Deserts

Do you think of a desert as a hot, dry place where sand stretches as far as you can see? The Sahara in North Africa is like that, but most deserts are not. All deserts are dry, but some deserts are very cold. Plants and animals live in most deserts. People have learned to live in deserts all over the world.
Most deserts form because of air movements over the planet. Masses of dry air create deserts. Water gets squeezed out of the air as it passes over the mountains, and a desert forms on the other side of the mountains. Deserts can also form along some seacoasts where there are currents of cold water. There are two main bands of desert areas, one north of the equator and one south of the equator. The equator is an imaginary line that goes around Earth’s middle.

TAKING AIR WITH YOU

You can go to places where there is no air. There is no air underwater, but you can dive underwater. You can stay underwater a short time just by holding your breath. Air tanks let you stay underwater for a long time. Scuba divers wear tanks on their backs. The tanks are filled with gases that make up air. The divers breathe the gases through hoses.
There is less and less air the higher up you go. People gasp for breath at the tops of tall mountains. Airplanes must carry air. Once the airplane gets up high, air is pumped into the cabin where passengers sit. Astronauts have to take all the air they need with them—there’s no air in space!

AIR

Take a really deep breath. Feel how your chest gets bigger and bigger. Your chest gets bigger because your lungs are filling up with air. You cannot see air, but air is all around you. You can feel it when the wind blows. Earth’s atmosphere is made of air. An atmosphere is made up of the gases that surround a planet.
Air is a mixture of several different gases. The main gases in air are nitrogen, oxygen, and argon. Air also contains smaller amounts of hydrogen, carbon dioxide, water vapor, helium, and other gases. Oxygen is the most important gas for animals. Animals must breathe oxygen in order to live.
Carbon dioxide is the most important gas for plants. Plants use carbon dioxide and sunlight to make food. Plants give off oxygen. Animals turn the oxygen back into carbon dioxide when they breathe.

COAL

Coal is mostly made up of the element carbon. When carbon burns, it releases a large amount of energy as heat. That’s what makes coal such a useful fuel. Coal is a fossil fuel. That means it comes from the remains of ancient life buried deep in Earth’s crust. The coal we use today started out as plants that grew in swamps millions of years ago. When the plants died, they settled to the swamp bottom. Over time, layers of mud and rock formed. They compressed and hardened the plant material. Heat and pressure caused chemical changes. Gradually, the once-living matter became coal.

REEFS IN DANGER

Scientists fear that many coral reefs are in danger. Many reefs have had a problem called bleaching. This occurs when the zooxanthellae, the algae that live in polyps, die off and the reef changes color. Then the coral polyps die. No one knows what causes bleaching.
A starfish called the Crown of Thorns starfish is also a threat to reefs. Sometimes too many of these starfish suddenly show up on a reef. The starfish kill the coral. No one knows why there are so many starfish.
Pollution is also a threat to coral reefs. Conservationists and governments are working to protect the Great Barrier Reef and other coral reefs.

THREE KINDS OF REEFS

There are three kinds of coral reefs: fringing reefs, barrier reefs, and atolls. All three kinds grow in warm places. Coral reefs cannot form in water colder than about 68° Fahrenheit (about 20° Celsius). Reefs form in clear, shallow water. They need sunlight in order for plants and animals to live there.
Fringing reefs are close to shore. There is no lagoon or other body of water between a fringing reef and the shore. Barrier reefs are farther out in the ocean. There is usually a lagoon between a barrier reef and the shore. Barrier reefs can be huge. The largest group of reefs in the world is Australia’s Great Barrier Reef. It covers thousands of square kilometers. Atolls are really the tops of volcanoes that sank below the sea. The reef grows on the rim of the volcano. Inside the reef is a lagoon.

KINDS OF CORAL

There are hundreds of different kinds of coral. Hard coral or stony coral is the kind of coral that builds reefs. These corals have hard outer skeletons. Other corals do not have hard outer skeletons and look like fans or flowers. Some kinds of coral look like the branches of a tree. Other kinds can sting you if you touch them.

Coral Reef

Imagine you are snorkeling in the warm ocean water around a tropical coral reef. You put on your mask and swim fins. You float around looking at the reef below you. The coral reef looks like strangely shaped rocks sticking up from the ocean floor. Crooked coral that looks like tree limbs twists up from cracks and caves in the reef. A big piece of coral that looks like a brain lies on the bottom. Orange and pink coral shaped like fans wave in the currents. Fish of all different shades and varieties of color swim near the reef.
Coral reefs play an important role in ocean life. Many kinds of plants, fish, and other animals live on and around a coral reef. Even the reef itself is made mostly of tiny coral animals, both living and dead.

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.

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.

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.

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.

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.

COULD WE RUN OUT OF WATER?

Nature recycles water all the time. Water in oceans, lakes, and rivers evaporates, or turns into a gas and rises into the air. The water vapor eventually turns back into a liquid and falls as rain. The water cycle keeps the total amount of water on Earth the same. But most of this water is salt water in the ocean. People need fresh water for drinking and for growing food on farms. Ocean water is too salty to drink. It is too salty to use for watering plants. Some places have more fresh water than others. People who live near big lakes or rivers have more fresh water than people who live in the desert. Places where a lot of rain falls have more fresh water. Places that usually have enough fresh water sometimes have a drought. Very little rain falls during a drought. People run short of water. People everywhere should be careful not to waste water.

