November 24, 2012 9:09 am

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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.
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.
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.
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.
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.
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.
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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