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Old Wednesday, September 09, 2009
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Default Weather

ATMOSPHERE

Every time you take a breath, you are inhaling Earth’s atmosphere. You cannot see, smell, or taste Earth’s atmosphere. It is the air all around you. Other planets also have an atmosphere. An atmosphere is a blanket of gases that wraps around a planet or any other object in space.

EARTH’S ATMOSPHERE IS AIR

Earth’s atmosphere is made up of a mix of gases called air. Air contains more nitrogen than any other gas. Nitrogen makes up 78 percent of the air. Oxygen, the gas that is most important for keeping you alive, makes up 21 percent. Earth is the only planet to have so much oxygen in its atmosphere. Water vapor and other gases are also present in small amounts in Earth’s atmosphere.

The pull of gravity holds the atmosphere in place. Without gravity, the air in Earth’s atmosphere would float off into space. Gravity is the force that also keeps you from floating away from Earth.

THE WEIGHT OF AIR

Air has weight. You cannot feel the weight of air, but all the air in the atmosphere presses downward. This weight is called atmospheric pressure. Atmospheric pressure depends on how much gas is in the atmosphere. The higher you go, the less air there is and the lower the atmospheric pressure gets. The atmosphere is heaviest and the atmospheric pressure highest close to Earth.

Differences in air temperature close to Earth form areas of high and low pressure. Warm air is light and rises upward. It makes low-pressure areas. Cold air is heavy and sinks. It makes high-pressure areas.

WEATHER AND THE ATMOSPHERE

Air in the atmosphere is always moving. You can feel air blowing on your face. You can see air scattering autumn leaves and making tree branches sway. Moving air is called wind. The wind blows where areas of high and low atmospheric pressure meet. As warm air rises, cold air rushes in to take its place.

Big areas of high and low atmospheric pressure cause storms. Thunderstorms often occur where big areas of high and low pressure come together. Huge thunderclouds form in these places.

Water vapor in the atmosphere makes clouds. Water vapor is a gas. As the gas cools, it turns to liquid water. The water falls to Earth as rain or snow.

A LAYER CAKE OF AIR

Earth’s atmosphere extends about 6,000 miles (9,600 kilometers) above the surface of Earth, where we live. You can think of the atmosphere as having several layers. Most of our weather comes from winds, temperature changes, and water vapor in the layer nearest Earth’s surface. This layer is called the troposphere. Most of the clouds you see in the sky are floating in the troposphere.

The stratosphere is the layer above the troposphere. Jet airplanes fly in the stratosphere because there are few clouds up so high and the ride is usually less bumpy. Earth’s ozone layer is in the stratosphere. The ozone layer absorbs, or soaks up, harmful rays from the Sun. These harmful rays would probably destroy life if they reached Earth’s surface.

The atmosphere gets thinner and thinner in the next layers up, the mesosphere and thermosphere. The top layer of Earth’s atmosphere is the exosphere. The atmosphere ends here, about 6,000 miles (9,600 kilometers) above Earth’s surface. The thin air here gradually merges with outer space.

ATMOSPHERES ON OTHER PLANETS

Any planet that has gas around it has an atmosphere. Mercury, the planet closest to the Sun has almost no atmosphere. Pluto, the planet farthest from the Sun, is so cold that sometimes its atmosphere freezes. The gases in Pluto’s atmosphere turn to ice.

There are colorful bands of clouds in the atmospheres of some planets. A gas called methane makes the atmospheres of Neptune and Uranus look smooth and blue. Jupiter has a swirling storm in its atmosphere called the Great Red Spot that may be like a giant hurricane.
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Default Clouds

CLOUDS

Have you ever played the cloud game? You lie on the grass and look up at the clouds. You try to see different shapes in the clouds. One cloud might look like the head of a horse. Another might look like a car. You watch as the clouds move across the sky. You watch the clouds change shape as they move.

WHAT ARE CLOUDS MADE OF?

