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Translations of Encyclopedia about Geology

 

Wind

Wind are in general horizontally moving air currents. Winds originate in the atmosphere as a result of the difference in air pressure as well as due to temperature fluctuation. Coriolis force (the bending or deviating force of the earth's rotation) bends winds in the northern hemisphere to the right, and in the southern hemisphere to the left.

Winds are a consequence of different pressure of neighbouring air masses. Dense air of the area of high pressure flows into the less dense and therefore lighter air of the area of low pressure. When the difference increases, the wind grows stronger. Cold front forms when cold air collides with warm air. Cold air slides under its warm neighbour and pushes it upwards.

Considerable horizontal temperature difference in temperature and air pressure creates zones of strong wind, called jet stream. Jet streams regulate the weather in the upper geographical latitudes of the northern hemisphere. In the upper troposphere, these wind zones are several thousand kilometres long and more than 300 kilometres wide.

The wind direction is indicated by the direction from which the wind is coming. The strength of the wind is indicated in metres per second, in accordance with the 12-level Beaufort scale. The strength or speed of the wind in the lower air layers is lower due to friction. The friction is caused by the contact with forests, mountain ranges, buildings, and other obstacles. Winds are stronger over open spaces, such as oceans or deserts. Winds shape the face of the earth. They reduce the surface of the earth by carrying off sand or snow.

When particles transported by air collide with an obstacle, they work as a sand blasting device. Airborne sand grains can "sand" a rock. When wind diminishes, the transported material forms sediments. When wind carries water particles, it creates waves. At a constant wind speed on the surface, wind creates water currents, such as Gulf Stream.

The energy of the sun determines the weather on the earth. The regions most heated by the sun are the regions around the equator. Hot, humid air masses rise in swirls into the atmosphere. There the air cools down and water vapour condenses into drops, which in the tropics results in heavy downpours. These tropical air masses, however, also move north and south. When this air reaches the troposphere, it is turned away from the pole and at approximately 30 degree latitude begins to descend again. In this zone, the air is exposed to higher pressure and loses its humidity. This dry, warm air creates conditions for the formation of deserts.

Part of the descending air from the high altitude returns as north-easterly trade wind, called passat in some parts of the world (in the southern hemisphere this trade wind is south-easterly) back to the equator. The constant circulation between the equator and 30 degree latitude is called the "Hadley Cell." The name "trade wind" or "passat" comes from an old marine expression meaning "regular path." There is a relatively narrow zone between the trade wind zones, which is called the calm belt. In this zone, there is often no wind. The position of the calm belt changes with the seasons.

Parts of the descending air masses stream as warm wind towards the north, collecting moisture over the ocean. Between the 50 and 70 degree latitudes, this humid, warm air collides with the cold and dry polar air. Areas of low pressure form at this polar front, creating mild weather in the middle latitudes.

Monsoons are constant winds that change their direction every six months. They are found mainly between the latitudes of 30 degrees N and 30 degrees S. Monsoons form because the land and the ocean are heated at a different rate. The larger the land mass, the stronger is the monsoon. The strongest monsoons occur in south and southeast Asia. In the northern hemisphere during winter, dry winds blow from central and northern Asia to the south. The air is heated over the Indian subcontinent and produces dry seasons lasting several months.

In the summer, winter winds are replaced by southerly winds. This summer monsoon blows over the ocean. As a result of the warming, there is a considerable increase of air humidity, producing enormous number of clouds. This ultimately leads to the well[-known, long-lasting heavy monsoon rains. These rains cause regularly flooding.

The highest amount of precipitation in one day, almost 900 millimetres, fell in Cherrapunji, in the Himalayas. That is more than the average annual precipitation in Germany.

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