快猫短视频

ONE of the most surprising things about our planet, to anyone who has sat on the beach and watched the ocean breakers roll in, is just how insignificant a puddle even the greatest ocean is. Even the floors of the deepest ocean trenches lie only 11 kilometres below the sea鈥檚 surface, and the average depth of the ocean is just 3.7 kilometres.

Oceans really are very shallow puddles indeed. The North Atlantic, for example, averages a depth of 3 kilometres, and is about 4800 kilometres wide. For a 5-metre wide puddle to have the same proportions, it would have to be a mere 3 millimetres deep. The thinness of the life zone on Earth also applies in the other direction 鈥� upwards. Even the atmosphere of the Earth 鈥� at least, the breathable part in which life exists 鈥� extends no more than about 10 kilometres above our heads. (Mount Everest rises nearly 9 kilometres above sea level, and breathing is pretty difficult on its summit.)

Some ecologists have graphically described the zone in which life can exist on Earth (oceans and atmosphere put together) as a 鈥渢hin green smear鈥� on the surface of a planet which is itself nearly 13 000 kilometres across. The power of this image is brought home by the realisation that the average distance from the top of the tallest mountain on Earth to the bottom of the deepest ocean trench is only about 20 kilometres, a distance which, in horizontal terms, you could cover by car in less than 20 minutes 鈥� given an open road -and in a reasonable day鈥檚 walking. If the Earth were reduced to a ball 13 centimetres across (about the size of a grapefruit), the 鈥済reen smear鈥� would be a film just 2 millimetres thick over the surface of the ball. At a little more than 19 kilometres, the distance from the bottom of the deepest oceanic trench to the top of the tallest mountain corresponds to little less than a half-marathon, a distance thousands of people run each year. Put in those terms, it is easy to see how thin the green smear really is; and the breathable atmosphere itself represents just half of that.

Atmospheric scientists, though, like to extend their domain outwards by including not just the breathable atmosphere but everything out to the region of space around the Earth that is dominated by the influence of particles streaming past from the Sun 鈥� the solar wind. The boundary between the region dominated by the Earth鈥檚 magnetic field and the region dominated by the solar wind lies about 10 Earth radii out into space in the direction of the Sun, with a long, tadpole-shaped tail streaming away on the other side of the Earth (Figure 1 and Inside Science No. 26).

Temperature trends

Atmospheric layers

THIS atmosphere is today made up of 75 per cent molecular nitrogen, 23 per cent molecular oxygen, 1 per cent water vapour and 1.3 per cent argon, by mass (the corresponding figures by volume are slightly greater for nitrogen and slightly smaller for the other main constituents). There are traces (less than 0.1 per cent by mass) of carbon dioxide, neon, helium, methane, krypton, carbon monoxide and sulphur dioxide, with even smaller traces (less than 0.0001 per cent by mass) of molecular hydrogen, nitrous oxide ozone, xenon, nitrogen dioxide, radon and nitric oxide. The composition has changed radically since the atmosphere first formed, largely through the influence of life (1. The history of the atmosphere)

The overall structure of the lower atmosphere is a series of layers in which the layer nearest to the ground, the troposphere, and the layer just above, the stratosphere, play key roles in determining patterns of weather and climate, and have the biggest direct influence on life on Earth. This layered structure can best be seen (Figure 2) by looking at how the temperature of the atmosphere varies with height above the surface of the planet. 快猫短视频s nowadays extend the terminology of this layering to the ice-covered parts of the Earth, or cryosphere, and to its oceans, or hydrosphere.

The source of energy which warms the atmosphere (apart from a tiny amount of geothermal energy released from hot radioactive material, which can be ignored) is heat from the Sun, in the form of electromagnetic radiation. Most of the Sun鈥檚 energy is radiated in the visible part of the spectrum, between 0.4 and 0.7 micrometres (it is visible to us because our eyes have evolved to make use of what is available 鈥� Figure 3). This radiation passes through the atmosphere without being absorbed, and warms the surface of the Earth. About 7 per cent of the Sun鈥檚 energy is radiated at shorter wavelengths, below 0.4 micrometres, in the ultraviolet. This radiation is absorbed by molecules of oxygen and ozone in the stratosphere, and warms that layer directly. A little solar energy is radiated at longer wavelengths, above 0.7 micrometres, in the infrared. Some of this is absorbed in the atmosphere, but it plays only a minor part in keeping the air warm.