Clouds are made of small drops of water or tiny pieces of ice. The water and ice in clouds come from Earth’s surface, especially from lakes, rivers, and oceans. Water on the surface evaporates. It changes into a gas called water vapor. Water vapor goes up into the air.

The higher up you go, the colder the air gets. When water vapor gets cold, it changes back into drops of water. If it gets cold enough, the water changes to ice.

The water droplets and bits of ice are so small that they float on the wind. You see the water and ice in the sky as clouds. Strong winds and gentle breezes make different kinds of clouds.

WHAT ARE THE DIFFERENT KINDS OF CLOUDS?

Clouds that form in calm air look like flat sheets or layers of sheets. Clouds that form in strong winds tower up high in the sky.

Meteorologists (scientists who study weather) group clouds into four families. Three families are called high clouds, middle clouds, and low clouds. These three families form at three different levels in the sky. The fourth family is made up of towering clouds that go up through the three levels. Meteorologists have grouped about 100 types of clouds into these four families.

HIGH CLOUDS

High clouds are made of bits of ice. High clouds can be 5 miles (8 kilometers) or more above Earth’s surface. There are three main types of high clouds. Cirrus clouds are thin and feathery looking. Cirrostratus clouds look like a misty, flat veil or sheet. Cirrocumulus clouds look like groups of small, white cotton balls. Rain and snow do not fall from these clouds.

MIDDLE CLOUDS

Middle clouds are made of water droplets. They form about 2 to 4 miles (about 3 to 6 kilometers) up in the sky. There are two main types of middle clouds. Altostratus clouds are like a gray or bluish sheet of frosted glass across the sky. You can see through them, but they make the Sun and the Moon look blurry. Altocumulus clouds look like big wads of white fuzz.

LOW CLOUDS

Low clouds are made of water droplets. Most low clouds are less than a mile (1.6 kilometers) high. There are three main kinds of low clouds. Stratocumulus clouds are soft, gray rolls of clouds. They often cover the whole sky. Stratocumulus clouds are not very thick, so blue sky sometimes peeks through the clouds. Nimbostratus clouds are thick, dark, and shapeless. They are gloomy-looking clouds. Rain or snow usually falls from these clouds. Stratus clouds are really fog up high. They look like gray blankets. These clouds often break up, letting you see blue sky beyond.

TOWERING CLOUDS

Towering clouds stretch through the low, middle, and high cloud layers. There are two main kinds of clouds in this group. Cumulus clouds have flat bottoms and rounded tops that look a little like cauliflower. Cumulonimbus clouds are dark, heavy-looking clouds. They rise up like mountains high into the sky. Sometimes the tops of these clouds spread out at the top. The top is made of ice. Cumulonimbus clouds are also called thunderheads. These clouds bring thunderstorms and sudden, heavy rains
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Default Erosion

EROSION

Why is river water sometimes brown? The water turns brown when it’s full of mud. A brown river is an example of erosion in action. Flowing water carries away, or erodes, tiny bits of dirt and rock from the ground it passes over. Erosion moves rock and dirt from place to place. Earth’s surface can be greatly changed by erosion. Over tremendously long periods of time, erosion can carry whole mountains into the sea!

EROSION STARTS WITH WEATHERING

The dirt and pieces of rock that erosion carries away come from weathering. Weathering is the process by which nature breaks rocks up. Heat from the Sun can make rock expand and crack into pieces. Ice can also break up rock. Water gets into cracks in the rock and freezes. Ice expands when it freezes and breaks the rock. Plant roots can also grow into rocks and crack them. Rainwater can dissolve, or absorb, some rocks, turning them into liquid. Rainwater can turn other rocks into sand or clay.

Once rocks break into pieces, erosion takes over. Water, wind, and ice carry away the pieces left behind by weathering.

