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Nature Gallery (Earth - Climate)

Monsoons, Tropical Storms and Tornadoes

Without the atmosphere, there would be no life. The cushion of air surrounding the earth protects animals and plants from harmful rays and keeps heat and essential gases from escaping into space. Yet the air in the atmosphere is constantly in motion, and winds blow across the surface of the earth, sometimes with tremendous force.

Colliding cold and warm air masses create storms, resulting in high winds and heavy rains. The violence or duration of these storms can lead to extensive property damage and loss of life. The world’s most significant and disastrous climatic events are monsoons, tropical storms, and tornadoes.

Monsoons

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The monsoons are seasonal winds that bring torrential rains in the summer and sunny and dry weather in the winter. These winds blow in response to differences in temperature between air over the land and air over the sea. The monsoons influence the climates of India, Bangladesh, Southeast Asia, and, to a lesser extent, northern Australia and Central Africa.

The classic and most prominent form of the monsoon climate is to be found on the Indian subcontinent. At the beginning of winter, the subcontinent cools rapidly, while the Indian Ocean is still warm because the temperature of water takes longer to change than does the temperature of land. The warm water heats the air over the oceans, forcing the air to rise. This movement draws cool air from the Himalayas and northern India to the Indian Ocean. These northeast winter monsoon winds bring cool, sunny, and dry weather to India during the winter.

As summer returns, the pattern reverses. The temperature in southern Asia rises faster than that of the Indian Ocean. Some areas of northern and central India reach temperatures of more than 40°C (100°F). Hot air rises over the land, drawing masses of cool, damp air from the ocean towards the land. These southwest winds mark the return of the summer monsoon rains. Moisture blown in by these winds condenses, resulting in sustained, heavy rains, which normally begin in June and last until September.

The Himalayas form a barrier that forces the warm air to drop its moisture over southern Asia. The southern flanks of the Himalayas receive large amounts of precipitation, while the northern slopes receive small amounts. This orographic effect also occurs along the southwestern coast of India as a result of the Western Ghāts Range. Thus, certain areas of India receive tremendous amounts of rain during the summer monsoon season. Cherrapunji, in northeastern India, receives about 10,920 millimetres (430 inches) of rain each year, most of that during the summer.

Summer monsoon rains cause widespread flooding in Bangladesh and other monsoon regions. Many people in these regions build their homes on stilts in preparation for the annual floods. Yet the summer rains are essential for crops, especially after the six months of dry winter monsoon weather. In some years, summer monsoon rains are not plentiful, leading to drought and crop failure.

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Tropical Storms
Tropical storms are large rotating storm systems that form in the Atlantic, Indian, and Pacific oceans. Those in the Atlantic and eastern Pacific are called hurricanes. Those in the western Pacific are known as typhoons, which means “great winds” in Chinese. Those in the Indian Ocean are called cyclones, or—by Australians—willy-willies. monsoons3.gif (143419 bytes)

Hurricanes and typhoons form during the hottest months of the year as moisture evaporates from warm oceans in the calm doldrums near the Equator. The humid air rises, cools, and condenses to form thunderclouds, while a current of air rushes inwards to replace the rising air. As the winds strengthen, they begin to swirl around a low-pressure centre called the eye of the storm. Hurricanes and typhoons rotate anticlockwise in the northern hemisphere and clockwise in the southern, a result of the earth’s rotation. From the edge of the storm towards its centre, the atmospheric pressure drops sharply and wind velocity rises.

Tropical storms drift westwards with the trade winds, gaining strength as they accumulate moist air. Over the ocean, the storms create violent winds, torrential rains, and high seas. The storms begin to dissipate after they reach land, because they are cut off from their source of ocean moisture.

Hurricanes and typhoons can be very damaging to low-lying coastal areas, causing widespread destruction from surging seas, flooded rivers, and high winds. They also cause mass erosion along coastlines, decimating homes and businesses and washing beaches out to sea. Gilbert, the strongest hurricane to strike the western hemisphere in the 20th century, devastated Jamaica and parts of Mexico in 1988 with winds up to 350 kilometres per hour (218 miles per hour).