The atmosphere is primarily heated from below, by the warm surface of the Earth. This is partly due to the direct conduction of heat from the warm surface to the gas above it, but mainly because the warm surface of the Earth radiates in the infrared part of the spectrum, and infrared radiation is absorbed by molecules such as water vapour and carbon dioxide in the lower atmosphere. The infrared radiation that is absorbed in the lowest layer of the atmosphere makes the air warm, so the air itself radiates heat in turn, still at infrared wavelengths. Some of this radiation goes back down to the surface and keeps it warmer than it would otherwise be. This is the so-called greenhouse effect (Inside Science No. 13). The rest works its way upwards through the atmosphere, being absorbed and reradiated successively until it eventually escapes into space (Figure 4).

The warmth of the lowest layer of the atmosphere causes convection, because the heated air expands, becoming less dense than the cooler air above it, and rises. This is a key process in the determination of weather and climate, contributing to the overall circulation of the atmosphere (Inside Science No. 44). But the warm air cannot rise upwards to the top of the atmosphere, because it is held down within the troposphere by the presence of warmer air in the stratosphere above. The stratosphere 鈥� which is essentially synonymous with the ozone layer 鈥� is the region where direct solar heating occurs as ultraviolet radiation is absorbed (Inside Science No. 9). Ozone, a triatomic form of oxygen, is produced in the atmosphere by the effect of ultraviolet radiation on ordinary diatomic oxygen molecules, splitting them apart and providing single oxygen atoms which can attach to other diatomic molecules. The ozone itself also absorbs ultraviolet radiation, at slightly different wavelengths. Both processes extract energy from the solar radiation passing through the atmosphere, and thereby warm the stratosphere.

The stratosphere can be thought of as a lid on the troposphere, holding down convection and keeping weather confined within the lowest layer of the atmosphere; it operates in this way because the stratosphere is an inversion layer: temperature increases with height and therefore convection cannot occur.

The average temperature at the surface of our planet is about 15 掳C (which is some 33 掳C warmer than it would be if the Earth had no blanket of air, and therefore no greenhouse effect, to keep it warm). Rising through the troposphere, the temperature initially falls with increasing altitude by about 6 掳C for every kilometre. The fall slows at a height of about 15 kilometres, and stops at 20 kilometres. This is the boundary between the troposphere and the stratosphere, known as the tropopause. The exact height of the tropopause varies with latitude, with the seasons, and from day to night, so the numbers given here are only approximate. The troposphere contains about 75 per cent of the atmosphere of the Earth by mass; at the tropopause, the density of the atmosphere is about 25 per cent of the density at sea level.

Ionised air

Electric layers

FROM an altitude of about 20 kilometres to about 60 kilometres, temperature then increases through the stratosphere, from about 鈭�60 掳C at the tropopause to a maximum of about 0 掳C at the top of the stratosphere (the stratopause). At the stratopause, the density of the atmosphere is only 0.009 per cent of the density at sea level; 99.5 per cent of the mass of the atmosphere lies below the stratopause, and only 0.5 per cent of its mass at higher altitudes.

In the altitude range from 50 to 80 kilometres another cooling layer, the mesosphere, occupies the now rapidly thinning atmosphere, with the coldest atmospheric temperatures, about 鈭�100 掳C, reached at the top of this layer (the mesopause). The density of the atmosphere at the mesopause is on average only 0.0007 per cent of the density at sea level, but is subject to relatively large fluctuations caused by changing solar activity. From there on outwards, temperature increases in the last thermal layer, another inversion layer called the thermosphere. As in the stratosphere, in this region energy (now in the form of both ultraviolet and X-radiation) is being absorbed directly from the Sun, but now so effectively that atoms are partly ionised, with electrons being knocked off them to leave positively charged ions behind.

A more useful way to describe the upper atmosphere is in terms of the extent to which the atoms have been ionised. The whole region is known as the ionosphere, with different layers within the ionosphere defined in terms of their differing degrees of ionisation. Above an altitude of about 400 kilometres, the atmosphere has become so tenuous that collisions between its component molecules, atoms and ions are too rare for it to be treated as a continuous gas, so the concept of temperature is no longer meaningful; in this region individual particles can escape into space, so it is sometimes referred to as the exosphere.

