Our Blue planet

 

The Earth is a planet of transformations. It may appear static, but its land masses and oceans are in a state of constant change. Earthquakes and volcanoes reveal the titanic forces still active within the planet, while on its surface the oceans and a protective atmosphere help to shape the land and provide an environment within which life can flourish. During a period of climate change it is even more important that we understand the processes at work in and on our planet to make sure it remains a good place to live for generations to come.

What controls the weather ?

Human beings have watched helpless and bewildered for thousands of years as they were confronted with many different weather conditions. However, people have always tried to discover the laws that govern the weather. At first they relied on rural folklore, which has often proved to be surprisingly accurate. Later on, scientists who kept regular records of the weather recognised some of its basic mechanisms and that the force driving all of it is the Sun.

How are vapour trails formed ?

The long white strips often seen criss-crossing the sky are caused by the exhausts of jet engines, and are made up mainly of water vapour. In addition to water vapour, aircraft engines emit what are known as aerosol particles, and these act as nuclei around which the water vapour can condense. As the water vapour rapidly expands and cools, ice crystals are formed and it is these that make the vapour trails so easy to spot from the ground. For vapour trails to form, the temperature of the air needs to be about -40°C, so the aircraft responsible must therefore be flying at altitudes of at least 10 km or more. At this height, the atmosphere absorbs so little vapour that the water quickly condenses and freezes into small ice crystals.

Vapour trails are thought to contribute to global warming. They prevent some of the Sun's rays from reaching the Earth. and also block heat flowing from the Earth back into space.

Why are there so many different cloud forms ?

Clouds occur mainly (but not exclusively) in the troposphere, the Cbottom layer of the Earth's atmosphere, at an altitude of up to 18 km. Within this band there is a constant interplay of wind, solar radiation, air pressure, condensation and evaporation, all of which results in great variety of cloud types.

Clouds may form small piles or be scattered across wide areas by the wind. In most areas, certain types of cloud seem to predominate but, in principle, all kinds of cloud formation can be seen almost anywhere on Earth. The study of clouds is known as nephology and forms part of meteorology.

From light and fluffy to black and threatening Approximately 27 types of cloud can be found distributed across all the layers of the atmosphere. They range from small, fleecy white clouds to the threatening, dark mountains we see when storms loom. All clouds consist mostly of ice crystals and water droplets.

What causes climate change ?

The Earth's climate has undergone many changes. Ice ages have alternated with long periods of warm weather, and events like volcanic eruptions and meteorite strikes have also caused the climate to change. Today, scientists know that human beings have a major influence on the climate. The next few decades will see the planet warming by at least 2°C, caused by the burning of fossil fuels like coal, gas and oil and the resulting emission of greenhouse gases. In order to distinguish between climate change brought about by natural events and that caused by human behaviour, it is crucial to look back at the history of the Earth's climate.

Where do weather balloons fly and why ?

Tethered weather balloons are filled with helium, a lightweight, inert gas, which enables them to rise to altitudes of up to 30 km. At heights greater than that the gas expands to such an extent that there is a risk the balloon will burst. Weather balloons carry numerous instruments aloft with them to measure temperature, atmospheric pressure, humidity and even the composition of gases in the atmosphere at different altitudes. A radio module transmits the data back to meteorologists on the ground, who then use the information to draw up a verical profile of the atmosphere. The data is not only useful to weather forecasters, it also allows scientists to better understand the atmosphere, and particularly the effects that humans may be having on factors such as ozone concentrations and the build-up of so-called greenhouse gases. Once all the data has been collected, the weather balloon can be pulled back to Earth using its anchor line.

How do oceans influence the weather and climate ?

Without the oceans, the climate on Earth would be quite different and certainly very much drier. Furthermore, if it were not for these large bodies of water, it is unlikely that any life could exist on the planet. Scientists don't yet fully understand the interaction between the oceans and climate, and, although the larger connections between the climate and the water cycle are now well understood, there are still many questions that remain unanswered. However, we do know for certain that it is not just the water itself that is important, but also the powerful ocean currents that supply life-giving warmth to entire continents.

What do we know about the effects of the weather ?

Lightning, dazzling displays of colour and devastating hurricanes - the weather can sometimes put on quite a show. Many phenomena remained a puzzle for a long time, and were often attributed to divine intervention. The actual causes involve a complex interplay of Sun, wind, the atmosphere and the nature of the Earth's surface. It is a surprising fact that explanations for some very rare phenomena, such as ball lightning, have only recently been discovered.

What causes thunderstorms ?

