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School was over for the day. Will watched his friends open their umbrellas and head home.

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Will had forgotten his umbrella. "It's a good thing Dad can pick me up today.

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The weather is terrible!" he thought and sat on a bench in the hallway. "I wonder why."

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Will's science class was studying weather, so he started to read the chapter about storms while he waited.

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After only a few paragraphs, he fell asleep.

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A gust of wind made Will open his eyes. All around him was the wide-open sky!

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Glancing down, he realized he was in the basket of a hot air balloon, high above the ground.

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Suddenly Will's phone beeped loudly. He pulled QT out of his pocket and said, "Hi! The people down there look like bugs!"

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"But they'll get bigger if we start to drop," QT warned.

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"Quick, pull the silver handles down to start the burners!"

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As soon as Will pulled the handles, huge flames shot out above him.

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"I don't want to burn the balloon!" he yelled over the burners' noise, letting go of the handles.

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An image of a hot air balloon lit up QT's screen.

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"We need the burners to heat the air in the balloon.

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Hot air is lighter than the cold air outside the balloon.

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That makes the balloon float," QT explained.

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"Hot air rising is very powerful. It also creates the weather, you know!"

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"Really?" Will asked. "How does it create weather?"

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"The sun heats the planet, but some areas get hotter than others.

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So the air in the hot regions rises, and cooler air moves in to take its place. We call this air movement wind."

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"So that's what makes wind!" Will watched a cloud pass by. "Does hot air make clouds too?"

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"Not really. When the sun heats water, some of those particles evaporate.

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That means they turn into a gas, called water vapor.

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The water vapor rises with the hot air. But the air cools with time.

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When it's cold enough, the water vapor condenses, meaning it becomes liquid water again.

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But the tiny droplets are so light that they float, forming clouds."

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Will pointed to a low, gray cloud in the distance. "That's water?"

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"Yes. The cloud looks dark and gray because the water is blocking the sunlight.

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That feathery one high above us is actually made of ice crystals, not water, because it's much colder higher up," QT explained.

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Gazing at the clouds, Will commented, "I like the ones that look like cotton balls best. Their shapes are always changing."

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"That's exactly what makes them dangerous! That harmless-looking, little cloud can quickly become a huge thundercloud, over 12,000 meters tall!

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Look to your left, and you'll see what I mean."

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Will turned and saw a tall, dark cloud in the distance. "Wow! That's huge."

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An image of a cloud appeared on QT's screen.

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"Thunderclouds form when there's a lot of warm, humid air.

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First a puffy, little cloud appears, like that one.

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If the cloud stays warmer than the air around it, it grows taller; warm air keeps moving up and up.

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The water droplets combine with other droplets until they get too big and heavy. Then they fall to the ground.

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Snow and hail also fall when they get too heavy," QT explained.

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"It's raining over there! I hope we don't get wet," Will said. "What makes it snow?"

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"Snowflakes form when a cloud's temperature is below freezing, so water vapor turns directly into ice without becoming drops of water first.

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The ice crystals give each snowflake a unique shape," QT answered.

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"Hail is another kind of precipitation. It forms in thunderclouds.

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Water droplets are carried by wind from warmer parts of the cloud into colder parts.

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The droplets freeze, and when they fall back through the warm parts of the cloud,

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the little balls of ice are covered by a layer of water condensing around them.

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If another wind carries the ice up again, that layer will freeze, making the hail bigger.

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After many cycles of this, hail can become as big as a baseball!"

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"I hope that thundercloud doesn't create such big hail," Will muttered, nervously watching the dark cloud. "It's coming this way!"

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"We'd better land. We don't want to be up here if that cloud produces lightning!

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Pull on that cord to release some hot air," QT instructed.

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Will pulled on the cord, and a vent at the top of the balloon opened. They began to descend slowly.

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"What causes lightning?" Will asked.

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"Well, when rain starts to fall from a thundercloud, it pulls some air down with it.

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The falling air and water rubs against the rising air and water, which creates electric charges.

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Lightning allows this energy to flow. The same basic thing happens when you touch a metal doorknob and feel a shock."

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Suddenly a lightning bolt escaped from the cloud, and thunder soon followed.

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Will tried not to seem scared, asking, "Why do we usually hear thunder after lightning?"

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"When lightning strikes, it heats the air around it to thirty thousand degrees Celsius!

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That makes the air expand very quickly.

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As it expands, it pushes the air around it, which creates a sound wave—thunder.

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We see lightning first because light travels much faster than sound."

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"I heard that you can measure how far away lightning is by counting the seconds between the lightning and thunder.

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If three seconds pass, the lightning struck about a kilometer away," Will said.

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"That's right, but we're almost to the ground! Get ready to land!"

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Will grabbed the side of the basket and closed his eyes. The basket shook as it touched the ground.

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Will opened one eye and saw his dad standing over him, gently shaking his shoulder. "Time to go, sleepyhead," he said.

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"I hope it's still storming," Will said as he got up. "Storms are really amazing, Dad!"