What is the mistral?


            This is the name given to a wind which blows in the Rhone Valley in France. The wind is caused by an exchange of air between the cold hinterland of France's Central Plateau and the warm Gulf of the Lion in the Mediterranean Sea.



            High Mountain ranges near flat country produce unpleasant winds, especially where the mountains descend to a warm sea. Atmos­pheric pressure is high above the cold mountains, but low above the sea. Air, therefore, flows towards the sea and is not warmed be­cause it has not crossed enough land.



         The north westerly mistral, funneling down the construction of the Rhone Valley, blows at a speed of 30 to 60 m.p.h. on at least 50 days each year. It bursts out on to the Mediterranean coast, filling holidaymakers' caravans with sand and capsizing yachts caught unaware at sea.



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How are rocks formed?


          Sedimentary rocks are formed from the sediment, or broken pieces of the earth's rock structure worn down by weather and erosion. The fragmented pieces become compacted and in time much of it is cemented in to form rock.



          Igneous rocks originate either from volcanic action as molten lava, which hardens, or from the slow cooling of molten masses beneath the earth's surface, which exposed after a volcanic eruption on or after much erosion.



          Many important minerals, not­ably uranium, have been found, enveloped in igneous rocks. The chief sedimentary rocks are sandst­one, shale, dolomite and lime­stone.



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Why do clouds have different shapes?


               Clouds vary in shape according to their height and temperature, and they contain minute drops of water or ice particles or a combination of both. And, of course, their forma­tion is greatly affected by wind changes.



               There are basically three groups of clouds: high clouds between 17,000 and, 45,000 feet (cirrus, cirro-cumulus' and cirro-stratus); middle clouds between 7,000 and 23,000 feet (alto-cumulus, alto­stratus and nimbo-stratus); and low clouds up to 7,000 feet (strato­cumulus, stratus, cumulus and cumulo-nimbus. Their height and temperature decide how much pressure is exerted on them by the atmosphere.



               Finally, the shapes of clouds differ according to the time of day. Towards evening Clouds tend to thin out, rise a little and flatten out.



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When does sheet lightning occur?


               Sheet lightning usually occurs during a storm as the result of a discharge of excess electricity within a single thundercloud. The outline is obscured and the result is a diffused light spread over a large area of the sky in contrast to the vivid spiral or ribbon-like flashes of chain, forked or zigzag lightning. The most favorable conditions for sheet lightning are provided when the electric field is equal throughout the area. Unlike other forms of lightning it does not reach the ground and the channel cannot be distinguished.



               What is often referred to as sheet lightning is merely the light­ing up of the sky by flashes occurring beyond the horizon.



               All lightning is the natural dis­charge of large accumulations of electric charges in the atmosphere. It may take place between neighboring clouds or between cloud and earth. Just before the dis­charge the cloud's electric poten­tial is often built up by the action of falling raindrops or other natural processes.



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What is twilight?


          Twilight is the faint light which appears a little before sunrise and again after sunset before it gets really dark.



          On the moon there is no twilight. Darkness comes suddenly as soon as direct sunlight ceases to reach the moon's surface. This does not happen on earth because of the halo of air, called the atmosphere, which surrounds it When the sun goes below the horizon, its light leaves the earth but is' reflected downward from the upper atmos­phere.



          Poets and writers have written of the evening twilight or “the gloaming”, as it is called in Scot­land - as an enchanted time. Per­haps one reason is that familiar objects become distorted in the half-light and we imagine we are seeing things that are not really there.



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Why do river deltas form?


               A river winding its way down to the sea, from its mountain source, will inevitably choose the lowest land through which to flow. By the time the river approaches the sea the speed at which it is travelling will have decreased considerably, thus allowing the water to drop its load of sediment and other solids.



               These solid particles (alluvium), therefore, form the land pockets which are characteristics of the various branches of a river delta. Owing to the slow pace of a river at this stage, it will wind its -way round any elevated land points rather than go over them.



               Deltas are most likely to form where the sea, into which the river flows, is particularly calm for most of the year. Notable deltas in the world include the Mississippi (the largest) the Ganges, end the Nile. A delta is so called because it is the name of the fourth letter of the Greek alphabet whose shape it resembles.



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When does a geyser occur?


            A geyser occurs when a hot spring erupts, hurling a column of water and steam high into the air. These springs are situated in regions which were formerly volcanic and which have retained considerable heat near the surface.



            They usually have craters with well-like shafts penetrating into the earth. The water which gathers deep down in these shafts becomes heated until the lower part is changed into steam, the pressure of the steam steadily mounts to a point when it suddenly hurls the water above it into the air, sometimes to a height of over 100 feet.



            The chief geyser districts are in Iceland (home of the Great Gey­ser), in the Yellowstone National Park, Wyoming, United States, and in New Zealand. For four years Waimangu in New Zealand, the greatest of all geysers, was capable of spouting jets up to 1,500 feet.



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How can scientists control the weather?


            Since the 1940s scientists have discovered techniques by which. Several weather conditions can be controlled. For example, it is pos­sible' to prevent lightning by using an electrical earth to diffuse the electrical content of a cloud. The American scientist V. J. Schaefer has shown .that it is feasible to produce greater concentrations of ice in clouds than occur under normal conditions.



            Weather experts already are taking advantage of these dis­coveries to increase snowfall on mountains for winter sports, to prevent damaging hailstones and to moderate, or even prevent, the development of dangerous storms. Scientists are now able, in some cases, to make a cloud burst to produce rainfall over parched areas.



            These local efforts may lead the way to large-scale weather con­trol. But before then scientists may' have to learn to cope with the damaging effect of air pollu­tion on weather conditions.



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Why does Holland have so many windmills?

          The large number of windmills in Holland, or The Netherlands, is due to the fact that they were needed to pump water into the canals off the rich, low-lying land reclaimed from the' sea. Windmills are still used for this purpose today, but pump worked by electricity are more usual. 

          There is an Old Dutch saying, "God made the world, but the Dutch made Holland". They certainly did make a great part of their land by dragging it from the sea, and the battle to hold it never ceases, The name Netherlands (from the Dutch Nederland) means low land, and more than one­ third of Holland's land area of 12,530 square miles lies below sea level.



          Along the coast are dunes of sand-nature's dykes-thrown up by normal tides. The Dutch plant them with marram grass, which holds the sand together with its long, strong, creeping roots. Behind the dunes the Dutch built three dykes of close-packed stone, clay and earth on wooden and concrete piles. The dyke nearest the sea is called a "waker' Behind it lies a "dreamer" and behind that again a "sleeper" Some of the dykes are 200-300 feet high and many have a road or, some, a railway running along the top.



          In 1170 the North Sea swept into the country and formed the bay called the Zuyder Zee (South Sea). In 1421, another high tide flowed in to form the Holland’s Diep (Dutch Deep). The great spring tide of 1953 (two feet higher than any previously re­corded) smashed-the waker dykes, overflowed the dreamers and drowned about 1,900 people. About 50,000 were forced to flee from their homes.



          A famous Dutch story tells of a brave boy who stood for hours with his hand thrust into a hole in a dyke and so prevented the sea from rushing in and widening the breach in the wall.



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