Rumbling thunder and flashes of lightning are the central features of thunderstorms. About 2000 of these violent weather events are occurring at any one time around the world. The principal requirement for the development of a thunderstorm is that there should be major temperature variations in the layers of the atmosphere. Warm moist air close to the ground rises vertically into colder atmospheric layers, especially after long periods of fine weather. This causes first droplets of water and then ice crystals to form. Friction develops between the ice crystals in the turbulent air currents, and water molecules bearing different electrical charges become separated. If this charge separation is sufficiently great it can create huge potential differences between areas that can amount to several tens of millions of volts. When a discharge occurs, the result is a flash of lightning. The roll of thunder is caused by the sudden rise in temperature and the explosive expansion of air along the path taken by the lightning.

Why are thunderclouds dark?

The colour and brightness of clouds is mostly determined by sunlight and water. Finely distributed water droplets or ice crystals absorb, diffuse and reflect light. If a cloud is impregnated with an exceptionally large amount of moisture that it is about to be released as rain, little sunlight can penetrate it and the cloud appears to be darker than usual. This is what happens with thunderclouds, which are known to meteorologists as cumulonimbus clouds.

These clouds often take on an anvil shape, formed by strong updraughts in the storm zone. However, unlike many other types of cloud, thunderstorm cells break up very quickly, usually after about an hour. Very large cumulonimbus clouds, or supercells, are an exception. They can produce tornadoes, large hail and flash flooding and affect the overall weather patterns for much longer than normal thunderstorms.

Where and when do most thunderstorms occur? 

Days during which observers at a weather station record at least one roll of thunder are defined as thunderstorm days. In Central European latitudes, meteorologists record up to 30 thunderstorm days per year, with the summer months of July and August seeing on average five times as many thunderstorms as during winter months.

Across the world, the frequency of thunderstorms varies widely. In the Midwest of the USA - an area popularly known as Tornado Alley - thunderstorms occur very frequently, and in tropical rainforests they occur almost daily. In the Polar regions, on the other hand. a peal of thunder is a rare event.

Can lightning be used in technical applications ?

The amount of energy produced by the dazzling displays of light Tthat are visible over such great distances may seem huge, but because lightning lasts for such a short time, a single bolt doesn't produce a large amount of usable energy. Despite this, scientists are looking for ways of producing cheap, environmentally friendly power from the 300 000 or so bolts of lightning that strike the Earth every hour-so far without success. However, the plasma from the hot charged particles which make up each lightning flash can be used to develop new kinds of surface coatings. This type of controlled plasma - a quasi-lightning bolt produced and captured in the laboratory may also form the basis for future fusion reactors.

Why is lightning attracted to metal?

A bolt of lightning is not attracted to metal, but instead is trying to find the easiest way to balance a voltage difference. Lightning therefore seeks out materials which offer the least electrical resistance. This is why metal - which is a good conductor of electricity-is struck more often than insulators, such as dry wood.

People in open spaces run the same risk of being struck by lightning as a metal rod does in the same place. This is because the shape and position of a lightning conductor is more important than the material it is made of. Anything that protrudes from the ground is more likely to be struck by lightning because of the shorter distance required to reach the discharge point.

How is a tornado formed ?

A tornado is a compact and often devastating whirlwind. In the complex build-up to a fully fledged tornado, warm, humid air spirals upwards beneath a large thundercloud. The rotation of air about the tornado's central axis becomes faster and faster, like that of figure skater. Finally, a funnel appears on the underside of the cloud which gradually descends towards the Earth's surface. This creates a vortex, and as soon as this column of rotating air touches the ground, everything in its path is hurled skyward. Scientists still don't know exactly what the mechanisms are that lead to the creation of a tornado. While their effects are usually limited to relatively small areas, tornadoes can form in almost any part of the world where heavy rain and thunderstorms occur. In the USA they occur most frequently in an area of the Midwest known as Tornado Alley, and it is here that they have been most intensively researched. In Tornado Alley, scientists can observe between 500 and 600 tornadoes each year. Because of the publicity given to American storms, the term tornado has gradually replaced the European term whirlwind.

What lies beneath the Earth's surface ?

The interior of our planet is far from solid. Masses of liquid rock are in constant movement beaneath its solid crust. The force that keeps the interior in constant motion is the heat of the planet's core, which exceeds 5000°C. So far, researchers have only been able to bore about 12 km into the Earth's outermost crust, but fortunately earthquakes and seismic waves provide a surprisingly precise picture of the Earth's interior, which is layered, like an onion.

Will the continents eventually get back together one day ?

When the German scientist Alfred Wegener published his theory of continental drift some 90 years ago, it caused a major controversy. We now know that he was right, and that the Earth's great landmasses really do move. About 200 million years ago, they came together to form Pangaea, the supercontinent. This later broke up, and gradually the continents arranged themselves into the pattern we see now.

