Thursday, November 15, 2012

Science Questions From Kids (and Parents): What Makes Wind?

by Matt Shipman, This article originally posted at Carolina Parent.

Do your kids ask you science questions that are difficult to answer? We're tackling one here: What is wind? 

Wind is moving air. Specifically, it is when the air in our "atmosphere" moves – and the atmosphere is what we call all of the air that surrounds us on Earth.

Because air is all around us, and because it's always there, we kind of take it for granted. But air is actually made of stuff – tiny combinations of elements (like oxygen) called "molecules." For example, when plants breathe, they take molecules of carbon dioxide out of the air. And the molecules in the air love to move around – they can't sit still (more on this later).

Here's something that's pretty amazing: all of the tiny, invisible molecules that make up the air have weight. They don't weigh very much (you couldn't put one on your bathroom scale), but their weight adds up, because there are a LOT of molecules in the air that makes up our atmosphere.

Why Air Moves: Weight and Pressure

All of that air is actually pretty heavy, so the air at the bottom of the atmosphere (like the air just above the ground) is getting pressed on by all of the air above it. That pressure pushes the air molecules at the bottom of the atmosphere a lot closer together than the air molecules at the top of the atmosphere.

And, because the air at the top of the atmosphere is pushing down on the air at the bottom of the atmosphere, the air molecules at the bottom REALLY want to spread out. So if there is an area where the air molecules are under high pressure (with a lot of weight pushing down), the air will spread out into areas that are under lower pressure (with less weight pushing down).

Picture a piece of Jello on a plate. If you push down on it with your hand, the Jello smushes out to the sides (where nothing is pushing down on it). Air does the same thing.

How Temperature Comes Into Play

Remember how I said that molecules in air like to move around? Heat speeds that up. The warmer the air, the more the molecules want to move. When the air is warmed up by sunlight, the molecules really get active, which pushes them apart from each other. That means that there are fewer molecules in the same amount of space. Think of it this way: if you had a shoebox full of cold air, it would have way more molecules in it than a shoebox full of warm air.

Areas that have a lot of colder air can pack a lot more molecules in, making the air heavier. The heavier air increases the air pressure on the ground. Areas with warmer air have fewer air molecules, which makes the air lighter, which means they have lower air pressure. So the colder, high-pressure air pushes down on the warmer, lower pressure – which gets shoved out of the way to either side (like the Jello that gets smushed on the table). All of that air moving around is what we call wind.

Thanks to David Hamilton, senior scientist at WeatherPredict Consulting, Inc., for helping me answer this question.

About this contributor: Matt Shipman  is a Raleigh-based science writer, father of three and founder of the First Step Project.

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