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NARRATOR

Listen to part of a lecture in a meteorology class.

MALE PROFESSOR

Now, Earth's atmosphere is sort of like a giant weather machine, right? With air and water being its key components.

A machine powered by energy from the Sun.

So, we need to consider the role that air, or more specifically, wind, plays in this machine.

So wind is really nothing more than moving air, right?

Now, as air is heated and becomes warmer, it expands, it becomes less dense.

When air in a particular area is heated, you get a concentration of warm air, an area of low pressure.

Likewise, when you get a concentration of cold air, that air's gonna be very dense, so it's gonna create an area of high pressure.

If you have an area of high pressure next to an area of low pressure, the colder, high pressure air will start moving toward the area of low pressure, right?

And the warmer air will move away, rising above the cooler air. Okay?

Uh...yes. Linda?

Linda，请说？

FEMALE STUDENT

[unsure] So wind is actually generated by the Sun?

MALE PROFESSOR

Well, Earth's rotation plays a role, and there are other factors and we'll come back to all that, but principally, yes.

The Sun creates the temperature differential that creates the areas of high pressure and low pressure that create wind.

FEMALE STUDENT

Um...l don't get what...how it causes a temperature differential.

MALE PROFESSOR

Right. Good. This will take us to the role of wind in the climate.

The key is that the Sun warms up different parts of Earth at different rates, and to different degrees.

For example [searching for examples], at sunrise, the land heats up faster than the ocean…that's why you get morning sea breezes.

The air over the water heats more slowly than air overland, so during the early morning, it's cooler and denser than air overland, so it moves in toward land -- a sea breeze.

What else?

The Sun's energy is more intense near the equator than it is near the poles.

So you've got masses of warmer air over the equatorial regions and masses of cooler air over polar regions and these masses are constantly interacting with each other, which is critically important for Earth's climate.

Uh, one result of these interactions is that equatorial air masses move away from the equator and in the process those equatorial winds actually take heat away from the equator and transfer it to some cooler part of Earth.

And by redistributing this energy—the Sun's energy really—winds play a critical role in maintaining a temperature balance from the poles to the equator.

Now, winds also help Earth maintain its balance in another way: by transporting water from one part of Earth to another.

Water is contained in the air in the form of vapor—mostly through evaporation, mostly from the oceans—and so when the air moves, it carries the vapor with it to some other parts of Earth, where it can deposit it as rain or snow, or some other form of precipitation.

Now, actually, the transfer of heat and water by way of the wind are very closely related… because a primary way that heat energy is transferred by wind is mediated by the process of evaporation.

What happens is that a certain amount of heat energy is required to convert liquid water into vapor.

So when water evaporates from the ocean, it takes energy to convert that water into a gaseous form, into water vapor.

But that heat energy, that conversion energy, doesn't raise the temperature of the water vapor or the air, it's just stored in the water vapor.

Then later when the water vapor converts back to liquid water, that energy is released.

[recapping] So: when water evaporates, energy’s taken from the ocean, and it’s stored in the water vapor, in the air.

Then the air—the wind—transports the water vapor to some other part of Earth.

Then the water vapor converts back into liquid water—It rains, in other words—and the heat energy that was stored in the water vapor is released into the new environment.

Okay? So the transfer of heat and the transfer of water are very closely related.

And what's the primary vehicle for this transfer?

The wind! So wind is a very critical element in the redistribution of both the Sun's energy and Earth's water.

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