However, the processes that cause continental drift are still active. In approximately 100 million years' time Antarctica will have moved into warmer areas and become ice-free. In 250 million years, all the continents will have drifted back together again to create a new supercontinent, Pangaea Ultima. Scientists believe that there will be a vast and very hot desert at its centre.

Why do the continents move?

Most wood is not as dense as water and so can float on its surface. It is the same with the continents. Although they are largely composed of granite, they actually float - in this case not on water, but on the molten rock of the Earth's mantle. The density of the molten matter is 3.5 g/cm³, considerably more than that of granite at 2.7 g/cm³. The ocean floors, which are also part of the Earth's crust, float on the molten mantle as well. Since they have a density of about 3 g/cm³, somewhat greater than that of the continents, they sink deeper into the molten rock. However, the tectonic plates don't just float, they also move. The reason for this lies in the Earth's mantle, which is almost 3000 km deep. Temperatures at its base are several thousand degrees, causing currents to rise slowly towards the surface. As they do so, they gradually cool and sink back into the depths. This creates a giant conveyor belt of circulating molten rock which provides the force that moves the continents.

How fast do the continents move?

The speed at which continental plates move is not constant and changes over the course of time. It also varies from plate to plate. Under normal circumstances, a plate can move at a rate of anywhere between 2 and 10 cm/year.

Where is the Earth's magnetic field located ?

In principle, we can detect the Earth's magnetic field all around the planet. In simple terms, the Earth has a north and a south pole, just like a small bar magnet. For this reason, the needle of a compass will point in a north-south direction, with the arrow pointing northwards, and this will happen anywhere on Earth, apart from areas close to the poles. However, because opposite poles attract, the geographic North Pole is actually the magnetic south pole and vice versa. On closer examination, we find that the magnetic field deviates slightly from the geographic north-south axis - at present it tilts away from the Earth's rotational axis by about 11.5°. To make navigation more accurate, maps usually show the small correction that needs to be made to take into account the local difference between magnetic and geographic north.

Why are tsunamis so dangerous ?

Tsunamis tend to be completely unexpected when they come crashing ashore. In most cases they are caused by distant earthquakes on the seabed, so they can appear suddenly, even on the calmest, sunniest days. With mountainous waves as high as 10 to 50 m, they can devastate coastal areas in a matter of moments. Even observant locals have no more than a few minutes to escape after they notice the typical sign of an approaching tsunami - water receding from the seashore, sometimes by several hundred metres. It is very difficult to set up an effective early warning system because in the open sea the waves are only a few centimetres high and don't reach their full, fatal size until they enter shallow waters close to the shore.

The last major tsunami occurred on 17 July 2006 following an underwater earthquake off the coast of the Indonesian island of Java, claiming 700 lives. The tsunami on 26 December 2004 was also triggered by an underwater earthquake and killed at least 231 000 people in eight Asian countries. This makes it one of the worst tsunami catastrophes ever recorded.

Are tsunamis only caused by earthquakes?

The majority of tsunamis are caused by earthquakes on the ocean floor, where they are known as seaquakes. For a tsunami to develop, the quake needs to be at least a magnitude 7 and must cause the ocean floor to suddenly shift vertically. Fortunately, only about one in every 100 earthquakes produces a tsunami.

Volcanic eruptions and sudden landslides in coastal regions can also unleash tsunamis. More rarely, meteorite strikes and even atomic bomb explosions can be responsible for triggering this phenomenon. The largest tsunami ever observed occurred in 1958 in an Alaskan fjord. There the waves were funnelled into great walls of water between 150 m and 520 m high.

How fast do tsunamis move?

The enormous speed of a tsunami is what makes it so dangerous. It allows barely any time for detection, or to warn imperilled coastal populations so that they have enough time to leave the area for higher ground. The speed of a wave is determined by the depth of the sea. The deeper the water, the faster the tsunami. Typically, the Pacific Ocean is about 5000 m deep, and waves travelling across it can reach speeds of about 800 km/h. Therefore a tsunami may take only a few hours to travel thousands of kilometres and cross entire oceans. By way of contrast, waves produced by even the severest storms rarely travel faster than 100 km/h. As water close to a coast becomes shallower, the tsunami wave slows down. At the same time its wavelength - the distance between successive crests - gets shorter and the wave begins to grow rapidly in height. Just for the record, the water that makes up a tsunami - and any other ocean wave for that matter-does not actually move through the ocean. Instead, the water molecules pass their momentum on to adjacent molecules.

Where do tsunamis occur most frequently ?

Tsunamis tend to happen most frequently in earthquake-prone areas. Most tsunamis occur in the Pacific Ocean, because this ocean is encircled by a belt of volcanoes, known as the Ring of Fire. Here, on the edges of the continental plates that surround the Pacific, earthquakes and volcanic eruptions are common.

Are any other coastlines under threat?