EROSION CAUSED BY WATER

Rainwater runs down hillsides and carries dirt with it. Over time, flowing water can carry away so much rock and dirt that it cuts into the ground and forms a channel. The Colorado River eroded a huge cut in the rock of the southwestern United States called the Grand Canyon.

Ocean waves crash against seashores. The waves take sand away from some beaches and pile it up on other beaches.

EROSION CAUSED BY WIND

Wind is the main cause of erosion in deserts. There are few plants to hold the dry dirt in place. Wind blows away loose sand and dry dirt.

Windstorms blew away a lot of dirt on the Great Plains of the United States in the 1930s. There had been no rain for years, and the dirt was dry. The farm areas destroyed by wind were called the Dust Bowl.

EROSION CAUSED BY ICE

Glaciers are huge sheets of ice. Glaciers move slowly across the land. Ice underneath the glacier picks up and carries away sand, dirt, and pebbles. The sand, dirt, and pebbles grind away more rock as the glacier moves.

HOW DOES EROSION CHANGE EARTH?

Water flowing through rivers can carve out deep canyons. The water carries sand and dirt to other places. The Mississippi River dumps tons of dirt where it empties into the Gulf of Mexico. The dirt eventually piles up to create land where once there was ocean. This process makes rich farmland called the Mississippi Delta.

Wind can blow away sand and dirt leaving bare rock. It can pile up the sand it erodes to make hills called sand dunes.

Glaciers can make big changes in Earth. During the last ice age, glaciers covered much of North America. Glaciers carved out many lakes. They made the five Great Lakes.
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Default Hurricanes

HURRICANES

A hurricane is coming! Boards are nailed over big store windows. Families pack their cars and leave town. A hurricane is a powerful storm with pouring rains and strong winds. Hurricanes can do a lot of damage. They cause floods. They blow down trees. Windows break and roofs fly off of houses.

WHERE DO HURRICANES COME FROM?

Hurricanes start over the ocean. They form over warm water near the equator. The equator is an imaginary line that goes around the middle of the planet.

Some hurricanes start in the North Atlantic Ocean, the Caribbean Sea, and the Gulf of Mexico. These hurricanes can strike the East Coast or Gulf Coast of the United States. Hurricanes generally do not hit the West Coast, and rarely make it as far north as Canada. In the western Pacific Ocean, hurricanes are called typhoons. Hurricanes near Australia and India are called tropical cyclones.

The winds of a hurricane blow in a huge circle. There is a calm place called the eye in the center of the circle. All around the eye are thick, tall clouds. Heavy rain pours down from these clouds.

Hurricanes move across the water. They grow stronger if they stay over warm water. Some hurricanes head for land. Hurricanes can do a lot of damage if they come up on land. But hurricanes grow weaker the farther inland they go. They also grow weaker over cold water.

HOW STRONG ARE HURRICANES?

Weather scientists called meteorologists study hurricanes. They rank hurricanes in categories from 1 to 5. The weakest hurricane is a Category 1. It has winds that blow at least 74 miles per hour (120 kilometers per hour). The strongest hurricane is a Category 5. It has winds that blow at more than 155 miles per hour (250 kilometers per hour).

The strongest hurricane of the 20th century was Hurricane Gilbert. It hit the island of Jamaica and parts of Mexico in 1988. Gilbert had winds that gusted up to 218 miles per hour (350 kilometers per hour).

One of the most destructive hurricanes in recent history was Mitch, which killed about 18,000 people in Central America in 1998. Another destructive hurricane, Katrina, happened in 2005. Katrina killed about 1,300 people. It destroyed tens of thousands of homes in Louisiana, Mississippi, and Alabama.

CAN HURRICANES BE PREDICTED?

Meteorologists watch for hurricanes at sea during summer and autumn. When a hurricane forms out in the ocean they watch it very carefully. They use radar, weather satellites, and instruments in the sea. They try to figure out which way the hurricane is traveling. Sometimes special airplanes fly into the hurricane to make measurements.

