Vancie asked her horse first, but when she didn’t get an answer she asked me. Where does all this wind come from? That’s okay, I don’t mind being her second choice.
Answers to that question have been the subject of jokes and tall tales since the first puff of wind messed up the first caveman’s beard. I know this, I was there. People have imagined everything from the wings of a million butterflies stirring it up to a vast conspiracy orchestrated by the hair spray industry. Sometime the inhaling and exhaling qualities of surrounding states get the blame.
Where to start with an explanation that even Vancie’s horse will like? Remember that we are really creatures walking around on the bottom of an ocean, an ocean of air. Normally, we don’t usually think of air as anything at all, and a fish probably doesn’t believe that water is anything either. We say a glass full of air is empty, not even half full. When we say “look at that wind,” we really mean look at what the wind is doing to our roof or our carport or our garbage can or the dust; we don’t really see the wind itself. But the ocean of air that we live in is always moving, and that movement is wind.
What starts it moving has to do with air pressure, but air pressure has to do with temperature. Warm air rises and creates low air pressure. If all the warm air rose like a big hot air balloon, we’d be left with a vacuum on the ground and nothing to breathe. Luckily that doesn’t happen because, as one of my science teachers used to say, “Nature hates a vacuum.” So air rushes in to fill the spot where the warmer air rose, and that’s what gets the wind started.
How much wind is a matter of scale. In the sea there are little swirls and big ocean-wide currents. It’s the same in our ocean of air. The equator of the earth is generally warmer than the north and south poles, so air generally moves from the poles to the equator to fill in the blank that would be left if all warm air at the equator rose. But when warm air rises, it eventually cools off and falls again, so we end up with belts of rising and falling and moving air around the earth. One of those belts of moving air, the jet stream, is often responsible for the wind when it comes on strong and lasts for days. The jet stream is a strong high altitude wind, but here in the Tetons we are a high elevation location, so sometimes the jet stream and Teton Valley meet up, or come pretty close. We have very good information about where the jet stream is, how strong it is, and which way it’s wiggling thanks to the large number of aircraft in the sky. This helps forecasters give you a heads up that windy weather is on the way and it’s time to put some extra sand in your small children’s shoes.
Not all wind is caused by the jet stream, but it’s still caused by differences in air pressure, but these are smaller scale differences. It can be something as small as a plowed field or a parking lot that warms up in the sun, and the rising air, prized by hawks and glider pilots, gets replaced by a surface wind blowing toward the hot spot. It can also be higher pressure cold air rolling down from the mountains into the valley creating that cold early morning breeze we often see. But this kind of wind is very local and hard to predict. It may show up as dust devils making things fly in your back yard while not a leaf stirs in your front yard. Which brings up another thing about wind. It hardly ever blows in a straight line. Just like moving water tends to swirl, so does moving air. This can happen at the place where wind speeds and directions change, we know that as wind shear, or when the wind encounters obstacles like mountains or your house and swirls around them like the river does around rocks, turbulence.
We often caution in a forecast that wind could be “higher near storms.” When rising air is moist enough and rises fast enough to cause a thunderstorm, or when warm air is forced up by heavier cold air plowing into it during a cold front passage, that air and rain and hail and whatever else is in it comes back down in a big way. When that downdraft hits the ground, it has nowhere to go but out away from the storm. That’s what they call outflow, and it can be so strong that people swear it was a tornado. It might as well be; outflow winds can be just as strong as some tornadoes. They are sometimes called straight line winds, but like all wind, they swirl around too. It often takes teams of weathermen days to look at the patterns of damage to tell the difference between strong outflow wind and a tornado.
So what causes the wind? The answer changes in size and scale, but it’s the same basic idea. Air moves generally from areas of high pressure to areas of low pressure but lots of things happen to it along the way, and these areas can be as big as a continent or as small as your back yard. And now that can come right from the horse’s mouth.