My Science School

 

Why does snow fall?

Snow forms when water vapour inside a cloud freezes into tiny crystals. The water vapour usually crystallizes around a tiny dust particle called a nucleus, and the snow crystal continues to grow. Crystals of snow clump together to form snowflakes, which fall from the clouds. As many as 100 snow crystals group together to form a 2.5 cm snowflake. Snow crystals are very beautiful in form. They are flat and six-sided, and can grow in the shape of elaborate stars. 

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What is the snow line?

The snow line marks out the area above which a mountain is permanently covered with snow. Because it gets colder as you move higher up, the snow will never thaw on a high mountain. However, the snow line moves higher up during the summer when some of the snow melts. The snow line is close to ground level in cold regions, but it is very high near the Equator where the air is much warmer. 

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What is a tsunami?

Tsunamis are tidal waves that are often caused by an undersea earthquake. Usually there is some warning of a tsunami because scientists can detect the seismic waves caused by the earthquake. The tsunami rushes along at up to 970 km/h, building into a wall of water 30 m high as it approaches land. Tsunamis are most common in earthquake zones, particularly around Japan.

Another form of tsunami is called a storm surge, in which giant waves are whipped up by a storm. In 1970 a storm surge and cyclone hit Bangladesh, killing 266,000 people. It returned again in 1985, killing another 10,000 people. 

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How fast can the wind blow?

The fastest wind speed ever recorded was measured on a mountain-top in New Hampshire, USA. It reached 597 km/h. However, the wind inside a tornado probably blows much faster. Wind speed is measured according to the Beaufort scale, which was invented in 1805 by a British admiral. On this scale, the strength of the wind is measured by a series of numbers from 0 to 12. Wind speed 0 means that the air is calm wind speed 9 is a gale strong enough to damage houses. A wind of 12 on the Beaufort scale means a hurricane of over 118 km/h.

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How big are hurricanes?

Hurricanes can measure between 320 and 480 km across. They travel at speeds of 16 to 24 km/h, growing larger and stronger as they move. Usually they travel west, and then swing east as they reach cooler regions, before gradually dying out. Repeated hurricanes occur during the storm season, and there may be as many as 15 in a single year. In the USA the National Weather Service tracks hurricanes coming out of the Caribbean and the Gulf of Mexico.lt forecasts the time when they will reach land and the strength of the accompanying wind. 

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The eye of storm

What would you see inside the centre of a hurricane? If you stood in this area, called the eye of the hurricane, you would be in for a surprise. After the violent winds and torrential rain pass over, the wind suddenly drops. A wall of dark churning clouds, hundreds of metres high, surrounds you. There are light breezes and the Sun shines brightly. But you don’t have long to enjoy this peaceful sunny weather, because as the hurricane continues its destructive path it carries the eye with it. You will soon be enveloped in the storm once again. 

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How are hurricanes formed?

Hurricanes are powerful tropical storms that can cause serious damage. They form near the Equator, where warm, moist air rises, drawing in cooler air below. The air moves in a spiral, eventually forming a whirling mass. Rising warm air continues to cool and drop down in the centre of the spiral to repeat the process. The winds inside a hurricane travel at speeds up to 200 km/h. Hurricanes form in the Atlantic Ocean, passing over the West Indies and the southern United States. Similar storms in the Pacific Ocean are called typhoons, whereas in the Indian Ocean they are called cyclones. 

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Which world cities are at risk from flooding?

Many of the world’s cities are low lying and threatened by flooding. Bangkok, in Thailand, and Venice, in Italy, are typical old cities built by water because they relied on shipping. Both cities are built on mud and are gradually sinking, increasing the risk of flooding. Sea levels around the world are predicted to rise during this century, with the possibility of devastating flooding in places. Many cities in the Netherlands are below or at sea level. They are always in danger of flooding although elaborate defences help to protect them. The Thames Barrage has been built to prevent high tides flooding the city of London. 

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What are tornadoes?

Tornadoes are violent destructive whirlwinds whose force is concentrated into a much smaller area than that of a hurricane. They are very common in the central United States, where they cause enormous damage. Tornadoes travel across the land at high speed, and the roaring noise they make can be heard 40 km away. A tornado happens when huge masses of clouds moving in different directions meet. The air begins to spin in a spiral, and a funnel of twisting air reaches out towards the ground. The low pressure in the funnel sucks up soil, dust and anything else it touches — even cattle.

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What causes floods?