Meteorologists warn people when a hurricane is headed their way. They tell people to board up their windows. They tell people to leave the area. Predicting hurricanes can save many lives.
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Default Rainbow

RAINBOW

Rainbows appear when the Sun is shining and it’s raining at the same time. That doesn’t happen very often. But you don’t have to wait to see rainbows—you can make them. It’s easy. Go out with a spray bottle on a sunny day. Stand with the Sun behind you and spray water into the air. You should see your very own rainbow!

WHAT MAKES A RAINBOW?

A rainbow is made by light bouncing back to you from the insides of raindrops. Ordinary light, like that from a light bulb or from the Sun, is called white light. White light is actually a mixture of seven colors. Light bends when it passes through water. Each color bends a different amount. When white light enters a raindrop, the colors get separated. The white light splits into seven colors that you can see.

These seven colors always appear in the same order: red, orange, yellow, green, blue, indigo (bluish-purple), and violet (purple).

DOUBLE RAINBOWS!

Sometimes you see two rainbows, a bright one inside a faint one. Here’s why double rainbows form. When light enters a raindrop, a lot of it bounces off the back surface of the drop. This light comes back out of the drop and makes a bright rainbow. Some light, however, bounces around inside the raindrop a couple more times before it manages to escape. This smaller amount of light makes the faint second rainbow. Every rainbow is actually a double rainbow, but the bigger one is usually too dim to see.

POTS OF GOLD?

It looks as though rainbows touch the ground at each end. People sometimes say there’s a pot of gold at the end of a rainbow. Unfortunately, you can never get to the end of a rainbow. You can’t see a rainbow when you’re too close to it. The light comes at you from the wrong angle. If you move toward a rainbow it just seems to get farther away and disappear.
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Default Seasons

SEASONS

Spring flowers, summer heat, autumn leaves, and winter snow. These are familiar images of the four seasons. Seasons are not the same everywhere. But for people around the world, the changing seasons help mark the passing of the year.

WHY ARE THERE SEASONS?

We have seasons because Earth is tilted! Earth is tilted on its axis as it orbits around the Sun. The axis is the imaginary line that runs between the North Pole and the South Pole. Earth rotates around this axis.

Earth’s tilt means that for half of the year, the North Pole is tilted toward the Sun. The South Pole is tilted away. The other half of the year, the South Pole is tilted toward the Sun and the North Pole is tilted away. The North Pole is in the Northern Hemisphere, the part of Earth where North America is found. The South Pole is in the Southern Hemisphere.

When the Northern Hemisphere is tilted toward the Sun, it gets more direct sunlight for more time each day than does the Southern Hemisphere. This makes the days longer in the north. The air and the oceans warm in the sunlight, and the temperature goes up. These are the months of spring and summer.

When it’s summer in one hemisphere, it’s winter in the other. When it’s spring in one hemisphere, it’s autumn (fall) in the other.

If Earth wasn’t tilted, the sunlight would be the same all year round. The Sun would rise and set at the same time each day. There would be no seasons. This is what it’s like near the equator, the imaginary line that circles the planet around its middle. Earth’s tilt has only a small effect there, so the seasons change very little.

SOLSTICES

Have you ever heard of a solstice? There are two solstices every year, one in June and one in December. On about June 21, the North Pole is tilted farther toward the Sun than it is at any other time of the year and the South Pole is tilted farther away from the Sun. The June solstice is the beginning of summer in the Northern Hemisphere. The Northern Hemisphere gets more sunlight on the June solstice than on any other day of the year. On the same day, it’s the beginning of winter and the shortest day of the year in the Southern Hemisphere.

On about December 21, the positions of the North and South poles are reversed. The December solstice marks the beginning of summer in the Southern Hemisphere. That’s why Australians can celebrate Christmas sunbathing on the beach! The same day marks the start of winter in the north.

EQUINOXES

Two times each year, day and night are the same length all over the world. These days are called the equinoxes. The equinoxes mark the beginnings of spring and fall. They occur on or about March 21 and September 23.