Some ionisation occurs at altitudes as low as 50 kilometres, at the top of the stratosphere, and everything from an altitude of 50 kilometres to 400 kilometres is regarded as the ionosphere. The presence of ionised particles in this region was first suspected when Guglielmo Marconi showed that radio waves can be transmitted 鈥渞ound the corner鈥� of the spherical Earth; radio waves travel in straight lines, and the long-distance transmission of signals around the world (without the aid of satellites) is possible because the ionised layers reflect radio transmissions at wavelengths longer than about 15 metres.

The lowest layer of the ionosphere, called the D-layer, has only a low concentration of free electrons, and only reflects long wavelength radio waves. It lies between an altitude of 50 and 90 kilometres. The next layer, the E-layer, lies between an altitude of 90 and 150 kilometres, and is also known as the Heaviside layer, or as the Heaviside-Kennelly layer. It was predicted by the British physicist Oliver Heaviside and the Indian-born electrical engineer Arthur Kennelly, independently of each other. It is more strongly ionised than the D-layer, and reflects medium wavelength radio waves. The F-layer, from an altitude of about 150 to 400 kilometres, is also known as the Appleton layer, after its discoverer, the British physicist Edward Appleton. It is the most strongly ionised region of the ionosphere, and the most useful for radio communications. As well as being less strongly ionised, the E-layer is distinguished from the F-layer by its variability 鈥� its weak ionisation almost disappears at night, which is why it is possible to pick up a different range of AM radio stations at night than in the daytime.

By and large, electromagnetic radiation with wavelengths between 8 millimetres and 15 metres is not reflected at all by the ionosphere, and escapes into space. So the ionised layers cannot be used for bouncing very high frequency FM radio or television signals around the Earth, and such signals can usually only be received if the receiver is within the line of sight from the transmitter. Sometimes, however, conditions in the atmosphere produce a phenomenon known as ducting, which is associated with the development of an inversion layer, with warmer air higher up, in the troposphere itself. The most common way in which such an inversion is produced is by a stable, long-lived region of high pressure (an anticyclonic 鈥渂locking high鈥�); descending air in the anticyclonic system warms as it is squashed by the weight of air above, in much the same way that air in a bicycle pump warms up when it is compressed.

Atmospheric bends

A radio mirage

THE warmer air is less dense and therefore has different refractive properties, compared with 鈥渘ormal鈥� conditions at the appropriate altitudes, so that very high frequency waves moving up through the troposphere are gradually bent over in an arc, and deflected back down towards the surface of the Earth. This sort of ducting is responsible for television interference caused by foreign stations during spells of settled fine weather; on occasion, Spanish FM stations can be heard in the British Midlands, and the record is about 4000 kilometres (2500 miles), from Hawaii to California.

Both E and F layers are strongly affected by changes in solar activity, including individual flares on the surface of the Sun, the 27-day rotation of the Sun, and the roughly 11-year long 鈥渟unspot cycle鈥� of solar activity.

Beyond the ionosphere, above an altitude of about 500 kilometres, the magnetosphere is the outermost region of the Earth鈥檚 atmosphere, where the ionisation is so complete that the particles form a plasma (a mixture of positively charged ions and negative electrons) constrained by the Earth鈥檚 magnetic field (Figure 1).

The magnetosphere forms the absolute limit of the Earth itself 鈥� the 鈥渉ull鈥� of 鈥淪paceship Earth鈥�. On the upstream side of the solar wind, charged particles flow outward past our planet. The interaction between the solar wind and the Earth鈥檚 magnetic field produces a shock wave at a distance of about 14 Earth radii, but the magnetosphere itself extends only to a distance of about 60000 kilometres. The boundary of the magnetosphere is called the magnetopause; beyond the magnetopause lies in interplanetary space. Closer in, two doughnut shaped zones of high particle density are centred above the equator at heights of 3000 kilometres and 15 000 kilometres; these are the radiation belts, named after James Van Allen, the American physicist who discovered them in the 1950s.

In the 1970s, a later generation of satellites mapped the magnetosheath, a long tail of plasma streaming away downstream in the solar wind. In shielding the Earth from the charged particles of the solar wind, the magnetic field deflects them into the Van Allen belts, but some spill over into the polar regions of the upper atmosphere (the polar cusps) where fast-moving electrons from the solar wind interact with atmospheric atoms to produce the coloured lights of the polar auroras. Associated activity alters the ionosphere and affects radio transmission at high latitudes.

