Kate: Welcome to the Sound of Science on WNIJ, I’m Kate Powers from NIU STEM Outreach and I have a question for Jeremy Benson. Jeremy, with cooler weather on its way I think Ellie’s question is very pertinent right now. Ellie wants to know why wind blows.
Jeremy: That's a great question because wind does seem to come out of nowhere. A good way to begin is with a smaller example of wind. I want everyone to think about being in your home on a hot day with the air conditioning on. Now think about the attic in the house. If you open the door to the hot attic you would feel the cold air from the house rush past you up the stairs into the hot attic. This is exactly how wind works.
K: I think I sorta get it, but what does the attic represent? There are no levels outside.
J: Well, before we talk about levels I want to remind you that hot air rises. When you heat a gas, you energize the gas particles and the particles spread out as far as possible. Now, this makes hot air less dense than cold air. Density of gas particles also relates to the pressure of the gas. Hot air is less dense and exerts less pressure; there are fewer particles of hot gas in a given space than there would be of cold gas in the same space.
K: Okay, so hot air raises and has lower pressure than cold air. That makes sense, but where does wind come into it?
J: Well, the sun heats the earth unevenly. As the air warms and raises, leaving behind pockets of low pressure, cooler air from another location (say from above a lake) rushes in to fill the low pressure.
K: Oh, I see! The air that is rushing in to fill the low-pressure areas is wind!
J: Exactly. And depending on how large these low-pressure systems are determines how windy -- and for how long -- an area is windy.
K: Why does air want to fill in the areas of low pressure though?
J: Well, that is due to the law of diffusion. If there are two areas, one of high pressure and one of low pressure, the pressures will begin to interact to even out the two areas. And when the areas of pressures are always changing and are very large areas (like our earth!) the different areas are always trying to even out.
J: Remember to keep your great science questions coming to us at email@example.com. This is the Sound of Science on WNIJ.
K: Where you learn something new every day.