Flooding occurs when water cannot drain away fast enough in the rivers. In areas of non-porous rock, water runs off the land very quickly and streams and rivers soon overflow. Flooding also happens when winter snows thaw in spring. Huge floods cover parts of Siberia every spring, when snow melts while the rivers are still iced up. Low-lying coastal lands are vulnerable to flooding, especially when gales and high tides cause water to flow inland. Low-lying Bangladesh is particularly liable to this kind of flooding. In addition, melting snow in the Himalayan Mountains adds huge amounts of water to Bangladesh’s rivers, increasing the flood risk. 

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Where were the worst floods?

Some of the most terrible floods known were in China along the Hwang-he, or Yellow River. The river burst its banks in 1931, killing more than 3.5 million people. For centuries, dykes have been built to prevent flooding, but these burst when the water flow is too great. Huge dams and sluice gates have been installed to reduce the risk of flooding. 

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Can sand dunes move?

Sand dunes can move slowly, causing deserts to spread over more fertile land. Continuous winds blow the sand to form crescent-shaped dunes. Sand from the top of the dune is blown farther away, and it gradually collects to form a new dune.

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How can people live in extreme climates?

Over millions of years, the human body has altered to suit the climate of the regions we inhabit. In general the hotter the region, the darker the skin of its inhabitants. Also, many people from Asia have a fold of skin in their eyelid to protect them from strong sunlight. People living in hot climates do not need a fat layer to keep warm, so they are usually slim. The Inuits of the Arctic, however, are mostly shorter and stockier to help conserve heat. 

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Which are the wettest places on the Earth?

Tropical rainforests are among the wettest places in the world. In general, most rainfall occurs on the sides of hills and mountains. The wettest place of all is Cherrapunji, in Assam, India. It faces the full force of the monsoon winds that sweep in from the Indian Ocean in July. Their warm, damp air rises in the Himalayan foothills, causing torrential rain. Mount Wai-‘ale-‘ale in Hawaii has the wet days each year. It rains on 350 days on average, and the annual rainfall is the highest recorded anywhere.

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What is permafrost?

Permafrost is a layer of ice and frozen soil that never thaws. It lies beneath nearly one-quarter of the Earth’s surface, throughout Alaska, Canada and Russia. Sometimes the soil in these places is frozen to a depth of up to 1,500 m. The surface layers may melt enough in summer for plants to grow, but the soil beneath is permanently frozen.

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Are there places where nothing can live?

Hot volcanic lava is probably the only place on the Earth’s surface where there is no life at all. Living things have evolved to survive in the harshest environments. Tiny bacteria can be found everywhere on the Earth, even buried in thick ice at the poles or in the very deepest parts of the ocean. 

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Where are the most extreme temperatures found?

Libya and the Antarctic have recorded the most extreme temperatures. The hottest shade temperature was in Libya in 1922, when the temperature in the Sahara desert reached 58 °C. Temperatures nearly as high as this were recorded in Death Valley in the USA in 1913. The coldest ever recorded temperature was in Antarctica in 1983, when Russian scientists measured a temperature low of - 89.2°C.

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The desert process

Deserts were once green and fertile areas, until a climate change altered them permanently. Just a small reduction in rainfall causes plants to die off. Without plant roots to bind and nourish the soil, the land gradually becomes barren. Soon the animals move away, and only desert remains. Very few deserts are completely barren, and plants and animals have evolved to live in the driest conditions, conserving their body water so they do not need to rely on rainfall.

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Are there cold deserts?

Antarctica is the biggest cold desert in the world. All of its water is locked up in ice and snow, so nothing can grow. The Gobi desert in Mongolia and western China is also very cold in the winter, when temperatures drop below freezing. However, it is hot in the summer.

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What is a mirage?

Mirages form in hot deserts, where the air is so hot it bends and distorts light rays. The shimmering images that a mirage produces have often tricked travellers in deserts. People think that they can see a town or oasis on the horizon, but in reality there is none. 

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Where is the world’s driest place?

The world’s driest place is the Atacama Desert in Chile. It is a narrow strip between the Andes and the Pacific, where the first rain for 400 years fell in 1971. Like other hot deserts, the Atacama lies in a region where air pressure is constantly high, with little air movement or cloud. Rainfall is very low in other deserts. Near Cairo, Egypt, annual rainfall averages about 28 mm each year, while in Bahrain, on the edge of the Arabian Desert, there is as much as 81 mm of rainfall. The rain may come in a single heavy storm, and some years there is no rainfall at all. 

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How much of the world is covered by desert?