This is the limit to which even the most ambitious atmospheric scientists can extend their domain 鈥� but that domain includes the air we breathe, the workings of the weather, our shield against ultraviolet radiation and what is still a major means of global communication. Enough, surely, to keep them busy.

1: The history of the atmosphere

VERY early on in the history of the Solar System, any traces of primordial gas around the young Earth would have been swept away by outbursts from the young Sun. So the present day atmosphere has evolved from a mixture of gases released from the interior of the Earth. Geologists and astronomers try to understand what happened in the early atmosphere of the Earth by studying the kinds of gases emitted by volcanoes today. The main gas released by volcanism is water vapour, which makes up 64 per cent by weight. Carbon dioxide provides 24 per cent of the total, sulphur dioxide 10 per cent and nitrogen just over 1.5 per cent. How has this mixture, leaking from volcanoes over billions of years, produced an atmosphere which is today mainly nitrogen and oxygen, with just a trace of carbon dioxide?

There are two reasons. First, the distance of the Earth from the Sun is in the range where surface temperatures allowed the primordial water vapour to condense and make oceans. Great quantities of carbon dioxide dissolved in the oceans and were eventually laid down in limestone beds. Because Venus, which in many ways is almost the twin of the Earth, is closer to the Sun, oceans never formed and the carbon dioxide remained in the atmosphere 鈥� there is roughly the same amount of carbon dioxide in the air on Venus as in the Earth鈥檚 rocks.

Secondly, life emerged on Earth and altered the composition of the atmosphere. Life did not develop photosynthesis until about 3 billion years ago (Inside Science No. 81). Unlike most modern plants, the first photosynthesisers, from about 3 billion to about 2 billion yeas ago, did not release oxygen (which would have been a deadly poison to primitive plants) directly into the air, but locked it up in compounds with iron. The result was the production of layers of iron oxides, known as banded iron formations (BIFs), which are found around the world in strata between about 3 billion and 1.5 billion years old.

But then the depositing of BIFs ceased as new forms of life arose that were able to live with free oxygen. These photosynthesisers released the oxygen they produced directly into the air, saving the effort required to lock it up in iron compounds and, as a bonus, killing off many competitors. As the supply of oxygen built up, more highly oxidised deposits of iron were laid down as red beds as the Earth literally rusted.

From about a billion years ago there was enough oxygen in the air for the ozone layer to develop, and over all this time the trickle of inert nitrogen from volcanic activity had been building up in the atmosphere because it had nowhere else to go. Since about 600 million years ago the atmosphere has had essentially the same composition we are familiar with today.

2: Dust in the air

THE behaviour of the atmosphere is affected by the changing amount of particulate matter it contains 鈥� both natural dust, largely in the form of material ejected from volcanoes, and pollution resulting from human activities. Fine particles floating high in the stratosphere (either dust finer than talcum powder, or minuscule liquid droplets known as aerosols) can alter the heat balance of the globe both by reflecting away incoming solar heat (thereby making the surface cooler) and by reflecting 鈥渆scaping鈥� heat down towards the ground (thereby making the surface warmer). The exact balance of these two effects depends on the nature of the dust, the season, and the time of day 鈥� for example, a dust layer may tend to make daytime and summer temperatures less, while making nighttime and winter temperatures more.

The overall effect of the injection of large amounts of volcanic debris into the stratosphere in explosive eruptions is to cool the globe. This was shown clearly after the eruption of Mount Pinatubo, in the Philippines, in 1991. The world cooled by about half a degree Celsius over the next 18 months but by the end of 1994 as dust cleared from the air it warmed up to the level that existed before the Pinatubo eruption. Most climatologists use a dust veil index, calculated on the basis of known volcanic eruptions throughout history, as one of the factors in their calculations of natural climate change.

Both industry and agriculture also add particles to the atmosphere, including soot and wind-blown soil, and for these purposes 鈥渄ust鈥� is used to include, for example, aerosol droplets of sulphuric acid produced by the combustion of dirty fuel in power stations. Such anthropogenic aerosols tend to stay in the lower atmosphere and be washed out by rain in a month or so -but because there is a constant source of new material, they can have a big overall effect. Western Europe failed to warm up as quickly as the rest of the world during most of the 20th century but then warmed more rapidly as it caught up. 快猫短视频s suggest that this is because the region had suffered considerable industrial pollution but it has now been greatly reduced.