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Translations of Encyclopedia about Geology

 

Windstorms

Windstorms are violent, generally hurricane-type air turbulences, which occur mostly in the tropic regions. These tropical storms reach high speeds. Their names change according to the individual tropical region. In the Caribbean and the region of the West Indies, they are called hurricanes. In the Bay of Bengal, it is a cyclone, in the Japanese and Chinese waters they are called typhoon, while in Australia their name is willy-willy. However, their damaging, catastrophic force is the same in all the affected coastal regions.

Tropical storms form over the tropical seas, where the surface temperature of the ocean is at least 27 degrees Celsius. Their characteristic is a spiralling wind vortex moving at a high speed of up to 300 kilometres per hour.

Evaporation results in the formation of an enormous cloud mass (cumulonimbus). The streaming wind produces a rotating motion around the cloudless centre, the so-called eye of the hurricane. In the northern hemisphere, the storm travels first to northwest, later it often changes its direction to the northeast; in the southern hemisphere, it travels first to the southwest, turning later to the southeast. This rotating impulse does not exist directly at the equator and therefore no hurricanes occur there.

The rising air rotates towards the outside, while the air in the centre falls, is heated and thus loses its moisture. The so-called eye of the hurricane is its rotation axis, and its diameter may reach 50 kilometres. The hurricane proper may be as large as 1000 kilometres or more.

These storms may have wind gusts of more than 300 km/h. They fall on the land bringing tremendous amounts of water. Wind and water have catastrophic impact on the land. Houses are scattered by the wind vortex as if they were toy building blocks, giant waves flood entire regions, and then the big, heavy clouds release masses of rain. In densely populated areas of southeast Asia, thousands of people lose their lives every year, while hundreds of thousands lose their simple homes.

A hurricane loses its strength relatively fast when it makes a landfall and continues inland, because it loses the supply of water vapour. Some of the recent hurricanes were andrew, which in 1992 destroyed more than 2000 buildings and killed 40 people in the region of the Bahamas, in the Gulf of Mexico; in 1997, hurricane Pauline took 210 lives on the Pacific coast of southern Mexico; and in 1998, Mitch destroyed homes of several hundred thousand people in Central America.

The greenhouse effect supports the formation of tropical hurricanes. Nowadays, larger and larger areas of the ocean surface reach the critical temperature of 27 degrees Celsius. In addition, the oceans are warming up faster, which results in formation of hurricanes before their usual, late summer season.

Tornadoes form generally in the middle latitudes. They form in storm clouds and often occur in combination with the hurricanes, especially in May and June. A tornado forms when cold, dry air slides over warm and humid wind. This produces a strong turbulence.

This is followed by winds, which start rotating, first gently, then the forming vortexes coalesce, producing a pressure drop in the column. This faster and faster rotating column may reach the ground, causing considerable damages. Nothing can withstand the force of wind that streams across the surface of the earth, following an unpredictable path, and reaches up to 500 km/h.

A tornado cuts a path only through a limited section of the landscape. Its short duration, generally only a few minutes, as well as its smaller size, usually less than 100 metres in width, prevent the destruction of large areas. The region where most of the world tornadoes occur is the Midwest of the United States, with 700 to 800 tornadoes a year. In Germany, tornadoes are very infrequent (only about 60 since the end of the Second World War). More common here is their "younger brother," the whirlwind.

In contrast to tropical cyclones, which are also called hurricanes or typhoons, in the middle latitudes we find windstorms that do not have such a great energy capacity. They are the extra-tropical cyclones.

These are great areas of low pressure moving to the east. They affect weather of large regions for a prolonged period of time. They form on the polar front, which is the boundary of air masses separating the warm tropical air and the cool polar air. In this zone, the colliding air masses exchange the heat and create balance in the atmosphere.

When these two different air masses meet, they produce warm and cold fronts. Simultaneously, they move around each other and around the centre of the low pressure, in either a spiral, or as waves. Warm air then rises and slides over the cold air of the warm front, producing clouds. First, these are cirrus clouds, developing later into cirrostratus and altostratus clouds. Finally, small amounts of precipitation form in nimbostratus clouds.

The warm air wedged between the fronts contains only light clouds. In the now joined gliding phase, the cold front catches up with the warm front. The residual wedge of warm air is compressed and pushed upwards. The cold air occupies the entire region, while the warm air above acts like a cap. The clouds rise together with this formation and release the remaining precipitation. When all this air rises up, the clouds, as well as the storm surface fronts, are released. This cycle repeats itself for several days.

At that time, the cyclones move with the so-called beam current, which lies above them and guides them towards the northeast, at a speed of 40 to 55 kilometres per hour alongside the polar front. Ultimately, they turn towards the equator, slow down, and dissipate.

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