SEASONS CELEBRATED

Since ancient times, societies have celebrated the solstices and equinoxes to mark the change of the seasons. Traditionally, crops are planted with the arrival of spring. Many crops are harvested with the onset of fall. Both seasons are greeted around the world with festivals and celebration.
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Default Thunder and Lightning

Thunder and Lightning

A thunderstorm is coming. A bolt of lightning flashes across the sky. Thunder rumbles in the distance. The storm comes closer. The lightning bolts get brighter. They light up the clouds. The thunder gets louder. It crashes and roars. Thunder and lightning can be frightening.

WHAT IS LIGHTNING?

Lightning is a big electric spark. Sometimes the spark goes between a cloud and the ground. Sometimes the spark goes between two clouds.

Lightning is caused by a kind of electricity called static electricity. Did you ever feel a shock when you touched a metal doorknob? The shock came from static electricity. You can make static electricity if you scuff your feet across carpet and touch something metal.

Static electricity comes from tiny invisible electric charges. There are positive charges and negative charges. Too much positive or negative charge in different things makes static electricity.

Negative charges can build up in a storm cloud. Positive charges can build up in the ground or in another cloud. Negative and positive charges pull toward each other. Once enough of them build up, the charges jump toward one another. The jumping charges make a big electric spark. The big electric spark is lightning.

WHAT IS THUNDER?

Thunder sounds like an explosion. Lightning causes thunder. The sound comes from air that suddenly gets very hot.

A bolt of lightning can make the air around it as hot as 18,000° Fahrenheit (10,000° Celsius). That’s hotter than the surface of the Sun! The hot air rushes away from the lightning bolt. The air rushing away makes the loud sound of thunder.

You often hear the thunder after you see a bolt of lightning. This is because sound travels slower than light. The farther away the lightning is, the longer it takes for you to hear the thunder. You can tell how far away the lightning is. You can count the seconds between the lightning and the sound of the thunder it makes. Sound travels about 1 mile (about 1.6 kilometers) every 5 seconds. If you count slowly to 5 and hear thunder, the lightning bolt was about 1 mile away. If you count to 15 before you hear thunder, the lightning bolt was 3 miles away.

CAN THUNDER AND LIGHTNING HURT PEOPLE?

Thunder cannot hurt you. Lightning can hurt people. There are things you can do to stay safe during a thunderstorm.

Do not stand under a tree to stay out of the rain. Lightning often strikes trees. Do not stand close to a window or talk on a telephone. Lightning can make windows shatter and can travel through wires.

The best place to be during a thunderstorm is indoors. If you are outdoors, the safest place to be is inside a car.
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Default Tornadoes

Tornadoes

You need to take cover if you ever see a dark cloud that is shaped like a funnel and pointed toward the ground. Warn people that you have seen a tornado nearby. Tornadoes are powerful, swirling winds. Tornadoes can tear roofs off of houses. They can smash brick buildings. Tornadoes can pick up cars and trucks. People can be badly hurt or killed by flying wood, glass, and other things blowing around in a tornado.

WHERE DO TORNADOES COME FROM?

Tornadoes come from big thunderstorms. Tornadoes drop down from the bottoms of storm clouds. Tornado winds blow in a circle. You can’t normally see wind, but you can often see a tornado because its strong winds pick up dust and dirt.

A tornado looks like a big, dark funnel coming from the bottom of a storm cloud. Some tornadoes look like a swinging elephant’s trunk. They sound like a freight train going by. Tornadoes cause damage when the bottom of the funnel touches the ground.

HOW LARGE AND FAST ARE TORNADOES?

Tornadoes can be huge. The biggest tornadoes are almost 1 mile (almost 2 kilometers) across. The average tornado is about 160 feet (about 50 meters) across.

Tornado winds whirl very fast. They whip around at speeds from 75 miles per hour (120 kilometers per hour) to almost 300 miles per hour (500 kilometers per hour).

