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In this video lesson, you will learn about wind and some factors that affect it, such as air temperature and pressure, the Coriolis effect, and friction.

By the end of this lesson, you will also understand how Earth’s rotation plays a role in how winds travel.

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Air in Motion

Let’s do a quick activity. Hold your hand out a few inches from your mouth, with your palm toward your face. Now open your mouth wide and blow onto your palm.

The air that hits your palm is fairly warm. Now make your mouth into a tight ‘O’ and blow again. Were you surprised by the result? This time, the very same air that came from your mouth was cool!This happens because air cools as it expands.

When you closed your mouth in the second part of the activity, you restricted the space where the air could leave your mouth, and so it expanded outward as it made its way to your palm. It cools because as the space becomes larger the particles in the air run into each other less and therefore give off less heat.The same thing happens with air in the atmosphere. As warm air rises, it expands and cools. It then sinks back down to fill the space the warm air left behind.

This convection current, or circulation of warm air rising and cool air sinking, has some interesting effects on wind. Wind is horizontally moving air – so, any air that moves left to right instead of up and down.

Pressure Gradient Forces

Wind ultimately comes from temperature differences because, as we learned in another lesson, temperature differences lead to air pressure differences, and air pressure creates convection currents, which, as we just learned, create wind. Let’s back up and see how this works.

Say we have a warm location, like the equator, and a cold region, like the North Pole. Air at the equator is warmed with more solar energy than the air at the North Pole, so it rises and then moves horizontally toward the North Pole. As it cools, it sinks back down toward the warmer equatorial region. The air pressure difference between the two locations is called the pressure gradient, and the force that actually drives the air from high pressure areas to low pressure areas is called the pressure gradient force.

The Coriolis Effect

If Earth didn’t rotate (which we know it does, because we have cycles of day and night), this pressure gradient force would create two single-cell circulations of wind – one for the Northern hemisphere and one for the Southern hemisphere.

But since Earth does spin on its axis, we get multiple circulations of wind on Earth. What’s especially interesting is that this rotation of Earth affects the path of wind so that it appears to deflect to the right in the Northern hemisphere and to the left in the Southern hemisphere (if you’re looking down from one of the poles). This deflection of wind from Earth’s rotation is called the Coriolis Effect.

The Coriolis effect is like being on a merry-go-round. Imagine that you are on one side and your friend is directly across from you on the other. If you were to throw a ball to your friend while the merry-go-round was not spinning, it would go straight to them. This is our one-cell circulation pattern on a non-rotating Earth.

But now let’s get the merry-go-round spinning counter-clockwise. Throw the ball to your friend again and the ball misses them this time! You may not realize it, but the ball is still traveling in the same straight-line path as before, and you would see this if you looked straight down from above. However, from your perspective on the merry-go-round, it looks as if the ball is deflecting to the right as it misses your friend.

This is what happens to wind on Earth. As Earth rotates, all free-moving objects, like air, water, airplanes and even snowballs, appear to leave their straight-line paths. Nothing is free from the Coriolis effect!The Coriolis effect also differs depending on wind speed and latitude. When wind travels faster, it gets deflected more. Objects that are higher in latitude (so, closer to the Polar Regions) deflect more than those at the equator.

Friction Affects Wind

When we talk about wind, we certainly can’t forget friction. Friction is a force that restricts motion, and any time an object comes in contact with a surface, it experiences friction.

You can feel this when you slide down a slide – you definitely feel the friction on the back of your legs if you slide down in shorts instead of long pants!Wind encounters friction all over Earth because it’s constantly coming in contact with Earth’s surface. The rougher the surface the more friction it will experience. So a sandy desert will allow wind to move over it more easily, but a rough mountain range is likely to slow that wind down considerably. Friction also reduces the effect of the Coriolis effect, because as we learned before, slower wind deflects less than faster-moving wind.

Lesson Summary

Just like the air that comes out of your mouth, air in the atmosphere cools as it expands.

As warm air in the atmosphere expands and cools, it sinks to fill the space the warm air left behind, and this circulation of warm air rising and cool air sinking is called a convection current. Convection currents create wind, which is horizontally moving air. But convection currents are ultimately at the mercy of temperature differences because these are what create pressure differences in the first place. The pressure difference between two locations is called a pressure gradient, and the force that actually moves air as wind is called the pressure gradient force.

Earth’s rotation means that air does not circulate in a single-cell convection current for each hemisphere. Instead, we get multiple air cells and the Coriolis effect, which is the apparent deflection of wind due to Earth’s rotation. Like throwing the ball to your friend on the merry-go-round as it spins, wind actually travels in its same straight-line path but when observed from above appears to deflect to the right in the Northern hemisphere and to the left in the Southern hemisphere. Wind deflection is greater with higher wind speed as well as with higher latitudes.Friction also plays an important role in wind behavior on Earth because this is a force that restricts motion. When wind encounters rougher surfaces on Earth, it slows considerably. When wind travels more slowly, the impact of the Coriolis effect is also lessened, because slower winds deflect less than winds traveling at faster speeds.

Learning Outcomes

When this lesson is finished, you should be able to:

  • Define convection currents
  • Describe pressure gradient and pressure gradient force
  • Identify how the earth’s rotation helps create the Coriolis effect
  • Understand friction’s effect on wind

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