The diameter of the area affected by winds of destructive force may exceed 240 kilometres (150 miles). Gale winds prevail over a larger area, averaging 480 kilometres (300 miles) in diameter. Within the eye of the storm, which averages 24 kilometres (15 miles) in diameter, the winds stop and the clouds lift, but the seas remain violent.

The strength of a tropical storm is rated from 1 to 5. Category 1 storms, the mildest, have winds of at least 120 kilometres per hour (74 miles per hour). The strongest and rarest, Category 5, have winds that exceed 250 kilometres per hour (155 miles per hour).

In the northern hemisphere, the storms usually travel first in a northwesterly direction. As they drift into the higher latitudes, the storms are turned by the westerlies towards the northeast. In the north Atlantic, hurricanes affect the Caribbean Islands, eastern Mexico, and the southeast United States. Some hurricanes turn further north and travel along the entire east coast of the US and Canada. In the eastern Pacific, hurricanes frequently batter the coast of western Mexico. Typhoons affect Southeast Asia, China, and Japan in the western Pacific. In the north Indian Ocean, cyclones strike India and other countries in southern Asia.

In the southern hemisphere, cyclones generally head southwest and subsequently southeast. The coast of southeastern Africa, Madagascar, northern Australia , Indonesia, and the islands in the southeast Pacific are the areas hit by these cyclones.

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To study hurricanes and typhoons, scientists fly aircraft into storms to measure wind velocities and directions, the location and size of the eye, the pressures within the storms, and their thermal structure. Meteorologists also use radar, sea-based recording devices, and geosynchronous weather satellites. Improved systems of prediction and communication have helped minimize loss of life, but property damage is still heavy, especially in coastal regions.

Tornadoes

Observers report that the roaring sound made by a tornado is like that of many freight trains rushing forwards at great speed. Wind speeds may exceed 400 kilometres per hour (249 miles per hour), although higher speeds have been estimated for extremely strong storms. Tornadoes, although short-lived, are the most violent storms on the planet.

Huge and dark cumulonimbus clouds, created when a cold front meets a mass of moist, warm air, are the breeding ground for tornadoes. These clouds, which form quickly, generate thunderstorms in which the warmer air rises rapidly, creating a powerful updraught. In the upper part of the thunderstorm, strong crosswinds begin to turn the central area of updraught into a swirling vortex. The rotating winds increase the speed of the updraught, pulling more moisture into the storm. The vortex spirals in tighter circles, picking up speed and growing in height through the clouds. Finally, the funnel-shaped tornado descends from the bottom of the cloud and touches the ground with ferocious intensity.

The fierce winds destroy virtually everything in the tornado’s path. In addition to picking up dust, which makes the tornado visible, the strong updraft can lift cars, roofs, and people high into the air. Objects sucked into the tornado, such as uprooted trees and wood debris, become deadly missiles. The path of the tornado can often be detected by the destruction left behind.

A tornado may be a few metres to about a kilometre wide where it touches the ground. It can move over land for short distances or for distances of many kilometres. The duration of a tornado is usually only a few minutes, but strong ones may last over an hour. A single thunderstorm system lasting hours may generate several tornadoes while covering large distances.

Most tornadoes spin anticlockwise in the northern hemisphere and clockwise in the southern, but occasional tornadoes reverse this behaviour. Weaker sea-going tornadoes, called waterspouts, occur most frequently in tropical waters.

Tornadoes are most common and strongest in temperate latitudes, including the United States, western Europe, Japan, India, South Africa, Argentina, and Australia. The most violent tornadoes occur in the central US, where they often form in the early spring. The highest frequency of tornadoes occurs in an area called “Tornado Alley”, which extends from Texas and Oklahoma to Kansas and Iowa.

Although tornadoes are difficult to predict, scientists frequently detect large tornadoes with Doppler radar and warn residents in the area.