Sometimes a tornado almost stands still in one place, but tornadoes usually move. They can move forward at speeds up to 70 miles per hour (110 kilometers per hour).

HOW POWERFUL ARE TORNADOES?

It is hard to study tornadoes. Scientists never know quite when or where they will appear, and they move fast.

In 1971, a scientist named Theodore Fujita made up a scale to tell how powerful a tornado is. His Fujita scale ranks tornadoes by how much damage they do. The scale goes from F0 to F5. Weak tornadoes are F0 and F1. Weak tornadoes may damage chimneys and break tree branches. Powerful F5 tornadoes can pick up houses off the ground and strip the bark off of trees.

ARE THERE TORNADOES ALL OVER THE WORLD?

Tornadoes can form anywhere, but they are more common in some places than in others. The United States gets the most tornadoes. Australia also gets many tornadoes. There are more tornadoes in spring than in any other season.

Tornado Alley is a place in the Midwestern United States where many tornadoes form. Tornado Alley goes from parts of Texas northward through parts of South Dakota. Another place that gets many tornadoes is Dixie Alley. It goes from southern Texas to Florida.

WHAT SHOULD YOU DO IF A TORNADO COMES?

The National Weather Service sends out tornado watches and warnings on radio and television. A tornado watch means that tornadoes might form in the next few hours. A tornado warning means that a tornado has touched down, or that someone has seen a funnel cloud. It can also mean that scientists have seen a tornado on a radar screen.

You should find shelter right away if you hear a tornado warning. Go to a basement if you can. If you can’t get to a basement, get into a closet, an inside hallway, or a bathroom in the middle of the house. Stay away from windows. Get out of mobile homes and cars. Tornadoes can blow them away. Get out of auditoriums, gymnasiums, supermarkets, and other buildings with big roofs. The roofs could fall down in a tornado. If you are outside, lie flat in a ditch and cover your head to protect it from wood, glass, and other things flying around in the air.

Scientists are trying to learn more about tornadoes. They want to get better at predicting tornadoes. Better predictions would warn people early. Early warnings about tornadoes could save many lives.
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Default Water Cycle

Water Cycle

Maybe you recycle cans, glass, and paper. Did you know that nature recycles, too? One of the things nature recycles is water. Water goes from the ocean, lakes, and rivers into the air. Water falls from the air as rain or snow. Rain or snow eventually find their way back to the ocean. Nature’s recycling program for water is called the water cycle.

IN STORAGE

The water cycle has four stages: storage, evaporation, precipitation, and runoff. Most of the water on Earth is in the first stage, storage. Water on Earth gets stored in oceans, lakes, rivers, ice, and even underground. The oceans store the majority of this water.

EVAPORATION: INTO THE AIR

Water goes from storage into the atmosphere (the air that surrounds Earth) by a process called evaporation. When water evaporates, it changes from a liquid into an invisible gas. The gas is called water vapor. Water vapor goes up into the atmosphere.

Most of the water that gets into the air evaporates from the surface of the oceans. Water also evaporates from rivers and lakes. Water can also go from ice caps and icebergs into the air. Ice changing into water vapor is called sublimation.

PRECIPITATION: BACK TO EARTH

Water returns to Earth as precipitation—rain, snow, or other moisture. Precipitation requires ice or liquid water. Water vapor can change into tiny ice crystals or drops of water when the air gets cold enough. Clouds are collections of tiny ice crystals or water droplets.

When the ice crystals or drops of water in a cloud get heavy enough, they fall to Earth as precipitation. Rain, snow, sleet, and hail are all forms of precipitation. Most precipitation falls into the oceans and goes right back into storage.

RUNOFF: MOVING AROUND ON LAND

Water that falls on land always flows from high places to lower ones. This flow is called runoff. Precipitation that falls on land runs downhill. The water cuts channels as it flows.

Some water seeps into the ground. It fills cracks between rocks. Underground water also flows from places that are full to places that have less water.