DRINKING WATER FOUND ON EARTH

Not all water is safe to drink. Water in lakes and rivers can be polluted. It can have harmful chemicals or germs that cause disease. Long ago, many people living in cities got sick or died from drinking dirty water. Scientists have learned how to clean, or purify, drinking water. They learned that boiling water could kill germs. They learned that adding certain chemicals could kill germs. Engineers learned how to keep dirty water in sewers, away from drinking water. Governments passed laws to keep factories from polluting water. People still get sick from drinking polluted water. Many poor people still do not have pure drinking water.

DRINKING WATER FOUND ON EARTH

Not all water is safe to drink. Water in lakes and rivers can be polluted. It can have harmful chemicals or germs that cause disease. Long ago, many people living in cities got sick or died from drinking dirty water. Scientists have learned how to clean, or purify, drinking water. They learned that boiling water could kill germs. They learned that adding certain chemicals could kill germs. Engineers learned how to keep dirty water in sewers, away from drinking water. Governments passed laws to keep factories from polluting water. People still get sick from drinking polluted water. Many poor people still do not have pure drinking water.

WATER FOUND ON EARTH

Liquid water fills the ocean, lakes, ponds, rivers, and swamps. Water droplets form rain clouds. Liquid water makes beads of dew on the grass. It seeps down into the ground. It fills underground lakes and streams. Ice falls as hail or crystals of snow. Ice forms on ponds and frosty windowpanes. Huge sheets of ice make glaciers and icecaps at the North and South poles. Water vapor is always present in the air. Water vapor makes clouds in the sky. Water vapor makes fog that hangs close to the ground. It is the steam that comes out of a teakettle. v

WATER FOUND ON EARTH

Liquid water fills the ocean, lakes, ponds, rivers, and swamps. Water droplets form rain clouds. Liquid water makes beads of dew on the grass. It seeps down into the ground. It fills underground lakes and streams. Ice falls as hail or crystals of snow. Ice forms on ponds and frosty windowpanes. Huge sheets of ice make glaciers and icecaps at the North and South poles. Water vapor is always present in the air. Water vapor makes clouds in the sky. Water vapor makes fog that hangs close to the ground. It is the steam that comes out of a teakettle. v

WATER

Think about the different ways you use water. You drink water when you are thirsty. You take a bath and wash your clothes with water. You water the grass or other plants. You swim in water. Water pouring over huge dams may even make the electricity that lights up your home. About three-quarters of Earth’s surface is water. Living things are mostly made up of water. Without water, there Wooster is a chemical. Chemists say water is a compound, a combination of different materials. Water is a combination of oxygen and hydrogen atoms. An atom is a tiny bit of matter much too small to see. Water is made of one atom of oxygen and two atoms of hydrogen. The three atoms make the tiniest possible drop of water, called a molecule. Water can be a liquid, a solid, or a gas. Liquid water flows. Solid water is ice. Water in the form of a gas is called water vapor.uld be no life on Earth.

THE SUN’S ENERGY GETS INTO FOOD

The Sun provides the energy that starts the whole process. A plant uses sunlight to make its own food. The energy is stored in the plant. An insect eats the plant. The insect stores the energy in its body. Then the energy is transferred to a fish that eats the insect. When you eat the fish, your body gets the energy stored in that fish. When you eat salad, fruits, and potatoes, you take in the energy stored in these plants. When you eat fish, beef, or chicken, you take in the energy stored in these animals. When you eat your dinner, your body is getting energy that first came from the Sun. You use this energy to do your schoolwork. You use this energy to run and play.

FOOD WEB

A food web is made of many food chains in a community of plants and animals. There are many tiny animals near the beginning of a food web. There are fewer but larger animals higher up in a food web. There are many more insects than fish in a food web. There are also more small fish than big fish. Human beings are at the very top of the food web. No animals are higher up to make us their steady diet. Decomposers play important roles in some food webs. Bacteria and fungi are decomposers. Decomposers eat dead plants and animals and cause them to rot and decay. They also eat animal wastes. They break things down into chemical parts called nutrients. The nutrients go back into the soil. Plants take up water and nutrients to make food. Nutrients move through food webs over and over again.

Food Chain

A tiny insect nibbles on some leaves. The insect skims over a lake. A little fish swimming below spots the insect on top of the water. The fish zooms up and eats the insect. Later, a slightly larger fish eats the little fish. Then, the slightly larger fish gets eaten by a really big fish. You are in a boat on the lake fishing. You catch the big fish, cook it, and eat it for dinner.Food chain is the way energy goes from one living thing to another through food. Plants are the first step in most food chains. Plants use the energy in sunlight to make their own food. Plants store the energy in their leaves and stems. Plants are called primary producers in food chains.Animals eat the plants that use the Sun’s energy to grow. Animals are called consumers. Animals that eat plants are primary consumers. Animals that eat other animals are secondary consumers. Animals store the energy in their bodies. Energy flows from plants to bigger and bigger animals through the steps of eating and being eaten. Each part of the food chain is directly connected to the other, just like the links in a chain.

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.

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.

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 ATMOSPHEREAir 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.

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 AIREarth’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.