My Science School

A hurricane is a huge, whirling circle of wind and rain. But in the centre of all this wind and rain, the air is calm. This calm hole is called the eye of the hurricane.

A hurricane begins over the ocean, near the equator, where the air is very hot, wet, and still. The heated air begins to rise and whirl around. As the great masses of the air rise up, towering rain clouds form.

The winds swirl faster in a giant circle around the centre of calm, warm air. The storm is called a hurricane when its winds are stronger than 119 kilometres per hour.

Some people in the path of a hurricane experience what seems like two storms. First, the front of the circle hits. The blowing wind and rushing rain sound like steady thunder.

After the front of the circle passes over, the eye of the hurricane arrives. The wind dies down. The rain stops. The air grows hot and still. It may take an hour or more for the eye to pass over. Then the back of the circle arrives. Once more, the wind blows and the rain pours. Finally, the hurricane passes, carrying the wind and rain to another place.

Picture Credit : Google

Wind has many different names, depending on how hard it is blowing. In this question, you will find out how to tell how hard the wind is blowing by watching what it does. And you will learn some of the names we give to the different speeds of wind.

1. In calm air, smoke rises straight up. The wind is blowing less than 1.6 kilometres per hour.


2. In a moderate breeze, small branches sway, and dust and paper blow about. Wind speed is 21 to 29 kilometres per hour.


3. In a strong breeze, big branches sway, and umbrellas are difficult to use. Wind speed is 39 to 49 kilometres per hour.


4. In a moderate gale, whole trees sway, and it is hard to walk against the wind. The wind speed is from 51 to 61 kilometres per hour.


5. A fresh gale breaks twigs off trees and makes walking very difficult. Wind speed is 63 to 74 kilometres per hour.


6. A strong gale can blow tiles off roofs and damage buildings. Wind speed is 76 to 87 kilometres per hour.


7. A whole gale will uproot trees and often do much damage to buildings. Wind speed is 89 to 101 kilometres per hour.

Picture Credit : Google

Wind

When the air seems calm, and you can’t feel a wind, it doesn’t mean that the wind has stopped blowing everywhere. There are always many winds keeping the air around the earth moving. In one place, there may be a soft breeze from the north. In another place, there may be strong gusts from the south.

Two different winds may blow near the same area at once. Near the ground, one wind may be pushing flags in one direction. But higher in the sky, another wind may be making the clouds scurry in another direction.

The fastest winds are high above the clouds, several kilometres up into the sky. These winds are called jet streams. Often, jet streams are connected. They form one huge, rushing river of wind that circles the earth, sometimes at speeds of more than 320 kilometres an hour. When an aeroplane takes off for a long trip, the pilot may fly into the jet stream because it can give the plane a powerful push.

Picture Credit : Google

The wind moves over the land. It sways the tall grass in meadows and rustles leaves on trees. It ruffles your hair. In a storm, it whirls and roars. Wind can change a cloudy day into a sunny one by pushing clouds along. Wind is moving air. And it is the sun that makes air move.

The earth spins around like a big top. As it spins, each part of the earth, in turn, comes into the sunlight. The sun’s light warms the earth, and the earth warms the air. The heat makes the molecules of gas in the air move faster and spread apart. This warm air rises up like a big, invisible cloud. It rises because warm air weighs less than cool air.

As the warm air rises, cool air from other places flows in to replace the warm air. This moving cool air is the wind. When you feel the wind blow, you are feeling the movement of cooler air pushing in to take the place of the warm air that rose up into the sky.

What happens to the warm air? It cools and then sinks back to the ground. There it takes the place of warmer air, and all the same changes happen over again!

Picture Credit : Google

Air doesn’t seem to be made of anything. It has no colour, taste, or smell, and you can see right through it. But air is made up of things. It is made of many kinds of gases. And these gases are made up of tiny bits called molecules.

Actually, everything on the earth is made up of molecules. Solid things, such as plants, animals, and rocks, are made of molecules. So are liquids, such as water, and gases, such as the ones in air.

In solids, the molecules are packed close together and hardly move. In liquids, the molecules are further apart and move faster. In gases, the molecules are very far apart and zip about rapidly. That’s why gases are so light that you can’t see them.

For us, the most important gas in the air is oxygen. We breathe to get oxygen into our bodies. Almost every kind of animal and plant in the world must have oxygen, or it will die.

Only about one-fifth of the air is made up of oxygen. Most of the air - nearly four-fifths - is nitrogen. The rest of the air is made up of many different gases. Water vapour and dust float in the air, but they are not part of the air.

