THIS LATE JUNE MORNING brought bright sunshine, without a cloud in the blue sky. Tom and Julie announced that they wanted to go over to the ocean beach, where the big waves are, and do some boogie boarding. We made a picnic lunch and arrived at the beach just after ten o’clock. The tide was low, and they caught some great rides. Afterwards we ate our sandwiches and drank apple juice, and we laughed about some spectacular wipeouts they took.

Boogie boarding is like surfing, only lying on your stomach.

Do you know how to boogie board? It’s a little like surfing, but the board is only about four feet long. You ride the wave lying on your stomach on the board. To get started, you stand in water up to your waist. When a big wave comes in, you throw yourself on the board and hang on tight. Timing is important. Just as the very top of the wave reaches you, you jump up, slap your belly on the board, and raise your feet up as high as possible. If you drag your feet through the water, it slows the board down and the wave outruns you.

Julie and Tom have gotten really good at boogie boarding, even better than I am. Sometimes they ride almost the length of a soccer field, zipping back and forth on the wave.

NOW WE’RE BACK AT MY HOUSE with some free time before supper. Tom has spent the past couple of hours on my computer, looking for answers to a big question: how does our atmosphere keep us alive and well? He’s been taking a lot of notes, and Julie and I are really curious to find out what he’s learned. Finally she can’t wait any longer.

Julie: “Hey, Tom! What did you learn about the atmosphere?”

Tom: “I’m the Gas Guy!” He giggles and rolls his eyes.

Julie, catching the giggles: “What do you mean, you’re the Gas Guy? You mean you’re full of it!”

Tom: “Yup. No, actually I mean the atmosphere is a mixture of gases. Since I’m the atmosphere expert, that makes me the Gas Guy!”

Julie: “Oh, I get it. Like I’m the Water Woman because I’m learning about the oceans. Well, since you’re that guy, maybe you can tell us what a gas actually is. Water is easy to understand because you can feel it and splash in it and boogie board on it. But gas is invisible. What exactly is it?”

Tom: “Okay, look. Suppose you have a pot of water on the stove. You heat up the water and pretty soon you see steam rising, right? That steam is still water until it disappears. Then it’s a gas! So a gas is just the same stuff as a liquid like water, or even something solid like ice—only it’s in a different form. When water turns into a gas, it’s called water vapor.”

Julie: “So, heating water turns it into a gas called water vapor. Does that mean you could turn the gas back into water again?”

Tom: “As a matter of fact, it does. What I read on the Web says you can reverse the process by cooling the water vapor. Like, if you hold a cold metal lid up above the steam, drops of water will form on the lid and fall back into the pot.”

I suggest that we head for the kitchen stove and try out what Tom’s talking about. Julie finds a pot and fills it with about an inch of water. She turns on the burner. Flame covers the bottom of the pot, and the water begins to warm up. Meanwhile, Tom finds the metal lid for the pot and puts it in the refrigerator to cool down.

Pretty soon we notice steam rising from the pot. “Water into gas!” Julie exclaims.

Tom takes the cold lid from the fridge and holds it over the steam, up high enough so the hot steam doesn’t burn him. Immediately drops of water form on the lid. When he tilts it, they roll down and drip back into the pot.

“Got it,” says Julie. “When the gas cools down, it turns back into water.”

Tom’s face shows the hint of a smile. He’s glad to find such a simple way of showing that a gas is really a liquid in a different form.

Tom checks his notes and goes on. “So the atmosphere is made of different gases. Water vapor is one of them. These gases are wrapped around the Earth like a kind of blanket. Actually …” He thinks for a minute, “… it’s more like five blankets, because the atmosphere has five layers.” He’s carried my laptop into the kitchen and now opens it. “Here, I’ll show you.”

Layers of the Atmosphere
The atmosphere is made up of five different layers.

Julie and I study the layers of the atmosphere in the picture. “I only see four layers, Tom,” she points out. “You said there were five.”

Tom is expecting the question. “The fifth layer is that blue area at the top of the picture,” he explains. “It’s called the exosphere, and it’s more than six hundred miles above the Earth’s surface.”

“Gramps,” Julie asks. “How far up in that picture did we go on our space trip?”

“We were in the exosphere, where Tom just showed us—a little higher than where you see that satellite.”

“That was so cool!” Julie remembers, her eyes sparkling.

Tom has another fact he wants us to know about the layers. “Remember on our trip, when we were coming back down through the atmosphere and it got really bumpy just before we landed?” he reminds us. “That’s because the air in the layer closest to the earth—it’s called the troposphere—is much thicker than the air farther out in space.”

“What do you mean by thicker?” I want to make sure both he and Julie are clear on this. “Do you mean like it’s a longer distance from the top of the layer to the bottom? Or do you mean thicker like a milkshake is thicker than milk?”

“I mean thicker like the milkshake,” Tom answers. “The lower layers of the atmosphere are thicker than the upper layers, because they’re being squished by the weight of the upper layers.”

I have an idea. From under the sink I bring out a big sponge, placing it in front of Julie. “What is this sponge full of?” I ask her.

“Holes, of course,” she answers, like it’s a silly question.