Not only are coastlines of the Pacific Ocean at risk, but most others also. Perhaps tsunamis do not occur in the Atlantic or Indian Ocean as frequently, but this makes them even more dangerous as the people are caught completely unawares. In the Atlantic, a likely starting point for a tsunami would be the Canary Islands off the west coast of Africa. These volcanic islands are subject to earthquakes that have triggered landslides in the past that set the sea in motion. About 300 000 years ago, part of the Canary island of El Hierro fell in to the sea and sent a tsunami racing across the Atlantic. When it reached the east coast of what is now the USA, it hurled rocks as big as houses several hundreds of metres inland. Scientists now warn that an eruption on the island of La Palma in the Canaries could cause a landslide resulting in a tsunami. The last major Atlantic tsunami, following a magnitude 8.7 earthquake, devastated the city of Lisbon, Portugal, in1755, killing more than 100 000 people. Active volcanic islands in the Caribbean arc have similar potential to generate tsunamis.

Even coastlines of smaller seas, such as the Mediterranean, are not completely safe. The Messina-Reggio magnitude 7.1 earthquake of 1908 generated a tsunami that devastated the towns of Italy's Messina Straits.

How reliable are tsunami early warning systems? 

Ever since the disaster of 26 December 2004 in the Indian Ocean, scientists have been seeking ways to warn local populations in time. The Indian Ocean nations have agreed to install an early warning system - a network of special microphones known as geophones which can detect seaquakes acoustically. The readings can then be carried electronically to any endangered coastline at a faster rate than the tsunami can travel. This should provide people with valuable time in which to prepare.

UN scientists are currently building a warning system off the coast of Sumatra in Indonesia. The network of sensors with its special buoys and satellite technology should help to avert tragedy, but one of the greatest challenges is to avoid false alarms, since not every earthquake causes a tsunami.

Why is the deep sea still such a mystery ?

It is probably true to say that we know less about the depths of the ocean than we do about the surface of the Moon. This is because it is very difficult to penetrate the cold, dark depths of the sea. It is only in the last 30 years that increasing numbers of diving expeditions have ventured into these last, unexplored regions of our planet. Scientists are constantly making surprising finds, discovering everything from amazing geological formations to previously unimagined forms of life.

How does the world look from the seabed?

The deep sea is an extreme habitat - it is cold and dark and the pressure is exceptionally high. Just surviving in such conditions is very difficult, which is why human beings have been so slow at discovering more about this world kilometres below the surface. For now, the region still belongs exclusively to the fantastic creatures that have adapted to this very hostile environment over millions of years. Some deep-sea fish, for example, have neither eyes nor gas bladders (also known as swim bladders). This is because they would not be able to see anything in the dark anyway, and a gas bladder would be damaged by the high pressure.

How deep does the light of the Sun penetrate the ocean?

Sunlight can only penetrate to a maximum of 300 m below the surface of the water. It therefore becomes darker and darker as you descend until total darkness reigns. The average depth of the world's oceans is a little over 4000 m, which means that most of these habitats are permanently pitch black. The reason for this is that rays of light in water spread out, refract and scatter differently to the way they do in the air. Colours under water are hard to distinguish, with red disappearing at 5 m, orange at 10 and yellow at 15 m. However, some deep-sea dwellers have developed ways of producing their own light, using a phenomenon known as bioluminescence. Angler fish belong to this group and have an appendage like a fishing rod, tipped by a luminous lure, in front of their mouths. This attracts small fish, which are snapped up as soon as they come within reach.

What causes marine luminescence?

When strange patterns of blue-green light appear on the surface of the water, the cause is generally marine luminescence. The creature usually responsible is the large, single-cell plankton Dinoflagellatum noctiluca miliaris. Marine luminescence is particularly impressive on dark nights when there is a new Moon. It is created by a chemical reaction between two enzymes. If the plankton is disturbed by the movement of a wave, a passing ship or a swimmer, the two enzymes react with oxygen from the air to produce a bluish-green light.

Is it true that more ships sink in the Bermuda triangle than anywhere else ?

Some people claim that an above-average number of ships and aircraft have gone missing in the Bermuda Triangle. This is debatable, and many of the numerous attempts to explain the supposed phenomenon belong in the realms of fantasy.

If there is indeed something to explain, scientists have come up with a fairly plausible theory as to why ships might be lost without trace in this area. On the seabed in the Bermuda area, there are deposits of methane hydrate, a gas which forms into ice. If deposits are damaged by an earthquake, large bubbles of gas could make their way to the surface where they might cause a sudden and dramatic reduction in the density of the water, causing ships to lose buoyancy and sink.

According to another theory, electromagnetic fields might be disturbing the navigational instruments on ships, causing them to lose their bearings in bad weather. However, no electromagnetic irregularities have so far been registered in the area.