If air is just floating gases, why doesn’t it float away into space? Because the earth’s gravity pulls at the air just as it pulls at you. The air can no more float off into space than you can!

If you could stand on the moon and look at the sky, it would always be dark black. This is because the moon has no air. If the earth had no atmosphere, our sky would stay dark, too. Instead, the sky over the earth is dark only at night when the sun doesn’t shine.

Sunlight is made up of red, orange, yellow, green, blue, indigo, and violet light. These colours scatter when they hit the earth’s air. Some of them scatter more than others. Red, orange, and yellow light scatter the least. Blue scatters so much that it spreads to fill the sky.

That line in the distance where the sky seems to meet the earth is called the horizon. When the sun is near the horizon, sunlight must travel further through our atmosphere. This makes blue and most other colours scatter too much to be seen. But red, orange, and yellow are scattered just enough to make a beautiful sunrise or sunset. Dust from pollution, forest fires, and volcanoes can help make a brighter red or orange sunrise or sunset. The dust makes the other colours scatter even more.

 

 

Long ago, people believed the sky was a roof that stretched over the earth. Today, we know that a thick layer of air surrounds the earth like the skin on an orange. But unlike an orange skin, the air moves around the earth, and it reaches far above the earth’s surface. This moving cover of air is a mixture of gases called the earth’s atmosphere.

Air covers the earth everywhere. The pull of gravity holds it there. Near the earth, the air is thick, or heavy. Further away from the earth, the air becomes thinner. Furthest away from the earth’s surface, the air thins and disappears altogether. Where this happens is where space begins!

Many scientists study the ocean, looking into the secrets of the sea. They study how the ocean moves and how it affects the atmosphere. They study the living things in the sea and the shape of the ocean floor.

Scientists who study the ocean are called oceanographers. Sometimes they work aboard ships. Some wear diving suits and air tanks to explore underwater. Others use small submarines. They use underwater cameras to take pictures of the ocean’s floor and the plants and animals that live in the ocean.

Sometimes they use robots to bring up samples of the mud and sand for study. Some scientists study the direction and strength of waves, tides, and currents.

Oceanographers called marine biologists study the plants, fish, and animals that live in the ocean, lakes, and rivers. They keep track of their health and the way they grow.

Oceanographers called seismologists study earthquakes that happen on the ocean floor. One cause of earthquakes is volcano eruptions, so seismologists often keep track of volcanic activity.

Picture Credit : Google

Water from Underground

Not all of the earth’s water is in lakes, ponds, rivers, and oceans. A lot of it is beneath your feet - down in the ground.

Rain falls. Snow melts. Much of the water seeps into the ground. It passes through holes and cracks in the soil until it reaches solid rock. The water can’t trickle any further down, so it spreads out, filling every nook and cranny underground.

The top of this underground water is called the water table. When there is a lot of rain, the water soon fills all the open spaces underground. Then the water table gets higher.

In some places, the water table comes all the way to the top of the ground.

Then, water bubbles out and makes a natural fountain called a spring. Sometimes a spring is the start of a river.

Underground water is usually cool and clean and good to drink. People often dig wells to get this water. There is some underground water almost everywhere in the world - even in deserts. But in a desert, the water is often very, very far down underground.

Picture Credit : Google

You probably know that lakes give us food and drinking water. But did you know that lakes offer transportation and energy sources?

Many lakes are important for fishing. People who live near Lake Titicaca in South America live simply by raising their own food crops and catching fish from the lake, such as trout. Other lakes, like Lake Winnipeg in Canada, support large fishing industries.

Lakes are important for shipping, too. North America’s Great Lakes are connected with each other and to the Atlantic Ocean. Ships from all over the world can use the lakes to bring things to the many large cities around the lakes.

Lake Maracaibo, in northwest Venezuela, the largest lake in South America, has many oil wells in its waters and along its shores. Under the bottom of the Caspian Sea, north of Iran, oil and natural gas have been found.

Finally, lakes are important to wildlife. For example, Lake Baikal in Russia, the deepest lake in the world, is home to many kinds of wildlife found only in that area. These include a fish called the golomyanka and the Baikal seal, one of the few kinds of seals that live in fresh water. Lake Victoria is the largest lake in Africa and the second largest freshwater lake in the world. Flamingos and other birds feed along the edges of the water. Lake Victoria is also known for its many kinds of tropical fish.

A lake is water that has land all around it. Some lakes are so big that we can’t see the other side. Lake Superior in North America is the largest freshwater lake in the world. It spreads over more than 82,100 square kilometres.

Some areas of water called seas are actually lakes because land surrounds them. The Caspian Sea, for example, is the world’s largest saltwater lake. This lake, which lies between Europe and Asia, stretches for 372,000 square kilometres.