“Well, the upper layers of the atmosphere are like this sponge. There are holes in the layers. That’s what Tom means by thinner. Now press down on the sponge.”

Julie mashes the sponge down with her hand.

“What happened to the holes?”

“They got much smaller.” She presses harder. “I can even make them disappear.”

I glance over at Tom, who likes my demonstration. “See how thick the sponge is now, almost solid?” I say. “You pressed down on the sponge, like the upper layers of atmosphere press down on the lower layers. Scientists have a word for that kind of thick. They call it dense. The sponge is denser when it’s squished than when it’s not. The reason we bounced around at the end of our space flight is because we were bumping into the denser air of the troposphere.”

Julie tosses the sponge up in the air, and Tom snatches it before she can catch it. She keeps right on with her questions.

“You said the atmosphere is like layers of blankets,” she prompts him. “Is that because the atmosphere keeps us warm like blankets do?”

Tom hands the sponge back to her. “That’s right. Some of the heat from the sun gets absorbed by the land and the oceans—kind of like a sponge absorbs water. A little heat bounces off the land and goes back out into space, like light bounces off a mirror. But the blankets of atmosphere do hold a lot of heat in the air close to the Earth.

“If there was no atmosphere, most of that heat would go back out into space. Then it would be too cold to live here, like it is up on Mars.”

Tom pulls up another image on the computer. “Look, here’s another picture that shows how it works.”

What Happens When Sunlight Reaches the Earth
Some of the sun’s energy is trapped by gases inside the Earth’s atmosphere.

He points. “See that reddish area? That’s the atmosphere. And those arrows show how some of the heat gets through to the Earth, and some doesn’t. When it gets close to the ground, the blanket of atmosphere traps a lot of it.”

On the picture the gases of the atmosphere are labeled greenhouse gases. “Why are they called greenhouse gases?” I ask Tom.

“You know how a greenhouse lets sunlight in?” Tom explains. “The air inside the greenhouse gets warmer than the air outside, because the sunlight warms it up and the glass keeps the heat inside from escaping. Our blankets of atmosphere work the same way, keeping heat close to the Earth once it’s come in from the sun.”

Julie’s eyes wander from the computer screen to the window. She can see branches of the oak trees swinging gently in the afternoon breeze. She’s quiet for a minute, thinking. Then she turns back to us.

“Tom,” she says, “yesterday we were talking about the other planets next to Earth in the solar system. I remember you said that Venus is too hot to live on because its atmosphere is so thick. Is it too hot there because so much heat from the sun gets in—but then it gets trapped and can’t get out again?”

“I think so,” Tom answers, looking over at me.

I nod. “Good thinking. The atmosphere blanket around Venus is much denser than the blanket around our Earth. Tom, you were telling us that the main gas in the atmosphere of Venus is carbon dioxide. So what kinds of gases is our own atmosphere made of, Gas Guy?”

Tom checks his notes again. “There are five or six gases in our atmosphere, but only three of them trap the sun’s heat. The biggest heat trap is water vapor, like clouds and stuff. The next biggest is carbon dioxide.”

“Hah!” Julie cries, her eyes flashing. “Water vapor! So water keeps us warm!” She hops out of her chair and does a little dance. “Water Woman rides again!”

I reach out and pull her into my lap. “Okay, smarty pants. If there’s so much water in our atmosphere, where does it come from?”

She pauses, then jumps up again. “The oceans! I bet it comes from the oceans!”

“You may be right,” I say. “And since you’re the Water Woman, it’s up to you to find out.”

Tom is more interested in where carbon dioxide comes from and what it does. He learned in his middle-school science class that this is really important, and he wants to find out why. He offers an idea.

“So … the blanket around Mars is really, really thin, and it’s much too cold to live up there,” he begins. “The atmosphere around Venus is really, really thick, and that planet is too hot to live on. So maybe our blankets of atmosphere around Earth are just about the right thickness. They trap some heat from the sun but not too much. The atmosphere keeps us warm but not too warm— kind of like with Goldilocks and the porridge.

“Just right!” I exclaim, giving him a playful punch on the shoulder. “We don’t want our atmosphere to get too thick, like it is around Venus.”

We’ve spent a lot of time inside on this beautiful afternoon, so I suggest that we go for a walk in the woods to sharpen our appetites for supper. The children like that idea. I close the laptop and lead the way out of the kitchen. But I have one more question for Tom.

“You’ve helped us understand that the atmosphere is made of gases, Gas Guy. And you showed us how carbon dioxide gas is important for keeping Earth’s temperature just right. Do you think you could find out where all that carbon dioxide comes from?”

Out in the hallway Tom and Julie flop down on the floor and start pulling on their sneakers. “Oh, I already learned that,” Tom remarks casually. “I’ll tell you all about it tomorrow.”

Julie wants the last word. “Okay, Gas Guy, you get the carbon dioxide! And I am Water Woman, so I’ll learn everything about water vapor.” She flashes me a big smile.

“We’re the Gaia Gang!” she announces. “Tom’s got the atmosphere, I’ve got the oceans, and you’ve got the Earth’s crust.” Then she chases Tom out the front door.