described in 1884 (the term is Latin for “luminous at night”). These can be seen only on a clear summer's night, after the Sun has set, but while its rays can still reach the high-level cloud against the dark sky. Widely reported in the past at higher latitudes only (from about 50° to 65°N and S), noctilucent clouds were observed in 1999 in Colorado and Utah, much further away from the poles than ever before. The sighting caused much speculation on how climate change might be affecting the mesosphere. One hypothesis is that methane from the lower atmosphere, believed to rise and react with sunlight to form the water vapour that makes up noctilucent clouds, is now being joined by carbon dioxide, which helps to cool the upper atmosphere – perhaps enough to move the range of the noctilucent clouds closer to the equator.

As we head even higher, into the thermosphere (80-650km/50-400 miles), the air becomes so thin that molecules have difficulty keeping each other in check. One molecule might travel a distance the width of the English Channel before colliding with another. Because the air thins out so drastically with height, the air “temperature” can climb dramatically to up to 1800°C/3200°F. However, with so few molecules buzzing around, the heat isn't that intense; in a pressurized suit, a space-shuttle astronaut would do just fine.

Like many other distinctions, the one that separates our atmosphere from outer space is rather fuzzy. Above a height of approximately 95km/60 miles, the air sorts itself out by weight, and the lightest molecules (hydrogen and helium) gradually rise to the top. The atmosphere doesn't so much end, as dribble down to an infinitesimal thinness through a zone a few hundred miles high. From about 650km/400 miles outward, the atmosphere is constantly being drained off. A few hundred tonnes of stray hydrogen molecules escape into space each year. The vast bulk of the atmosphere stays in place, though, held fast by gravity.

The thinness of the upper atmosphere allows forces other than brute mechanics to take over. The intensity of the unfiltered sunlight at these heights breaks up molecules and leaves them electrically charged. When an especially potent batch of magnetized plasma arrives via the solar wind, a geomagnetic storm may result. The strongest of these can last for days, often fouling up radio signals and satellite orbits. Such storms are now predicted and tracked by earthbound sensors and by satellites deployed as far as a million miles from home. (Geomagnetic storms also produce the ethereal beauty of the aurora borealis and australis. See “The Sun and Earth”, below.)