Factors That Affect Wind: Pressure Gradient Forces, Coriolis Effect & Friction

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  • 0:08 Air in Motion
  • 1:16 Pressure Gradient Forces
  • 2:02 The Coriolis Effect
  • 3:47 Friction Affects Wind
  • 4:33 Summary
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Lesson Transcript
Instructor: Sarah Friedl

Sarah has two Master's, one in Zoology and one in GIS, a Bachelor's in Biology, and has taught college level Physical Science and Biology.

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.

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.

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