About one-third of the world’s land surface is covered by desert. Desserts are found wherever there is too little water to allow much plant life to grow. This lack of vegetation leaves large areas of soil exposed. The largest desert in the world is the Sahara in Africa. It has an area of about 9 million sq km. 

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What are ‘highs’ and ‘lows’?

Air pressure varies across different parts of the Earth’s surface, and this difference causes winds. Air moves from an area of high pressure, or an anticyclone, to an area of low pressure, or a depression. Depressions are usually associated with worsening weather conditions and rain. These changes in air pressure can be measured by an instrument called a barometer. In a mercury barometer the air pressure pushes down on the mercury, which is forced up the barometer to give an accurate reading. 

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Why do we get thunder and lightning?

Lightning is a huge spark of electricity that is produced in a cloud. Thunder is the loud noise made by the lightning as it rips through the air.

During thunderstorms, enormous electrical charges build up inside a cloud. Eventually the charges seem to flow down to the ground when lightning strikes. In fact, the electrical charges flow up from the ground and down from the cloud at the same time. The tremendous heat generated by the lightning causes the explosive noise of thunder. Lightning frequently strikes in the same place repeatedly, and along the same path through the air. It can also strike between two clouds.

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Thunderbolts

A lightning strike discharges about 100 million volts of electricity, and heats the air in its path to more than 33,000°C.The lightning strike travels at 299,792 km per second, which is almost the speed of light. A strike between a cloud and the ground may be 14 km long, and a strike between clouds can be over 140 km long.

Ball lightning is a small fiery ball which occasionally appears during thunderstorms. There is no accepted scientific explanation for it. 

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How can forecasters predict the weather?

Meteorologists use a wide range of instruments and techniques to help them track changes in the weather and to predict future weather conditions. Weather forecasters have used simple instruments such as thermometers, rain gauges, barometers and wind gauges for many years, but the advent of satellite photography has transformed weather forecasting. Weather satellites can track the movements of clouds, and show the positions of high and low pressure areas and weather fronts. Radar measures the size, speed and direction of storms, so accurate warnings of severe weather can be given. With the use of computers, increasingly accurate forecasting is now possible.

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What causes mist and fog?

Tiny water droplets condensing from moist air cause both mist and fog. They can occur at ground level. The air can only hold a limited amount of water vapour. If the air suddenly cools, its capacity to hold water is reduced, resulting in mist or fog. When fog occurs visibility can be affected. Mist is less dense. It commonly occurs on calm, clear nights when heat rises, forming a thin layer of mist close to the ground. 

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How are hailstones formed?

Hail is produced when particles of ice bounce up and down inside a cloud. In cold temperatures, water droplets inside a cloud will freeze into small pellets of ice. As these begin to fall, they may meet warm air rising, which carries them back up into the cloud. There they cool once more, and the process is repeated. As more and more water freezes onto their surface, the ice pellets gradually grow in size. Eventually they become so heavy that they fall to the ground as hailstones. Some hailstones are as big as a clenched fist and can do serious damage to buildings and livestock. 

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What is a monsoon?

A monsoon is a season of very heavy rain falling in tropical countries. They are most common near the Equator, when seasonal winds spring up, carrying moist air in from the sea. Important crops such as rice depend entirely on the monsoon to provide the right growing conditions. If the monsoon rains fail, famine commonly follows.

 

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What is smog?

Smog is a serious form of air pollution that can cause breathing problems. The term smog was first used to describe a type of air pollution in London, when smoke from coal fires combined with fog to produce dense yellow clouds that sometimes persisted for days. The term now describes the hazy fog that is caused mainly by vehicle exhausts and smoke from factories. It builds up when there is little air movement, and can happen when a layer of warm air traps cooler air near the ground, preventing the smog from being blown away. Cities in bowls or sheltered valleys are particularly liable to this condition. 

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Who studies the weather?

People called meteorologists study the world’s weather and the changes in the atmosphere that produce our weather. They observe and analyze the weather, and try to understand the processes that lead to changing weather conditions. The study of weather is known as meteorology. 

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Why does rain fall?

Water evaporates from the Earth’s surface, condenses into water droplets and falls back down to the ground as rain. Heat from the Sun causes water to evaporate from the land, from rivers and lakes and from the sea. The water vapour rises with the warm air, and eventually reaches high altitudes where it cools and condenses into water droplets, forming clouds. The droplets grow larger until they are too heavy to stay in the cloud and they fall as rain. Rain usually falls from the largest and darkest clouds, which contain the most water vapour. 

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