But seriously, why does that happen? It's not as if the earth suddenly starts spinning faster on March 1. Something is changing, at exactly this time of the year, which makes the wind pick up. But what is it?
(clip art from Idaho Public Television)
- I found only a couple sites that explained this. But before I could understand exactly what I was reading, I had to learn all over again what wind is.
- The air surrounding our planet is not all at the same temperature. In some places, it's warm, in others it's cold, still others, it's downright hot. The molecules in hot air behave differently than those in cold air.
Molecules on the left are cold. Not moving much. Packed together. Molecules on the right are warm. Moving around a lot. Lots of space in between. Flying high.
(Diagram posted by AtownWxWatcher at AccuWeather.com's blog)
- Think of the molecules in the air like a bunch of little kids. They're all standing around on the playground. Warm up the playground, and they'll start running around. They'll get all giddy and excited. The space between each kid will get bigger and bigger. Just as the kids are running around, excited, putting more space between each other, that's what happens to air molecules as you heat them up. They move faster, spread out, get less dense.
- And when kids get a chance to run around and burn up their energy, they feel less and less pressure. They're happy, they're excited, they're under very little pressure. So, too, when the air heats up, its pressure drops.
- Now, instead of warming up the playground, let's cool it down. In fact, let's get it downright cold. The little kids are going to stop running around. They're going to huddle up next to each other. They're going to press their arms and legs together tight and stick close to each other for warmth. That's what the air molecules do, too, when it's cold. Slow down and stick together.
- When you put a bunch of little kids really close together, they won't like it very much. Packed in like that, they're going to start feeling the pressure. In the same way, the cold air molecules are sticking together at higher pressures.
- So we've got our various pockets of air. In the hot zones, all the little kids are running around excited under very little pressure. And they're right against the pockets of cold air, where all the little kids are packed together and surly and not liking it one bit.
- Now, it's just a fact of physics that when you put two extremes next to each other, they're going to try to balance each other out. (Specifically, according to the second law of thermodynamics, high energy states will move toward low energy states, yah yah yah.) So our excited, high-pressure kids are going to try to mingle with the surly, low-pressure kids. "Hey, you guys," they're going to shout, running over to the cold kids. "What's the matter with you? Come on, let's play! Tag, you're it!"
- Okay, I now have to abandon my metaphor because the air molecules are going to start moving in ways that little kids do not. Like I said, when the hot high-pressure air gets near the cold low-pressure air, the hot air will flow toward the cold to try to balance out the differences in pressure. That gets the air moving, starting up our wind.
Almost exactly what happens when two pockets of different air pressure encounter each other. Except the hot air blows into the cold.
(Art by Johanna)
- Because the pressure in a column of cold air gets lower and lower really fast as you go up the column, the hot air is going to shoot up that column of cold air to try to balance out the pressure. Which makes for more wind.
- Another factor that affects all this is the fact that the earth is rotating. Underneath the air, the earth is spinning. Where the air is closer to the ground, the friction of the moving ground slows down the wind speed. Up higher, where the temperature also happens to be colder and the pressures lower, the wind speed is faster.
- The motion of the earth below the wind also influences the way the air travels around the centers of pressure. Around our hot, low-pressure pockets, the wind turns in one direction; around low, high-pressure centers, it spins the opposite way. Where the two pockets of pressure touch each other, those two windstreams will join up and help to spin each other faster.
- Which way the wind spins around the pressure centers is determined by which hemisphere they happen to be in. In the Northern Hemisphere, the air around the high-pressure pockets spins clockwise and counter-clockwise around low pressure zones. In the Southern Hemisphere, it's exactly the opposite.
How the wind spins in different directions around high pressure versus low pressure centers.
(Diagram from weatherquestions.com)
- Finally, because of the earth's rotation, the wind isn't spinning in nice neat circles around the pressure centers, but is flowing around them in a stream. The streams flow against each other or collide, and any place they come into contact, they're going to try to balance each other out -- more wind.
- So we've got all kinds of things creating wind. But what causes the most noticeable increases in wind speed is when two very different pockets of air pressure and temperature get up next to each other. The hot is going to rush into the cold, and it'll get windy.
- Specifically in March, in the Northern Hemisphere, there's still a lot of cold air hanging around, left over from winter. That cold air can be really dense and cold, same as during a storm in January. However, because the Northern Hemisphere is starting to get more sun, the sunlight is heating the surface of the earth and it's creating more pockets of warm air.
Due to the revolution of the earth around the sun and the earth's tilt, by the time it's March, the Northern Hemisphere will start getting more sunshine, which means more warm air.
(Diagram from Scholastic.com)
- The new warm air pockets start flowing toward the big cold air masses that are still hanging around. The pockets of warm air will rush toward them, trying to balance out the pressure. The warm air rushes up into the pockets of cold air, mixing the pressures and creating lots of wind.
- Throughout March, the earth is steadily working its way around so that still more sunshine falls on the Northern Hemisphere. By the time we hit April, there's a lot more warm air, and the cold air masses are less severely cold. So the wind isn't as gusty and forceful come April.
- I think it's pretty much a guarantee that March will always come in like a lion and go out like a lamb.
(Image by Karen Craig, from the picture book directory of children's illustration)
Weather Questions.com, What causes wind?
theweatherprediction, What Causes Wind? by meteorologist Jeff Haby
Weather Dudes, Why is March such a windy month?
USA Today blog, diagram labeled Why is March so windy?