Most lakes are just holes in the ground that are filled with water. Glaciers dug many of these holes. Long ago, these huge rivers of ice flowed out of the north and covered many parts of the world. As the gigantic glaciers slid slowly along, they cut out great pits and made valleys wider and deeper. Then, when the glaciers began to melt, the water filled up many of the holes, forming lakes.

Some lakes form when part of the earth caves in, leaving a hole. This happens mostly in places where the ground is limestone. Year after year, rain dissolves away the soft limestone, forming caves and tunnels.

Finally, the tops of these tunnels cave in, leaving what is called a sinkhole. Rain or water from underground springs and streams fills the sinkhole, and it becomes a lake.

Part of a river can also become a lake. Sometimes a river deposits so much mud and sand that the water backs up and forms a natural lake. People may make a lake by building a dam. A dam causes the flowing water to spread out over the river’s banks and form a lake.

Picture Credit : Google

People are shouting, “The water is rising. The river has reached the streets! Get to higher ground. It’s a flood!”

A flood happens when water runs over land that is usually dry. Rivers most often flood. Normally, much of the rain that falls on land runs into the nearest river. Water from melting ice and snow also runs into rivers. So, when there is a long, heavy rain, or lots of melting ice and snow, millions of tonnes of water may pour into a river.

Just as a bath will overflow if you keep running water into it, the river soon spills over its banks and floods the land around it.

Some rivers flood regularly. The people who live near them prepare for floods by piling bags of sand along the riverbanks. This keeps some of the water from spilling over the banks.

Sometimes lakes and seacoasts flood. Hurricanes and other bad storms can cause floods along the seacoast. Their strong winds push great waves far onto the land. Soon, much of the shore is underwater.

Picture Credit : Google

High on a mountain, snow melts. Some of the melted snow trickles down the mountainside, finding the easiest path. It is so narrow you could step across it.

Another trickle of water bubbles out from under a rock from underground water called a spring. This trickle joins the melted snow, making a wider, faster-moving stream. It flows down the mountain increasing speed. More streams or tributaries, come together to form a river.

Soil and stones, carried along by the rushing water year after year; cut a groove into the mountainside. The bottom of this groove is the bed of the river. The high sides of the groove are its banks.

The rushing river hurries to the edge of a cliff in the mountainside and falls in a roaring, tumbling, splashing waterfall.

In a steep place near the bottom of the mountain, the fast-moving river has worn away the soft rock. Only bumps of hard rock are left sticking up as the river swirls and foams around them. This part of the river is called the rapids.

Past the rapids, the land slopes gently, so the river moves more slowly. The river leaves the mountain and flows out onto a plain.

Other rivers from other mountains join the first river. Together they become a great, broad river that winds slowly across the plain on its journey to the ocean. If the river overflows its banks, it leaves behind mud, sand, and silt that form a flat area called a flood plain.

At the edge of the ocean, the river’s mouth is often a sort of dumping place. The river carries soil and sand. If the water is calm at the river’s mouth, the sand and soil sink to the bottom of the riverbed. Over time, they pile up and form tiny islands. The river flows around the islands and splits into branches.

Over time, a large piece of land shaped somewhat like a triangle has built up at the mouth of the river. This land is called a delta.

Those first trickles of melted snow have travelled far from the river’s head high on the mountain to its mouth in the ocean.

Picture Credit : Google

The ocean shapes the land

When water pokes its way into the land, it creates many different kinds of bodies of water and land areas. Here are some words used to describe such places.

A bay is a place where a part of the ocean or a lake pokes into the land. Seen from an aeroplane, a bay often looks as if a giant has taken a big bite out of the land and water has come in to fill the hole.

An inlet is a narrow body of water that pokes into a piece of land or runs between islands. An inlet tends to be finger-shaped.

An area of land that is almost completely surrounded by water is called a peninsula. One part of it connects to the mainland.

When waves knock pieces of rock into the water, sometimes they wash up on a beach or settle under the shallow water along a coast. When many pieces of rock collect, a new strip of land called a sand bar or spit rises from the ocean.

 

The ocean meets the land

Often where the ocean touches the land, whether it’s the edge of a tiny island or the coast of a continent, there is a beach.

A beach is a stretch of sand, pebbles, or rocks. The sea makes beaches. Waves crash into a rocky shore for thousands of years, tossing the rocks around and breaking them into pebbles. Then, for hundreds or thousands of years more, the waves grind the pebbles together. In time, the pebbles are ground into tiny grains of sand. Many lakes make beaches this way, too.