Water from land flows into streams. Streams flow down mountains. Streams join together to make rivers and eventually the water flows into storage in the ocean. Then the water cycle starts all over again.
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Default Weather

WEATHER

Crack! With a flash and a bang, a bolt of lightning splits a tree in half. Hurricanes bring the ocean onto the land and carry houses away. Tornadoes pick up cars and throw them through brick walls. Floods turn roads into rivers. What do all these extreme events have in common? They’re all examples of weather.

Usually the weather is less extreme. Maybe you listen to the weather forecast in the morning. You want to know whether it will be cold enough to wear a sweater or jacket, or warm enough to wear shorts. Weather is important for many things that you do. Hot and cold temperatures, wind, rain, and snow are all part of the weather.

WHAT MAKES WEATHER?

Weather comes from the Earth’s atmosphere and the Sun’s heat. The atmosphere is made of air. Wind is moving air. Heat from the Sun makes the air move around. Hot air rises, leaving a space behind. Cooler air flows into that space. Air is always moving around Earth. The moving air carries clouds along with it.

WHERE DO CLOUDS COME FROM?

Clouds come from air that is warm and full of water vapor. Water vapor is water in the form of a gas. It gets into the air when the Sun heats water in lakes, rivers, and oceans. The water evaporates, or turns to water vapor. As the Sun heats the air, the warmer air rises. But it’s colder higher up, and the water vapor turns to tiny water drops or bits of ice. The water and ice make clouds in the sky. The particles of water or ice are so small that they float in air.

Some clouds are white and fluffy. Some clouds are dark and gray. Some clouds are close to Earth. They lie across the sky in sheets. Some clouds go up high like mountains. Watch out for dark clouds. They could mean a storm is coming.

WHY DOES IT RAIN OR SNOW?

Rain can fall from clouds when the air temperature is above freezing. Sometimes water vapor in clouds cools and becomes raindrops. The drops get bigger and heavier until they fall from the cloud.

If the temperature is below freezing, tiny bits of ice called crystals form in a cloud. The ice crystals can fall. Snowflakes form from ice crystals. The ice crystals stick together to make snowflakes.

Rain and snow are important for life on Earth. Rain brings water to plants so they can grow. Rain and snow put water back in lakes, rivers, and oceans.

WHAT CAUSES A STORM?

Swirling winds and warm and cold air make storms. In some places, the air is warm. In other places, the air is cold. Cold air and warm air running into each other make storms. Weather forecasters call moving cold air a cold front. They call moving warm air a warm front.

Storm clouds can form when fronts meet. Tall clouds make thunderstorms. Rain pours down from the cloud. Sometimes balls of ice called hail come down from the cloud. Lightning flashes across the sky. The spark of lightning going through the air makes the crack of thunder.

WHAT ARE TORNADOES AND HURRICANES?

Sometimes powerful, swirling winds called tornadoes drop down from thunderstorms. Tornadoes can do a lot of damage. They can flatten houses. They can tear trees out of the ground.

Huge storms called hurricanes form over the Atlantic Ocean. They have swirling winds that blow very hard. Sometimes hurricanes reach land. The high winds can do a lot of damage. The heavy rains can cause floods. Hurricanes over the Pacific Ocean are called typhoons.

FORECASTING THE WEATHER

It is not easy to forecast the weather. Meteorologists—scientists who study weather—use many kinds of tools. They study the kinds of clouds that form in the sky. They measure how fast winds are blowing. They send up weather balloons to measure temperature and humidity (moisture in the air) up high. They use satellites to take pictures of clouds and storms swirling around Earth.

All of this information goes into computers. Computer programs tell weather forecasters what might happen. You listen to your radio or television weather forecast to learn whether you should carry an umbrella or wear a jacket.

Sometimes the weather forecast is right and sometimes it is wrong. Meteorologists are always looking for better ways to predict the weather.
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