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As this warm air rises due to its lower density, it cools. Once it cools past the dewpoint, condensation occurs and clouds form. With continued rise and cooling, the air cannot hold the moisture and precipitation falls.
In response to that rising air, surface air must flow in to fill the vacated space. The rising air results in a low-pressure center. This is why when you hear about low pressure in the forecast, is typically associated with rising air masses and therefore with crummy weather. The air rushing in toward the equator defines the trade winds. These winds converge on the equator but blow to the West because of Earth’s rotation. This rotational effect is known as the Coriolis effect. We won’t get into that in detail here, but if you are interested, check out the video below.
Video: The Coriolis Effect (02:43)
[thundering]
NARRATOR: Picture a circle. Here's its center, here's point A, and here's point B. Point A is twice the distance from the center of the circle than point B. Oh, yeah, and it spins from its center. In two seconds, both points do one full revolution. But to go all the way around, point A has to go this far, while point B only has to go this far. And we all know if something travels a greater distance in a shorter amount of time, it must be going faster. So, point A must be moving faster than point B.
Okay, now swap out this flat circle for the Earth, and the same thing is true. All points closer to the center, say like someone in Greenland, will be spinning slower when compared to points spinning further away from it, say like people in Brazil, closer to the equator. So, if we look at it all flattened out, we can picture something like this. Arrows at the equator travel faster than arrows at the 45 degree line, like we just observed. Now, imagine you're a cloud that formed here on the equator. You'll have the same velocity as the Earth. But then a gust of wind sweeps you to the north, where the Earth isn't spinning as fast.
Due to inertia, your speed remains the same. You don't get any faster, but everything around you is literally traveling slower, so you, relative to the ground, move ahead of everything else. If you're a cloud that forms at the 45 degree line, you'll also have the same speed as everything around you. But if you drift down to the equator, you'll be moving slower than the ground underneath you, so you'll fall behind. And the same thing for the Southern hemisphere. Moving towards the equator always results in falling behind, while moving away results in pushing ahead.
Okay, now imagine a low-pressure cell. That means all the air around it will get sucked into the center. But the air coming from the equator will be traveling faster, so it will deflect to the right, while the air coming from the poles will be moving slower, so they'll fall behind and deflect to the left. What this results in is a circular air current spinning counterclockwise. And that's exactly what hurricanes are, low pressure cells spinning because of the Coriolis effect. Moving this example down to the Southern hemisphere, things are reversed. A low pressure cell will still suck in the surrounding air, but now the air coming from above will be moving faster, again deflecting to the right, while the air coming from below is moving slower, again falling behind by moving to the left.
This results in a clockwise spin, which is why storms spin, which is why storms in the Southern hemisphere spin this way. And that's about it. It's a short video, and that's the point. I hope you got what you came for. And the Coriolis effect doesn't really influence toilets. They're just really too small. And the direction of the spin more depends on the placement of jets inside the toilet. But that's it. That's the Coriolis effect. If you like this short and to-the-point video, give this video a like. And if you want to see more videos like it, why not subscribe? I'll be back next week with another video. So until then, thanks for watching.
These flows drive convection cells, with dimensions that are controlled by the viscosity and density of air, and by the thickness of the atmosphere. The air that rose from the equator flows North and South at the top of the cell and eventually descends at around 30° N or S latitude. As the cool, now dry air descends, it warms. Sound familiar?
Just as occurs when air descends on the leeward side of mountain ranges and causes rain shadows, the amount of water that the descending and warming air could hold increases. But there is no additional moisture to be found, so the actual amount of water vapor in the air mass remains more-or-less fixed. These descending limbs of the Hadley cells form high-pressure centers and would be regions where persistent dry conditions should prevail – leading to the Earth’s desert belts that include the Gobi, Sahara, Arabian, and the Australian Outback (not just a steakhouse!).
The equatorial convection cells are known as Hadley Cells. There are two more in each hemisphere, also driven by the uneven distribution of incoming solar radiation density; these are Farrell and Polar cells. Check out the diagram of this process below.
